FOUS11 KWBC 052119
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 PM EST Fri Mar 05 2021

Valid 00Z Sat Mar 06 2021 - 00Z Tue Mar 09 2021


...Pacific Northwest/California to the Northern Rockies...
Days 1-2...
Ongoing precipitation associated with a frontal band moving inland
across the Pacific Northwest and Northern California is expected
to continue through the evening, with additional mountain snows
likely from the Olympics and Cascades to the northern Sierra.
While accumulations are expected to be light across most areas, a
well-defined shortwave moving through the base of the broader
scale trough is expected to amplify near the Northern California
coast overnight. This will likely support some heavier
accumulations, especially across the mountains of northwestern
California into the southern Cascades. WPC PWPF suggests
widespread additional accumulations of 4-8 inches are likely
across this region during the evening and overnight hours. Snows
are expected to quickly diminish across this area as the shortwave
lifts northeast of the region on Saturday.

Shortwave energy rotating out ahead of a closed low centered west
of British Columbia will drive another front into the Northwest
Sunday morning, with heavy snow accumulations likely over the
Olympics. The potential for organized heavy amounts is expected
to diminish as the front moves east and weakens on Sunday.

...Eastern Great Lakes...
Days 1-2...
Deep northwesterly flow with embedded energy aloft will support
multi-band lake effect snow showers south of Lake Ontario and
eastern Lake Erie Friday night into Saturday. While widespread
heavy amounts are not expected, at least a couple of inches of
snow are likely, especially north of and along the Finger Lakes
region. Snows will likely diminish as a surface high pressure
shifts east into the region by late Saturday.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira



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FOUS11 KWBC 060829
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Sat Mar 06 2021

Valid 12Z Sat Mar 06 2021 - 12Z Tue Mar 09 2021


...Pacific Northwest/California to the Northern Rockies...
Days 1-3...
Frontal band precipitation moving inland across Northern
California is just reaching the eastern side of the San Joaquin
Valley, but will be winding down over for the northern Sierra
Nevada by 12Z (when Day 1 snow probabilities begin) as the
mid-level shortwave trough axis lifts northeast into the Great
Basin with veering low level flow in the approaching ridge cuts
off Pacific moisture.

A cold front from a low pressure system drifting south well off
the BC coast reaches the WA/OR/far northern CA coast late tonight
with snow levels around 2000ft. Organized precip/mountain snows
diminish as the front moves east and weakens on Sunday and onshore
flow backs southerly, cutting off moisture influx. The next
trough/frontal boundary from this low that should be off the WA
coast by Sunday night reaches the PacNW coast Monday with a decent
fetch of Pacific moisture pushing across far northern CA. Day 1.5
snow probabilities are high for 6 or more inches for the Olympics
and higher WA Cascades. Day 3 snow probabilities for 6 or more
inches are moderate for the Klamath/CA Cascades and
northern/central Sierra Nevada as well as some low probabilities
for the Wallowa Mtns of OR and the Salmon River/Clearwater Mtns of
ID.

...Eastern Great Lakes...
Day 1...
Deep northwesterly flow with embedded energy aloft and a saturated
DGZ will continue to support multi-band lake effect snow showers
south of Lake Ontario and eastern Lake Erie today. At least an
additional couple of inches of snow are likely, especially north
of and along the Finger Lakes region. Snows will diminish as a
surface high pressure shifts east into the region this evening.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson



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FOUS11 KWBC 061945
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Sat Mar 06 2021

Valid 00Z Sun Mar 07 2021 - 00Z Wed Mar 10 2021


...Pacific Northwest/California to the Northern Rockies...
Day 1-3...
Shortwave energy moving through the base of an upper trough
associated with a closed low centered west of British Columbia is
expected to drive another another frontal band into the Northwest
as it lifts toward Vancouver Island Sunday morning. This will
bring another round of locally heavy mountain snow into the
Olympics and northern Cascades Saturday night and Sunday.
Shortwave ridging will then likely bring a brief period of drier
weather across the Northwest Sunday evening. Then Sunday night
into Monday, the latest guidance disagrees on the timing and
extent to which wet weather will return to portions of the
Northwest and northern Rockies. Overall, models are in generally
good agreement with the synoptic setup -- showing a well-defined
shortwave lifting northeast from Northern California to the
northern Rockies, ahead of the previously noted low as it drops
farther to the south off of the Washington and Oregon coasts.
Left-exit region upper jet forcing along with a low-to-mid level
baroclinic zone dropping southeast is expected to interact with a
modest increase in moisture to support increasing precipitation
coverage from Northern California to western Montana. However,
models differ on the extent of the more organized heavier
precipitation, with the NCEP guidance generally wetter than the
non-NCEP consensus. While forecast confidence is generally
limited, probabilities for heavier snow accumulations have
increased some, especially over northeastern Oregon into northern
Idaho during the Day 2 period. As this leading shortwave lifts
northeast across the northern Rockies and High Plains into central
Canada on Tuesday, upstream energy moving through the base of the
trough will draw it farther south across California, with
increasing potential for heavier mountain snow developing across
the Klamath Mountains into the southern Cascades and northern
Sierra.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira



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FOUS11 KWBC 070833
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 AM EST Sun Mar 07 2021

Valid 12Z Sun Mar 07 2021 - 12Z Wed Mar 10 2021


...Pacific Northwest/California to the Northern Rockies...
Day 1-3...

Three shortwave troughs round a low pressure system shifting south
from off the BC coast and down the West Coast, tracking into The
West through midweek.

The first wave is reaching the WA/OR coast early this morning and
will lift northeast over WA today. 0.5" PW air pushing to the WA
Cascades will make for moderate mountain snow today with moderate
Day 1 snow probabilities for 6 or more inches for the Olympics and
high WA Cascades with snow levels dropping to around 1500ft this
morning.

The second shortwave trough is currently just reaching the low
pressure system from the central Gulf of Alaska, it amplifies the
surrounding trough through tonight before swinging east-northeast
across northern CA Monday, the northern Great Basin to MT Monday
night. Left-exit region upper jet forcing along with a low-to-mid
level baroclinic zone dropping southeast is expected to interact
with a modest increase in moisture to support increasing
precipitation coverage from Northern California to western
Montana. Day 2 snow probabilities for 6 or more inches are
moderate in the Klamath and CA Cascades, the Wallowa Mtns of
northeast OR, the Salmon River and Clearwater Mtns of ID as well
as ranges in central MT.

The third shortwave trough ejects south from the eastern Gulf of
Alaska tonight, amplifying the trough even further south before
ejecting east-northeast across the central CA coast Tuesday,
crossing UT Tuesday night. High level Pacific moisture wraps
around the lee-side surface cyclone that develops over the central
High Plains Tuesday night, aiding a swath of higher elevation snow
(snow levels around 5000ft) to develop across WY to the Black
Hills that continues through Wednesday. Day 3 snow probabilities
for 6 or more inches are moderately high for the Klamath Mtns,
down the full extent of the Sierra Nevada and low for ranges in
east-central NV, the northern Wasatch in UT, and around the
Bighorn Mtns in WY.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson



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FOUS11 KWBC 072054
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EST Sun Mar 07 2021

Valid 00Z Mon Mar 08 2021 - 00Z Thu Mar 11 2021


...Pacific Northwest/California to the Northern and Central
Rockies/Northern Plains...
Day 1-3...
As a closed low centered west of British Columbia continues to
drop south, a series of shortwaves moving through the base of the
associated trough will eject eastward -- impacting the western and
central U.S. through the period.

Overall, models are in good agreement -- showing a well-defined
shortwave lifting from Northern California though the northern
Intermountain Region and into the northern Rockies on Monday.
Models continue to show favorable upper jet forcing, along with
low-to-mid level frontogenesis, interacting with increasing
moisture to support a southwest to northeast oriented swath of
light to moderate precipitation developing and shifting northeast
across the region. Guidance has been been moving into better
agreement over the past 24 hours with respect to the timing and
magnitude of precipitation -- with the GFS backing off some on its
more expansive heavier amounts. That said, probabilities for
widespread significant accumulations have decreased during the Day
1 period (ending 00Z Tuesday). However, WPC PWPF still indicates
some potential for locally heavier amounts across the Blue
Mountains into the northern Idaho ranges. As this energy
continues to lift northeast, snows are expected to continue into
the evening hours across northeastern Oregon and northern Idaho,
while developing farther northeast across western Montana. A
brief period of low-to-mid level northeasterly northeasterly
flow/frontogenesis is expected to support a period of light to
moderate snows from the northwestern Montana ranges into the High
Plains.

This leading energy is forecast to lift northeast into central
Canada, with precipitation diminishing across the northern Rockies
and High Plains on Tuesday. Meanwhile, the upper low will
continue to drop south off of the Washington and Oregon coasts,
with upstream energy ejecting farther south across California.
During this period, the potential for locally heavy snow is
expected to increase from the Klamath mountains in southwestern
Oregon and northwestern California to the northern Sierra.

On Wednesday, the threat for heavy snow will continue to spread
south across California. As energy continues to eject inland, the
upper low is forecast to continue its southerly track -- drawing
cold air and the axis of stronger inflow farther south across the
state on Wednesday. Periods of heavy snow are likely to extend
from the northern into the southern Sierra by early Wednesday,
with the likelihood of heavy snow increasing along the Southern
California coastal ranges during the day.

Meanwhile, energy ejecting across California into the Great Basin
on Tuesday is forecast to lift across Colorado and Wyoming early
Wednesday. As it does, models are showing a pretty good signal
for significant snows developing from central Wyoming into western
South Dakota. Favorable upper jet forcing along with low-to-mid
level frontogenesis/upslope flow is expected to support at least
light to moderate accumulations across the region -- with WPC PWPF
showing high probabilities for 4-inches of snow across the region,
with locally heavier totals likely across the Bighorn Mountains
and Black Hills.

Models have shown a fair amount of run-to-run variability and
spread with respect to QPF as this system moves downstream.
Therefore, while at least some measurable snow and/or ice does
appear likely across the Dakotas into Minnesota, details of the
forecast late in the period are far from certain.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

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FOUS11 KWBC 080840
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Mon Mar 08 2021

Valid 12Z Mon Mar 08 2021 - 12Z Thu Mar 11 2021


...The West and Northern Plains...
Day 1-3...
As a closed low currently centered west of Vancouver Island shifts
south off the Pacific Northwest Coast through midweek, a pair of
shortwave troughs moving through the base of the associated trough
will eject east-northeastward. These will cross the Pacific
Northwest and northern Rockies and then California, the Great
Basin, the central Rockies and the northern Great Plains.

The first shortwave trough axis lifts from Northern California
though the northern Great Basin today and across the ID/MT Rockies
tonight. Favorable upper jet forcing and low-to-mid level
frontogenesis, interacting with increasing moisture supports a
southwest to northeast oriented swath of light to moderate
precipitation shifting northeast across these regions. A brief
period of low-to-mid level northeasterly northeasterly
flow/frontogenesis is expected to support a period of light to
moderate snows from the northwestern Montana ranges into the High
Plains tonight before lifting into central Canada Tuesday.
Meanwhile, the closed low will continue to track southeast toward
the far northern CA coast, with upstream energy ejecting inland
farther south across California Tuesday. During this period, the
potential for locally heavy snow is expected to increase from the
Klamath mountains in southwestern Oregon and northwestern
California to the northern Sierra Nevada. Day 1 snow probabilities
for 6 or more inches are moderately high over the Klamath/CA
Cascades, far northern Sierra Nevada, Wallowa Mtns of northeast
OR, the Clearwater/Salmon River Mtns of ID, and west-central MT
Rockies.

The second shortwave trough ejects across south-central CA into
the southern Great Basin on Tuesday before shifting across
southern UT and central CO Tuesday night. Favorable upper jet
forcing along with low-to-mid level frontogenesis/upslope flow is
expected to support at least moderate accumulations across the
region. Day 2.5 WPC snow probabilities are moderately high for 6
or more inches for the Uinta Mtns of UT and over central and
northeastern WY and the Black Hills.

The threat for heavy snow will continue to spread south across
California Tuesday night/Wednesday as the closed low tracks south
along the coast, drawing cold air and the axis of stronger inflow
farther south across the state on Wednesday. Periods of heavy
snow are likely to extend from the northern into the southern
Sierra Nevada by early Wednesday, with the likelihood of heavy
snow increasing along the Southern California coastal ranges
during the day.

There is decent agreement above non-CMC 00Z guidance for the Day 3
track of the second shortwave trough from the central Plains to
the Upper Midwest with ample moisture and cold enough air likely
allowing a swath of snow to develop along the north side of the
low level pressure center. As of now Day 3 snow probabilities are
low for 4 or more inches in a broken swath across northeast SD and
northern MN with low probabilities for a tenth inch of ice for far
northern WI into the western U.P. of MI.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 090911
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
410 AM EST Tue Mar 09 2021

Valid 12Z Tue Mar 09 2021 - 12Z Fri Mar 12 2021

...Western U.S....
Days 1-3...

A deep-layer closed low currently centered off the OR coast tracks
southeast to the far northern coast of CA through tonight. The
next shortwave trough to round this low and track inland over the
CONUS crosses southern CA this afternoon and track across the Four
Corners and southern CO tonight.

A combination of improving upper-level jet support,
frontogenetical forcing, and orographics will favor areas of brief
heavy snow across the Uinta range of northeast UT, the high
country of northwest Colorado which have moderate Day 1.5 snow
probabilities for 6 or more inches. East of the Divide in WY,
increasing easterly flow from lee-side cyclogenesis over eastern
CO this evening allows the snow swath to expand with the southern
Wind River range, the Big Horns, the Laramie Range, and areas east
from there to across the Black Hills of SD where Day 1.5 snow
probabilities for 6 or more inches are 50 to 80 percent.

As the aforementioned closed low advances down across California
into Thursday expect increasing deep-layer Pacific moisture
transport along with a lowering of snow levels as the height falls
arrive. This coupled with rather strong dynamic ascent and
orographic forcing will favor heavy snow across the length of the
Sierra Nevada where locally 1 to 2+ feet of new snow are expected
through Wednesday. Moisture and energy will spread down through
the mountains of southern CA tonight through Wednesday night with
snow levels around 4000ft and Day 2 snow probabilities moderate
for 8 or more inches, especially for the San Gabriel and San
Bernardino mountains.

This moisture influx shifts inland over the Desert Southwest
Wednesday and continues through the rest of the week as the parent
low stalls over southern CA. Pacific moisture and good orographic
forcing will yield accumulating snow for the interior of the
southern Great Basin, the Wasatch of UT, northern AZ where Day 2
and 3 have probabilities for 6 or more inches. This rather
positively tilted trough promotes southern Rockies lee side
troughing by Friday which should bring about an extended period of
snow for CO, some of which could be quite prolonged and heavy for
the CO slopes east of the Divide this weekend.


...North-central Great Plains through northern Minnesota...
Days 2-3...

The shortwave trough that crosses CO tonight refocuses lee-side
surface cyclogenesis to the central Plains/KS with a low that
quickly tracks northeast to the eastern IA/MN border Wednesday,
then to the U.P. of MI Wednesday night before quickly tracking to
northern Quebec Thursday. This fast moving, but elongated wave in
the rather positively tilted western CONUS trough with the low
center between the left exit region of an intensifying
southwesterly jet under the trough and the right entrance region
of the southwesterly jet that crosses James Bay Canada. Moisture
influx from across the entire Gulf of Mexico streams up the MS
Valley to the Great Lakes while moderately cold Canadian air
(Arctic-sourced air does not arrive into ND until Thursday) is
drawn down the northern Great Plains. Locally enhanced and banded
snow develops north of the low track across northwest Neb and SD
Wednesday then tracks across north-central and northeastern MN
through Wednesday night before lifting into Ontario Thursday
morning. Dynamical cooling in the banded snow looks to be required
to get the column cold enough for accumulating snow, limiting the
breadth of the snow swath. Overall there is good Day 2 agreement
on placement among global guidance. The 00Z regional CMC (which
goes out 84hrs) tracks farther south than the 00Z CMCnh which is
the southernmost track (ECMWF/UKMET/NAM are on the north side with
GFS in between. The Day 2 snow probabilities contain the Neb/SD
(east of the Black Hills)/MN snow well with a swath of low to
moderate probabilities for 4 or more inches from northeastern WY
across SD and north-central/Northeast MN. One note is the
southwestern extent of the wrap-around snow looks to linger in
northeast MN into Thursday morning.

The surface baroclinic zone looks to drift south enough to have
some overrunning and a stripe of freezing rain and sleet potential
across north-central MN into the western U.P. of MI. The ice
accretion risk is rather low with low probabilities for a tenth
inch of ice or more limited to the western U.P. for Day 2.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson

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FOUS11 KWBC 102150
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
450 PM EST Wed Mar 10 2021

Valid 00Z Thu Mar 11 2021 - 00Z Sun Mar 14 2021

...Upper MS Valley through the upper Great Lakes...
Day 1...

Low pressure will continue to develop as it tracks northeast from
southern Minnesota to Lake Superior by Thursday morning and
produce a swath of moderate to heavy snow from near the SD/ND/MN
border to northeastern MN. Strong low/mid level moisture
transport/convergence allows a corridor of strong lift on the cool
side of the system...which will help dynamically cool the column,
resulting in a transition to a heavy wet snow in west central to
northeast MN. This snowfall should accumulate 4-8" of snow from
northeastern SD to northeastern MN. the probabilities show about a
30 percent chance of 8 inches of snow in northeast MN northwest of
Lake Superior, as precip may start as mixed (not all snow) along
the shoreline. The event winds down Thu afternoon and the low and
associated ascent move northeast into Canada.

Accumulations should struggle closer to the low track, where the
rain/snow line exists...due to marginal BL temps and a rain/snow
mix or heavy, wet snow with low snow to liquid ratio snowfall in
the MN arrowhead.

An area of icing is expected in northern WI and into the western
UP of MI where overrunning with sufficient surface cold to
freezing supports a stripe of icing near a tenth of an inch. There
is about a 30-40 percent chance of a quarter inch of ice in
northern WI close to Lake Superior. The steady rainfall rates may
lead to runoff, so the heavy icing threat seems low.


...Southwestern U.S/Southern Great Basin/Central to Southern
Rockies and Central High Plains....
Days 1-3...

A closed 700 mb low centered over northern CA near coast drifts
east, reforming over Nevada on Thu. Areas of snow will continue to
occur in higher elevations of central/southern CA as Pacific
moisture is adverted inland over terrain of the Sierra Nevada and
southern CA ranges. As the low moves inland Thu., snow coverage
and rates taper as ascent dwindles in CA. Most of the accumulation
in the Sierra Nevada and southern CA ranges will be above 3500
feet.

Moisture will spread eastward across the Great Basin and Desert SW
to the Rockies tonight through Friday. 700 mb convergence maxima
cross the ranges of central to northern NV, and then the UT
Wasatch, resulting in several inches of snow. Day 1 snow
probabilities are moderately high for 6 or more inches over
central/eastern NV ranges, northern AZ and the Wasatch.

On Thu night-Fri, the elongated circulation continues to move
east, with bands of 700 mb coverage and ascent both near the
circulation and also downstream from the circulation as the cold
front moves east from UT into CO and WY. As the warm front moves
across the front range, warm advection and frontogenesis results
in snow developing over central CO, with marginal temperatures in
valley areas in the I25 corridor
resulting in heavier amounts in the foothills and mountains/front
range of CO to southeast WY.

On Friday night through Saturday, the 700 mb low redevelops in
southeast CO according to the NAM/Canadian global/ECMWF/GFS. The
leads to both strong low level convergence near the low and also
wrap around strong/moisture advection extending across northeast
CO into southeast WY.
The wraparound jet leads to strong upslope flow in northeast CO to
southeast WY, where 300 mb divergence maxima increase in
magnitude, supporting strong ascent. Consequently, confidence is
increasing in heavy snow developing in these areas, and continuing
beyond this period.
Probabilities are high for 4 inches and moderate for 8 inches on
Day 3 in the front range of CO to southeast WY.

The slow motion of the low looks likely to result in a major snow
storm for eastern slopes of the northeast CO/southeast WY
mountains,
with snowfall expected to be 2-4 feet when added over a multi-day
period. The primary uncertainty will be the duration of snow on
the Plains as the east movement of the low results in cooling and
transition to snow on the high plains of CO/WY/western NE.


Petersen

$$
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FOUS11 KWBC 112142
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
441 PM EST Thu Mar 11 2021

Valid 00Z Fri Mar 12 2021 - 00Z Mon Mar 15 2021


...Great Basin/Southwest to the Central Rockies and High Plains...
Days 1-3...
...Major Snow Storm Coming to Colorado and Wyoming...

Confidence is high that a major winter storm will bring heavy snow
to portions of the central Rockies and High Plains beginning on
Friday and continuing through Sunday. This includes the I-25
corridor from Cheyenne through Denver. Multiple feet of snow are
possible in the foothills and front range of northern CO and
southeast WY.

This storm is associated with a deep upper trough/low currently
centered over Nevada. On Day 1, the primary snow is expected over
the UT Wasatch, mountains of southern NV, and Mogollon Rim of AZ.
The 700 mb circulation advects moisture north across AZ in NV and
UT, with lift occurring to produce the snow tonight starting in
the Mogollon Rim of AZ up to the UT Wasatch late tonight through
Fri. WPC PWPF indicates a moderate probability of 4 inches and
low probability of 8 inches most areas, with one max of moderate
probabilities for 8 inches in the Mogollon Rim.

On Day 2, as the upper low begins to slowly move east, snow is
expected to continue across many of these same areas through
Friday into early Saturday, with additional light to moderate
accumulations likely, especially along the Mogollon Rim and the
Utah Mountains.

By early Saturday, strengthening warm/moisture advection on the
high Plains will produce increasing integrated vapor transport
across the high Plains and into the foothills and front range of
northern CO and WY. The strongest persistent 700 mb jet develops
in NE coming into WY. Increasing moisture and large-scale ascent
will support widespread heavy snows developing across the central
Rockies and High Plains on Saturday, continuing into Sunday. The
strong low level jet leads to anomalously strong upslope
conditions in northeast CO and southeast WY, with good multi-model
agreement on heavy snow.

For the Day 3 period, WPC PWPF shows widespread probabilities of
50 percent or greater for snow accumulations of 12 inches covering
much of southeastern Wyoming into the foothills and front range of
northern CO. Heavy snow is expected in the urban I-25 corridor
from Cheyenne through Denver. The 72 hour WPC PWPF shows that the
highest probability for 2 to 3 feet of snow is in the foothills
into the Front Range of northern CO/southern WY.

One major component of uncertainty is are how far north the
QPF/snow extends out of NE into and across southern SD, with
remaining large spread in solutions based on the 850-700 mb low
track north-south differences. Another component of uncertainty
is further east on the CO Plains and NE Plains, temperatures are
initially too warm for snow. As the low drifts east, the
rain/snow transition line drifts east across the NE panhandle and
CO Plains. The operational NAM produces dynamical cooling and a
faster transition than other models. The NAM is offset by other
models plus the NAM COnus Nest, which remains warmer longer than
the parent NAM, and thus has shorter duration snow in eastern CO
and western NE.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 130948
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
447 AM EST Sat Mar 13 2021

Valid 12Z Sat Mar 13 2021 - 12Z Tue Mar 16 2021

...Southwest to the Central Rockies and High Plains...
Days 1-2...
A major storm system will bring heavy snow to portions of the
central Rockies and the central High Plains Saturday through early
Monday. This includes the I-25 corridor from Cheyenne to Colorado
Springs.

A deep upper low currently moving into the southwestern U.S. is
expected to move into the Four Corners region later today.
Ongoing high elevation snow is forecast to continue across
portions of Utah and Arizona. While widespread heavy amounts are
not expected, some areas including portions of the Mogollon Rim
and White Mountains in central to eastern Arizona may see several
inches of additional accumulation on Saturday.

Meanwhile, strengthening southerly, low-level winds will begin to
transport deeper moisture up through Texas into the central High
Plains, where a developing wave will begin to direct that moisture
back west into the high terrain. As the upper low moves farther
east, increasing moisture and large-scale ascent will support
widespread heavy snows developing across the central Rockies and
High Plains on Saturday, continuing into Sunday. For the Day 1
period (ending 12Z Sunday), WPC PWPF continues to shows widespread probabilities of 50 percent or greater for snow accumulations of
8-inches or more covering much of southeastern Wyoming and central
Colorado. This includes the urban I-25
corridor from Cheyenne to Colorado Springs. WPC PWPF shows that
heavier totals on the order of 18-inches or more are likely just
east of the corridor into the Front Range and the Laramie
Mountains. In addition to these areas, heavy snow accumulations
are also expected across the Uinta Mountains in northern Utah and
southwestern Wyoming, as well as along the Wind River Range and
southern Big Horn Mountain in Wyoming. Relatively lighter, but
significant accumulations will extend east into the High Plains,
with WPC PWPF indicating high probabilities for accumulations of
4-inches or more reaching out into the western Nebraska Panhandle
on Day 1.

Models show the low moving east of the central Rockies, with heavy
snow continuing across portions of the High Plains and Rockies on
Sunday. Additional accumulations of 8-inches or more are likely
for portions of the Front Range, southeastern Wyoming, including
the Cheyenne Metro, the Nebraska Panhandle, and southwestern South
Dakota, including the Black Hills. Snows will likely diminish
Sunday night as the low-to-mid level center moves east. Storm
total amounts of 2-feet or more are likely from the southeastern
Wyoming I-25 corridor back into the Laramie Mountains, as well as
along the Colorado Front Range. Storm totals of 2-feet or more
are also likely across portions of the Wind River Range and Uinta
Mountains.

...Northern Plains and Upper Midwest...
Days 2-3...
As the deep low centered over eastern Colorado Sunday evening
begins to lift farther northeast, precipitation will shift
northeast across the region, with rain changing to snow across
portions of eastern South Dakota and western Iowa, with mainly
snow developing across southwestern Minnesota Sunday night into
early Monday. Snow is expected to continue across the region,
reaching into southern Wisconsin and northern Illinois during the
day on Monday. Models have been trending cooler and wetter, with
higher probabilities for accumulations of 4-inches or more
expanding across southeastern South Dakota, southwestern Minnesota
and northwestern Iowa.

...Oregon and California...
Days 2-3...
Energy moving through the base of a sharp upper trough moving
across the eastern Pacific will develop a closed low west of
Oregon on Sunday, that will then drop southeast across California
into Nevada on Monday. This will support lower elevation snows
across southeastern Oregon into northwestern California with
significant high elevation snows possible from the Klamath
Mountains and southern Cascades to the Sierra.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 132125
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
424 PM EST Sat Mar 13 2021

Valid 00Z Sun Mar 14 2021 - 00Z Wed Mar 17 2021

...Southwest to the Central Rockies and High Plains...
Days 1-2...
A major snow storm is expected across portions of the central
Rockies and the central High Plains tonight through early Monday.
This includes the I-25 corridor from Cheyenne to Denver. Snow
amounts near 3 feet are expected in the ranges on northern CO and
3-4 feet in the Laramie Mountains of southeast WY.

A deep upper low over the four corners region this evening is
expected to continue moving east northeast tonight to near the
CO/NM border Sun morning , then turning northeast across eastern
CO tomorrow afternoon. A well defined upper jet wraps around the
low and moves east to west across northern CO and southern
WY,accompanied by 300 mb divergence maxima. The anomalously
strong easterly flow tonight support strong upslope flow coming
into the mountains of southeast WY. The combination of synoptic
lift and orographic lift is expected to result in heavy snowfall
rates tonight and Sunday, with potential for record setting snow
in southeast Wyoming.
For the Day 1 period, WPC PWPF continues to shows high
probabilities of a foot of snow across the Wind River Mountains,
Casper Arch, and then most of southeast WY to the NE border, and
the Fort Collins/Denver corridor. A secondary area of high
probabilities continues over the Uinta Mountains in northern Utah
and southwestern Wyoming. The primary uncertainty is
precipitation type as precip now rain in northeast CO and adjacent
NE gradually changes over to snow, with several inches expected by
Sunday evening.
Additional 2 to 4 inch amounts occur as moisture continues to wrap
around the circulation in northeast CO tomorrow evening,
continuing under the upper jet in western NE and southwest SD
tomorrow evening before the upper low and jet depart later
tomorrow night.

...Upper MS Valley to the Upper Midwest/Upper Great Lakes/Central Appalachians...

Day 2...
As the deep low centered over eastern Colorado Sunday evening
begins to lift farther northeast, precipitation will shift
northeast across eastern NE and SD, continuing across southern MN
and Iowa. Lowering heights results in precip changing over to all
snow.
The confluent low-mid level flow results in 850-700 mb theta-e
advection and convergence maxima, which supports liquid equivalent
amounts of half to one inch. Heavy snow is possible where the
longer duration snow is forecast across southern MN to norther
central IA.

The other uncertainty regard the models gradually tapering QPF and
resultant snow amounts as you go across WI as the 850-700 mb
theta-e advection gradually weakens as Mon progresses. Models
have been trending cooler and wetter, with higher probabilities
for accumulations of 4-inches or more across southeastern South
Dakota, southern Minnesota and northern Iowa. Probabilities
decrease the further south across NE/IA you go, due to shorter
duration snow that is forecast.

The models have indicated that the mid level warm advection
downstream in northern IL and IN should result in precip when sfc
temps are still at or below freezing. With warming aloft
occurring first, light freezing rain appears likely across much of
northern IL and IN. Several models forecast near a tenth of an
inch before the change over to rain. A low risk of a quarter inch
is shown as a small minority of ensemble members forecast freezing
rain amounts that high. Light icing is possible on Day 3 from
northern IN across southern lower MI, and in the mountains of
western PA to western MD, and straddling the WV/VA border.

...Oregon and California Day 2 and Southern Great Basin Day 3...

The next approaching upper level trough results in the forward jet
maxima arriving in western OR and CA late tomorrow
afternoon/evening. Increasing moisture and advection and
convergence results in snow developing late tomorrow
afternoon/evening and continuing overnight, starting in the ranges
of southwest OR and northwest CA and spreading inland as Sun night
progresses.

With lowering heights, temperatures aloft cool, so snow levels
fall in OR/northern CA Monday. This will support lower elevation
snows across southeastern Oregon into northwestern California with
significant high elevation snows possible from the Klamath
Mountains and southern Cascades to the Sierra. The models have
the highest QPF in the form of snow in the northern CA Sierra
Nevada, so the highest probabilities for heavy snow are there,
supported by amounts of a foot in the NAM, GFS, and ECMWF.

As the upper trough/embedded closed low move southeast tomorrow
night to Tue morning, snows taper in CA/OR and develop in the
ranges of southern NV Tue, and continue on to the ranges of
southern UT as the upper low crosses Tue during the day. Several
inches of snow are expected in these areas.

For Days 1 and 3, the probability of significant icing (0.25-inch
or greater) is less than 10 percent.

Petersen


$$
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FOUS11 KWBC 140828
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Sun Mar 14 2021

Valid 12Z Sun Mar 14 2021 - 12Z Wed Mar 17 2021


...Central Rockies/High Plains...
Day 1...
Ongoing snow associated with a deep closed low moving east of the
central Rockies is expected to continue through Sunday, with
additional accumulations of 8-inches or more likely across much of
southeastern Wyoming, north-central Colorado, southwestern South
Dakota, and the western Nebraska Panhandle into north-central
Nebraska. Within this area, locally heavier amounts of a foot or
more are likely along and west of the southeastern Wyoming I-25
corridor and along the Colorado Front Range. Portions of the
northern Nebraska Panhandle into southeastern South Dakota into
the Black Hills will also likely see locally higher totals as well.

...Northern Plains/Midwest...
Days 1-2...
As the upper low begins to lift northeast, precipitation over the
central Plains will shift northeast into the northern Plains and
upper Mississippi valley with rain changing to snow across eastern
south Dakota and western Iowa Sunday evening, with mostly snow at
the onset across southwestern Minnesota. Models still presenting
a good signal for at least a few inches of snow accumulating
across the region by early Monday, with WPC PWPF showing high
probabilities for accumulations of 4-inches or greater by 12Z
Monday across portions of southeastern South Dakota, southwestern
Minnesota and northwestern Iowa. As precipitation continues to
push farther east, a wintry mix is expected from Iowa into the
southern Great Lakes region on Tuesday. While some models are
indicating the potential for significant freezing rain
accumulations -- resulting in some low end probabilities for ice
accumulations of 0.25-inch or more across portions of northern
Illinois and Indiana, current thinking is that those numbers may
be overdone, with trends and thermal profiles showing greater
potential for accumulating sleet.

...Western U.S....
Days 1-3...
Energy moving through the base of a sharp upper trough moving
across the eastern Pacific will develop a closed low west of
Oregon on Sunday, that will then drop southeast across California
into Nevada on Monday. This will support lower elevation snows
across southeastern Oregon into northwestern California with
significant high elevation snows possible from the Klamath
Mountains and southern Cascades to the Sierra. Generally light
accumulations will spread across the Great Basin and Arizona into
the central and southern Rockies Monday and Tuesday.

Pereira

$$
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FOUS11 KWBC 142040
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
439 PM EDT Sun Mar 14 2021

Valid 00Z Mon Mar 15 2021 - 00Z Thu Mar 18 2021


...Northern Plains/Midwest...
Days 1-2...
As the upper low begins to lift northeast from CO across the
central Plains, precipitation over the central Plains will shift
northeast into the northern Plains and upper Mississippi valley in
tandem with an upper level coupled jet, which supports upper
divergence maxima and lift crossing southern South Dakota/northern
Nebraska tonight and then Iowa/southern Minnesota to Wisconsin
Monday.

Precip has started out as rain across much of the threat. As the
upper low approaches,falling temperatures result in rain changing
to snow , with the transition zone moving east roughly in concert
with the low.

The heavier snows are forecast in the coupled upper
divergence/lower convergence maxima across southern SD to southern
MN and northern Iowa.
WPC PWPF showing high probabilities for accumulations of 4 inches
or greater across southeastern South Dakota, southern Minnesota
and northern Iowa. As precipitation continues to push farther
east, a wintry mix is expected from Iowa into the southern Great
Lakes region on Tuesday. While some models are indicating the
potential for significant freezing rain accumulations -- resulting
in some low end probabilities for ice accumulations of 0.25-inch
or more across portions of northern Illinois and Indiana, a
mixture of both sleet and freezing rain will likely keep amounts
lighter than a quarter inch.

...CA/Great Basin/Central/Southern Rockies/High Plains of southern
CO, northeast NM to western KS....
Days 1-3...

An upper trough dropping southeast from the Pacific and moving
across OR and CA results in a 70 mb front progressing across OR/CA
with a period of enhanced moisture/ascent in advance of the front.
Strong vertical motions as the pre-frontal band of precip
crosses the Shasta/Siskiyou Ranges/CA Cascades and CA Sierra
Nevada mountains results in heavy snow across these ranges.
The probability of a foot of snow is moderate to high across these
CA Ranges. Snow tapers Monday afternoon as the front moves east
into NV and drying aloft occurs.

Monday night and Tuesday, the 700 mb low moves across southern NV
and UT,reaching southern CO/northern NM by Tue evening.
Low level convergence and pooled moisture ascent along the low
track results in several inches of snow across the ranges along
the cyclone's track. The steady movement of the low keep the
probabilities of 4 inches low to moderate and probability of 8
inches low in the southern UT Wasatch.

On Tue night to Wed., the 700 mb low crosses the Sangre DeCristo
Mountains in southeast CO to northeast NM and then turns east
northeast across the southern Plains. Several inches of snow are
expected in the Sangre de Cristo Mountains as the circulation
crosses the region, which should taper late Wed as he low departs.
The NAM, ECMWF, Canadian global, and several SREF members show
cooling aloft in the north side of the circulation in the Plains
of southeast CO to western KS, allowing a change over to snow.
There is a chance for several inches of snow according to the
ECMWF and SREF mean there. Due to the initial warm temperatures
limiting duration of snow, probabilities for heavy snow are
limited.

Petersen

$$
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FOUS11 KWBC 152057
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
457 PM EDT Mon Mar 15 2021

Valid 00Z Tue Mar 16 2021 - 00Z Fri Mar 19 2021

...Southwest across the Southern Rockies onto the Southern High
Plains...
Days 1-2...

A compact upper low shifting southeast from Northern CA this
afternoon will shift east to AZ late tonight and across NM into
the TX Panhandle through Tuesday night. Mountain snows will
persist into this evening for the far southern Sierra Nevada to
the Tehachapi along with Southern CA ranges with snow levels
around 3000ft and locally heavy amounts possible across the higher
terrain. Locally heavy snow in higher terrain also occurs farther
east tonight across southern NV/Utah mountains, as well as across
the higher terrain of northern AZ before tracking across the San
Juan Mtns of CO Tuesday and the Sangre de Christos of southern
CO/northern NM Tuesday night. Day 1 snow probabilities are
moderate for 6 or more inches in southern UT and the San Juans of
CO. As the system moves across New Mexico and into the Texas
Panhandle late Tuesday into early Wednesday, a period of strong
northeasterly flow along the northwest side of the low, along with
favorable forcing aloft, should support a period of heavy snow
along and east of the Sangre de Cristo Mountains (including the
Raton Pass and Raton Mesa) in southeastern Colorado. Day 1.5 snow
probabilities are high for 6 or more inches for the eastern slopes
in southern CO into northeastern NM.

The lee side surface low develops Tuesday night into Wednesday as
it moves east from the TX Panhandle across OK with TROWAL
development and potential for snow to accumulate despite initially
warm conditions over the southern High Plains near the CO/NM
border into KS and OK/northern TX Panhandles where Day 2
probabilities for 4 or more inches decrease to the east/lower
elevations.


...Central Appalachians...
Day 1...

A shortwave trough spinning off the occluding low over the central
CONUS will shift east across the Midwest tonight and off the
Mid-Atlantic coast later Tuesday. A wintry mix with ice accretion
is likely as precipitation spreads across the central Appalachians
this evening and continues into Tuesday morning. Day 1 ice
probabilities are 40 to 60 percent for 0.10-inch or more along
with 20 percent probabilities for a quarter inch or more for the
crest of the central Appalachians including the Allegheny
Mountains of WV down to the Blue Ridge Plateau of southwest VA.


...Northern California and Pacific Northwest...
Day 3...

The next system is a potent low that drifts toward the Pacific
Northwest coast Wednesday night with the associated cold front
reaching the coast from northern CA up through WA on Thursday.
Ample Pacific moisture is drawn from the sub-tropics ahead of this
low with PWs progged to top 0.75" which makes for snow levels
4500ft to 5000ft on Thursday. Day 3 snow probabilities are 50 to
70 percent for 6 or more inches for the higher Klamath Mtns and CA
Cascades with low probabilities for the Olympics (where the snow
level will be closer to 3500ft).


Jackson


$$
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FOUS11 KWBC 160912
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
512 AM EDT Tue Mar 16 2021

Valid 12Z Tue Mar 16 2021 - 12Z Fri Mar 19 2021

...Colorado and New Mexico to southeastern Kansas and the
Oklahoma/Texas Panhandles...
Day 1-2...
A vigorous upper low dropping southeast into Arizona this morning
is forecast to move east into New Mexico -- bringing mountain
snows into Colorado and New Mexico later today. There remains a
pretty good signal for locally heavy snows developing, especially
as the low begins to move east of the mountains into the High
Plains late Tuesday into early Wednesday. Strong northeasterly
flow along the northwest side of the low, along with favorable
forcing aloft, will likely support a period of heavy snow along
and east of the Sangre de Cristo Mountains in southeastern
Colorado and northeastern New Mexico. WPC PWPF indicates snow
accumulations of 4-inches or more are likely from the mountains
eastward along the Colorado-New Mexico border, with higher totals
likely in the mountains. As the low continues farther east, heavy precipitation may support rain changing to snow within the
system's deformation band -- producing at least a few inches of
snow across portions of southeastern Kansas and the northern
Panhandle region on Wednesday.

...Western U.S....
Day 3...
A frontal band associated with an upper low/trough approaching the
Pacific Northwest will bring the next round of mountain snows into
the western U.S. beginning Thursday and continuing into early
Friday. The areas most likely to see significant accumulations
through early Friday include the Olympics, the southern Cascades
and northern Sierra.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira



$$
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FOUS11 KWBC 171948
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 PM EDT Wed Mar 17 2021

Valid 00Z Thu Mar 18 2021 - 00Z Sun Mar 21 2021

...Kansas/Missouri...
Day 1...

An anomalously deep mid-level low will move eastward from OK
tonight to TN Thursday evening, accompanied by an upper divergence
maxima to drive slowly strengthening surface low pressure. As
these features shift east, pronounced warm/moist advection in the
form of a TROWAL will rotate cyclonically around the upper low and
back to the W/SW near the KS/MO border. While precipitation will
initially begin as rain across all of the area, an intense
deformation axis W/NW of the low combined with sharpening fgen
will cause rapid dynamic cooling of the column to cause a p-type
transition from rain to snow. The rate at which this occurs still
varies, but the preferred high-res guidance are more aggressive
owing to better CSI potential to enhance snowfall rates. With
low-level thermals remaining marginal, it will require these heavy
rates to accumulate efficient, and the recent HREF guidance has
increasing probabilities for 1"/hr during the period of most
intense ascent. WPC probabilities for heavy snow have increased,
and now indicate a narrow corridor of 10-20% chance for 4",
potentially including the Kansas City metro area.


...Northeast...
Day 2...

Models show the previously described low beginning to shear as it
moves east of the central Appalachians and begins to interact with
a northern stream trough amplifying over the Northeast Thursday
night driving a cold front through New England. Moisture
interacting with the boundary will support precipitation spreading
across the northern Mid Atlantic and portions of the Northeast on
Thursday into Friday. The guidance has trended a bit further north
with its precipitation today, likely due to some sheared vorticity
spreading northward from a subtly stronger mid-level low to the
south. There remains considerable spread in the accumulation
forecasts from the models, likely due to different handling of the
intensity of the mesoscale forcing. Since the antecedent
conditions are unfavorable for snow (warm with rain), the higher
res models are preferred since they depict intense ascent through
a deformation axis and possible CSI, which would be enough to
overcome the antecedent warmth. The heaviest snow is likely in a
stripe from near the Catskills eastward across CT and MA, where
WPC probabilities are as high as 20-30% for 4 inches due to 1"/hr
snowfall rates. Further to the south across PA and potentially
even into the higher terrain of WV/VA, light accumulations are
possible as the initially colder temperatures in the terrain will
allow for more efficient accumulation as the column cools behind
the cold front.


...Western U.S....
Days 1-3...

A closed mid-level low will pivot slowly eastward across the
Pacific Thursday and Friday, moving onshore WA/OR Saturday
morning. This feature will be accompanied by a slow moving surface
low, while at the same time shedding pieces of vorticity energy
eastward into the Northern Rockies. With the broad trough
surrounding the low maintaining its axis west of the coast,
prolonged 700-500mb SW flow will spread moisture inland, but with
relatively high freezing levels. At least one of these vorticity
spokes will spawn secondary surface low pressure developing across
the Great Basin, but in general waves of precipitation are likely
to occur across the region in response to the combination of
synoptic forcings. Rounds of snowfall are likely each day, first
in the northern CA ranges and then spreading northeast as far as
the Olympics and Cascades, as well as the Northern Rockies and
ranges of NW WY including the Tetons. High WPC probabilities for 6
inches are confined to the Shasta area on D1, and then expand into
the Sierra, Olympics, and Cascades D2, and continue across these
ranges while spreading into the Tetons on D3. Total snowfall may
exceed 2 ft in the highest terrain of the Olympics and Sierra.


For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 180826
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Thu Mar 18 2021

Valid 12Z Thu Mar 18 2021 - 12Z Sun Mar 21 2021


...Northeast...
Day 1...

Models show waves of low pressure moving across the central
Appalachians and mid Atlantic before moving offshore south of Long
Island and southern New England. Moisture is forecast to be
lifted by the mid level front north of the low pressure axis
across the southern tier of NY, northern PA, the Catskills,
Berkshires, and then to southeast MA.

There remains considerable spread in the accumulation forecasts
from the models, likely due to different handling of the intensity
of the mid level front. Since the antecedent conditions are
unfavorable for snow (warm with rain), the majority of solutions
have the event as mostly rain, and then ending as a 2-3 hour
period of snow before drying aloft leads to the event ending.

The heaviest snow is likely in the Catskills, Berkshires, and
southeast MA, where WPC probabilities are as high as 20-30% for 4
inches due to 1"/hr snowfall rates. The uncertainty involves both
duration of snow as noted, but also intensity. The WRF ARW/NSSL
WRF, 00z High Res Rapid Refresh and NAM CONUS Nest each show
potential for several inches in southeast MA tonight.
The low pressure and associated front move east on Friday,
bringing the event to a close.

...Western U.S....
Days 1-3...

A closed mid-level low will move east on Thursday and Friday
towards the Pacific northwest, shearing into an upper trough as it
moves inland across WA/OR/northern California Saturday morning.
Prolonged 700-500mb southwest flow will spread moisture inland Thu
and Fri, but with relatively high freezing levels. The combined
day 1 plus day 2 snow should lead to an area of 1-2 feet of snow
in the mountains of northern and central CA. Height falls with
the upper trough bring cooler temps on Day 2 and spreads heavy
snows across the WA Olympics. Total snowfall may exceed 2 ft in
the highest terrain of the Olympics.

On Day 2, several inches of snow are likely in the forward flank
jet maxima crossing southern Idaho to northwest WY, where 300 mb
divergence maxima promote ascent as they crosses the terrain. As
the upper trough moves east on Sat-Sun., coupled upper
divergence/lower convergence maxima cross the UT Wasatch,the WY
Tetons, continuing east across the WY Bighorn Mountains and the
ranges of western CO. Several inches of snow are expected in
favored terrain.

High WPC probabilities for 6 inches are confined to the
Siskiyous/Shasta area of northern CA on Day 1, in the Sierra
Nevada, Olympics, and Boise Mountains Day 2, and the Tetons/UT
Uinta Mountains Day 3.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 181954
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Thu Mar 18 2021

Valid 00Z Fri Mar 19 2021 - 00Z Mon Mar 22 2021


...Northeast...
Day 1...

A band of precipitation sinking southeast behind a cold front will
be chased by cold air approaching from the NW as CAA increases.
While cold air following moisture rarely leads to heavy snowfall,
this will be enhanced by a stripe of strong deformation to aid in
locally intense ascent which will help dynamically cool the column
more rapidly. The guidance has trended downward in its snowfall
this aftn, and it appears the best chance for moderate to heavy
snow will be in the terrain of the Catskills, Berkshires, and
Worcester Hills due to elevation dependency, as well as in a
narrow band just SW of Boston, MA which may have the most
prolonged period of precipitation with temps cold enough for snow.
Generally 1-2" of snow is forecast, but WPC probabilities do
indicate a 10-20% chance for 4 inches, highest in the Worcester
Hills.


...Western U.S....
Days 1-3...

A slow moving closed mid-level low will pivot slowly southeast
towards the WA/OR coast Friday into Saturday, shearing into an
upper trough as it moves inland during the weekend. This feature
weakens substantially by Sunday, but spokes of vorticity rotating
around the mean trough will continue to produce periods of
enhanced ascent across the West. In addition to these height falls
and rounds of PVA, prolonged 700-500mb southwest flow will spread
moisture across the region, but at the same time drive freezing
levels as high as 5000-7000 ft ahead of the trough axis, lowering
rapidly however as the trough swings eastward, especially by D2/D3.

For D1, the heaviest snow is expected in the Sierra, WA Cascades,
Olympics, and Sawtooth ranges which will be most orthogonal to the
moist mid-level flow and guidance indicates omega maxima across
these ranges due to the upslope enhancement. WPC probabilities for
6 inches are high on D1 in these ranges, slightly lower in the
Sawtooth. By D2 heavy snow continues in the Cascades and Olympics
as the upper trough swings onshore steepening the lapse rates
while maintaining moist flow, but also begins to shift east into
the ranges of NW WY and southern MT as diffluence within the LFQ
of an upper jet streak maximizes. WPC probabilities D2 are again
high for 6 inches in these areas, with locally more than 12"
likely in the highest terrain. The primary trough axis swings well
eastward to the Northern Rockies and Great Basin D3, shunting
moisture and forcing near the PacNW, but continuing to drive moist
ascent into the Central Rockies and ranges of WY/MT. WPC
probabilities for 6 inches are moderate to high D3, highest in the
Big Horn range.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 190745
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Fri Mar 19 2021

Valid 12Z Fri Mar 19 2021 - 12Z Mon Mar 22 2021


...Pacific Northwest/California/Great Basin/Rockies/central High
Plains...
Days 1-3...

A slow moving closed mid-level low will pivot slowly southeast
towards the WA/OR coast Friday into Saturday, shearing into an
upper trough as it moves inland during the weekend. For D1 Fri,
the heaviest snow is expected in the WA Cascades and Olympics,
with lighter amounts downstream in the ranges of OR to southern ID
and northwest WY/adjacent southwest MT. WPC probabilities for 8
inches are high on D1 in
the WA Olympics, and moderate in the Cascades . A secondary
maxima is for an additional several inches of snow in the Central
CA Sierra Neva range as a slow moving cold front crosses the area,
with moist pre-frontal southwest flow advecting moisture inland
from the Pacific into the terrain. Upslope conditions help augment
the frontal induced lift. The snows wind down as the 700 mb trough
moves further inland on Sat. The probability of 8 inches of snow
on Day 1 in the central CA Sierra Nevada is high.

The longer duration ascent on Day 2/Sat looks to be across the
Beartooth Mountains of far southwest MT to the Yellowstone
National Park and Tetons of WY. The long duration ascent
underneath 300 mb divergence maxima leads to amounts of 6-12
inches across the region, extending east into the Bighorn
Mountains of WY. The trailing cold front crosses the Wasatch, so
several inches of snow are excepted in the terrain of southwest WY
to northern UT. WPC probabilities for 8 inches are moderate ,
highest in the ranges of northwest WY to adjacent southwest MT.

On Day 3/Sun, the amplifying upper trough builds south from the
central towards the southern Rockies and Great Basin. An upper jet
progresses out of the southwest across the ranges of NM, providing
favorable difluent flow in the left exit jet region of CO.
As a 700 mb low forms in southeast CO, moisture pools near the low
and return flow advects moisture in to the front range of CO down
to the Sangre DeCristo Mountains and Palmer Divide. Snow develops
in the mountains, with uncertainty whether it will be cold enough
for accumulating snow on the CO high Plains. Up to a foot is
possible in Rocky Mountain National Park/CO front range, where the
probability of 8 inches is moderate.

In WA State on Sun, the upper trough approaching British Columbia
drives a zonal jet with moisture advecting onshore from the
Pacific across WA State, in conjunction with 700 mb ascent.
Another round of snow is likely across the higher elevations of
the WA Olympics and Cascades, with clusters of 12-24 inches across
the terrain with the best orographics. The 00z ECMWF has
increased QPF/snow amounts in the Olympics/northern WA Cascades,
so confidence is increasing in heavy snow potential on Sun. The
probability of 8 inches of snow Sun is moderate to high in the
northern WA Cascades.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen


$$
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FOUS11 KWBC 200827
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Sat Mar 20 2021

Valid 12Z Sat Mar 20 2021 - 12Z Tue Mar 23 2021


...Pacific Northwest/California/Great Basin/Rockies/Central High
Plains...
Days 1-3...

An upper jet extending southwest to northeast across NV to western
WY and central MT leads to a combination of a slowly moving
frontal boundary plus moisture advection from NV across southeast
ID , northwest WY and southern MT. Well defined and persistent 700
mb convergence maxima helps lift the available moisture, in
conjunction with upper divergence maxima and orographics in
windward areas to produce moderate to heavy snow. The heaviest
snow are forecast in the Beartooth Mountains of south central MT,
where up to a foot of snow is expected. WPC probabilities are high
for 6 inches across these mountains. Secondary, lighter maxima are
forecast in the Cascades and Wasatch, where 6 to 10 inches is
possible.

From Day 1 through early Day 2, heavy snow is likely in the Big
Horns where persistent mid-level WAA will produce snowfall that
accumulates 6 to 12 inches.

On Sunday, a 700 mb low forms in CO. 700 mb ascent and moisture
convergence leads to precipitation developing in the CO high
Plains to the foothills and front range. E/NE low-level flow
around the surface low drives upslope enhancement into the Front
Range and Palmer Divide. WPC probabilities on D2 are 40-50% for 8
inches in the Front Range, and as high as 50% in the Palmer
Divide. The column is marginally cold enough for snow into the
High Plains, so the primary uncertainty is how far east on to the
Plains light snow accumulations are likely Sunday night along the
I-25 urban corridor.

On Monday, the 700 mb low moves northeast across the central
Plains. Initial temperatures are too warm for snow, and cooling as
the low passes may lead to a brief period of snow before precip
tapers on the western side of the circulation in eastern CO to
western KS and Nebraska. Probabilities are low, up to 20 percent,
for 4 inches due to the short duration of snow.

On Sunday, the next shortwave and associated Pacific jet streak
will spread moisture and 700 mb convergence maxima across the
Olympics, Cascades, and later into the Northern Rockies. The
strong moisture advection and lift results in locally heavy snow
that accumulates 12-18 inches in the Olympics and northern WA
Cascades.

Several inches are expected Sunday into Monday into the Blue
Mountains of OR, Clearwater Mountains of ID, and Tetons of
northwest WY as the upper trough moves inland, with associated 700
mb convergence maxima crossing these mountains, aiding in
producing lift.
Because the convergence maxima are progressive, the probabilities
for each 24 hour period providing 8 inches of new snow are low.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen


$$
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FOUS11 KWBC 201946
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 PM EDT Sat Mar 20 2021

Valid 00Z Sun Mar 21 2021 - 00Z Wed Mar 24 2021

...Central Rockies into Central High Plains...
Days 1-2...

A positively tilted longwave trough will drop slowly southeast
from the Pacific Northwest towards the Four Corners through Monday
morning before amplifying into a negatively tilted trough over the
Southern Plains Tuesday in response to sharp shortwave energy
rounding the base. As this trough deepens, an upper jet streak
will intensify over the Southwest leaving the diffluent LFQ atop
the Central Rockies. The combination of these features will drive
lee cyclogenesis Sunday night, and this low is likely to lift
quickly northeast into the Central Plains by Tuesday morning.

Pacific moisture being transported inland will be wrung out by the
deep layer ascent, aided by WAA on SW flow and intensifying height
falls. As the surface low develops over far NE NM and lifts
northeast, low-level flow will turn E/NE, producing upslope
enhancement into the Front Range and Palmer Divide, especially
Monday morning. The heaviest snow D1-2 is likely in the Front
Range and Palmer Divide, as well as the terrain west of the Front
Range. WPC probabilities D1 are high for 6" in the western CO
terrain, shifting eastward D2 with locally more than 12" possible
in the Front Range. The thermal profiles east of the terrain are
marginal for snowfall, but persistent SW flow aloft and the
likelihood for a deformation axis to enhance ascent to dynamically
cool the column should produce a few inches of snow along the I-25
urban corridor and into the high plains of NE CO. WPC
probabilities are high for 2" across these areas, with potentially
higher amounts possible.


...Pacific Northwest through the Northern Rockies...
Days 1-3...

An upper jet extending southwest to northeast across NV to central
MT leads to a combination of a slowly moving frontal boundary plus
moisture advection from the Great Basin into Wyoming on D1. Well
defined and persistent 700 mb convergence maxima helps lift the
available moisture, in conjunction with upper divergence maxima
and orographics in windward areas to produce moderate to heavy
snow. The heaviest snow is forecast in the Big Horn range of WY
where WPC probabilities are high for 6", and locally more than 12"
is possible.

As this first system shifts southeast, a secondary jet streak will
dive out of the Gulf of Alaska and onshore the Pacific Northwest
coast Monday. This will spread moisture, upper diffluence within
the LFQ of this jet streak, and 700mb convergence into the region.
This will squeeze out heavy snow in the terrain of the Olympics,
WA and OR Cascades, OR Blue Mountains, and into the Northern
Rockies D2 with WPC probabilities high for 6 inches. Snow levels
will be such that even at the Cascade Passes accumulations could
reach 12". The jet streak and associated forcing weaken and drop
southeast on D3, leading to renewed light snow accumulations in
the Absarokas, Tetons, and Big Horns.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Weiss


$$
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FOUS11 KWBC 210848
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
447 AM EDT Sun Mar 21 2021

Valid 12Z Sun Mar 21 2021 - 12Z Wed Mar 24 2021

...Central to Southern Rockies/Southwest into Central High
Plains...
Days 1-3...

On Day 1, snow remains likely across the CO foothills and front
range as a mid-upper level trough slowly approaches, with the
forward flank upper jet maxima crossing eastern CO into KS. A
combination of a well defined and persistent 700 mb convergence
maxima helps lift the available moisture, in conjunction with
upper divergence maxima and orographics in windward areas to
produce moderate to heavy snow.
Lee cyclogenesis occurs Sunday night, and this low is likely to
lift northeast into the Central Plains by Tuesday morning.

As the surface low develops over far NE NM and lifts northeast,
low-level flow will turn E/NE, producing upslope enhancement into
the Front Range and Palmer Divide, especially Monday morning. The
heaviest snow D1-2 is likely in the Front Range and Palmer Divide,
extending south into the Sangre DeCristo Mountains of southeast
CO. Locally a foot of snow is possible.

The thermal profiles east of the terrain are marginal for
snowfall, but the likelihood for a deformation axis to enhance
ascent to dynamically cool the column should produce a few inches
of snow along the I-25 urban corridor and into the high plains of
NE CO.

On Day 3, Tuesday, the next upper trough crosses NV/AZ on the way
to CO/NM. The amplifying trough leads to increasing upper level
divergence maxima developing in southern CO and NM. A wave and
then circulation develops in southern CO, drifting south into
northern NM.
Well defined boundary layer convergence occurs along the path of
the circulation, peaking Tue night-early Wed morning in southeast
CO to northeast NM.

The GFS and ECMWF have increased QPF this cycle and potential snow
amounts in response to mid level frontogenesis, favorable
difluence aloft with embedded 3030 mb divergence maxima, plus east
northeast flow coming into the Sangre deCristo Mountains that
provide an orographic component to lift. The prolonged ascent
leads to locally heavy snow in southeast CO/northeast NM focused
along the Sangre DeCristo mountains.The
ECMWF/GFS/NAM/UKMET/Canadian global show potential or a foot of
snow, and a moderate risk for 8 inches of snow in these areas.

Secondary maxima are expected in the southern UT mountains south
into the Mogollon Rim of AZ and White Mountains of NM as the
trailing cold front and 700 mb trough move south across these
areas, with frontal convergence providing lift in the terrain.
Forecast amounts are not as high since the sfc-700 mb convergence
strength is not as strong as over northeast NM.


...Pacific Northwest through the Northern Rockies...
Days 1-3...

Well defined and persistent warm/moisture advection occurs across
WA into the northern Rockies today into Monday. Peak ascent in
the 850-700 mb layer and high layer relative humidity over 90
percent lead to conditions favorable for heavy snow in the WA
Olympics and Cascades. The lift is accented by a 300 mb jet max
max streaming onshore and across WA state today and tonight.
Difluent flow aloft crosses the northern Rockies as well,
combining with 700 mb ascent to lead to persistent snow in the
ranges of northern ID to northwest MT Sun night to early Mon .
Heavy snow remains likely in the terrain of the Olympics and WA/OR
Cascades, with 2 day totals locally as high as 2 feet in the
northern WA Cascades. WPC probabilities are high for 8 inches.

Secondary snowfall maxima are expected in the OR Blue Mountains,
and into the Northern Rockies with Day 1 plus Day 2 amounts
locally a foot. The jet streak and associated forcing weaken and
drop southeast, leading to renewed light snow accumulations in the
Absarokas, Tetons, and Big Horns.

On Day 3 the remaining snow is expected to be focused on the Wind
River Range of WY and Gillette Arch, continuing down to the
Laramie Mountains. The approach of a 700 mb ridge towards the
Pacific northwest leads to a dry period there.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen


$$
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FOUS11 KWBC 211941
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 PM EDT Sun Mar 21 2021

Valid 00Z Mon Mar 22 2021 - 00Z Thu Mar 25 2021

...Pacific Northwest into the Great Basin...
Days 1-3...


...Central and Southern Rockies...
Days 1-3...

Two distinct storm systems will bring heavy snow to the region
through the middle of the upcoming week.

Tonight through Monday night, a shortwave moving across the Four
Corners will deepen and close off as it pushes over the TX
Panhandle before lifting away to the northeast and across the
Midwest by Tuesday morning. At the same time, a subtropical jet
streak extending from the Pacific will arc eastward placing the
favorable diffluent LFQ for ascent more robustly atop the Central
Rockies. This enhanced diffluence combined with the height falls
and divergence associated with the closed mid-level feature will
drive surface low development in the lee of the Rockies early
tonight. This low is then likely to strengthen gradually as it
lifts northeast from NM into KS by Monday night, and then towards
the Midwest on Tuesday. As this low develops, ascent will be
enhanced by increasing E/NE low-level flow which will provide an
upslope component into the Front Range, Sangre De Cristos, and
Palmer Divide. Robust available moisture being acted upon by this
intense deep layer ascent will produce heavy snowfall much of
Monday and Monday night before the system pulls off to the
northeast and bringing an end to the forcing. WPC probabilities
are high on D1 for 6 inches in the Front Range, Palmer Divide, and
northern Sangre De Cristos with locally up to 12 inches possible.

After a brief respite on D2, yet another shortwave diving out of
the Pacific Northwest Tuesday will close off near the Four Corners
region late Tuesday night leading to height falls and intense
mid-level divergence across the Southern Rockies Wednesday while
yet another upper divergence maxima pivots overhead in response to
a strengthening jet streak. The setup for D3 appears very similar
to D1, but further south, with more abundant moisture, and colder
as the back door front should be well into New Mexico by this
time. A colder column with strong ascent aided by renewed upslope
will produce heavy snow across much of the High Plains of NE NM
and SE CO, with heavy snow extending back into the Sangre De
Cristos and San Juans. WPC probabilities are already high for 12
inches in these areas D3, with locally much higher amounts
possible. In the major cities of Santa Fe and Albuquerque,
confidence is lower due to shadowing, but several inches of
snowfall is possible there as well.


...Pacific Northwest through the Northern Rockies...
Days 1-3...

A shortwave dropping out of British Columbia will move into
Washington state Monday morning and then continue to progress
southeastward towards the Great Basin on Tuesday. This will be
accompanied by the LFQ of a jet streak dropping southward,
transporting abundant moisture into the region. Ascent within the
850-700mb layer becomes intense, within a region of high
RH/moisture. These features together will drive a wave of surface
low pressure and associated cold front southward as well,
providing some enhanced low-level convergence, along with some
upslope on the windward side of the terrain. Moderate to heavy
precipitation will follow these features southeastward, and WPC
probabilities are high for 6" on D1 in the Cascades, Olympics,
Northern Rockies, and Blue Mountains of OR. Locally more than 12"
is possible in some of these regions where upslope becomes
maximized. As this system continues southeast D2, moderate to
heavy snow is likely in the Great Basin ranges, Wasatch of UT,
Absarokas, Wind Rivers, and Big Horns where WPC probabilities are
moderate for 6 inches.

While less of an accumulation impact, guidance is coming into
better agreement that as the shortwave digs southeast and steepens
the mid-level lapse rates atop the cold front, scattered to
widespread convective snow showers or snow squalls may occur.
0-2km theta-e lapse rates fall as low as -7C/km coincident with
the cold front and at least modest instability. These snow squalls
could impacts parts of WA/OR/ID/NV/MT Monday evening and Monday
night. The HREF probabilities suggest a high potential for
snowfall rates briefly exceeding 1"/hr, so while this should not
accumulate much in any squall, severely restricted visibility and
briefly dangerous travel is possible.

After a brief dry period, another shortwave and associated jet
energy may approach the Pacific Northwest coast late on D3. This
could reintroduce heavy snow to the Cascades and Olympics with
more than 6" of snow possible.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.


Weiss


$$
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FOUS11 KWBC 230833
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Tue Mar 23 2021

Valid 12Z Tue Mar 23 2021 - 12Z Fri Mar 26 2021



...Great Basin and Southwest to the Central and Southern Rockies...
Days 1-3...
A well-defined deep trough develops over the Southwest late today
into early Wednesday. This will bring snow across Utah and
northern Arizona and through the Four Corners region today.
Locally heavy accumulations are possible along the central
southern Utah mountains and along the northern Arizona high
terrain such as the Mogollon Rim.

As a broad 700 mb low and area of increasing ascent associated
with the left-exit region of the upper jet interact with an area
of deepening moisture, the potential for heavy snow is expected to
increase from the eastern Mogollon Rim and White Mountains of
eastern Arizona into western New Mexico and the San Juan Mountains
of both Co and NM. Meanwhile, post-frontal, northeasterly winds
along with the supportive upper level dynamics will encourage
heavy snows developing farther east along the Sangre de Cristo
Mountains and the adjacent high plains of southeastern Colorado
and northeastern New Mexico Tuesday night into Wednesday. Snow is
expected to continue into late Wednesday before diminishing as the
leading shortwave ejects northeast into the central Plains.
Heaviest storm totals are expected along the eastern slopes of the
Sangre de Cristo and the adjacent mesas and plains, with WPC PWPF
showing high probabilities for snow accumulations of a foot or
more.

On Day 3, Thu., the next 700 mb wave crossing NV and the CO River
Valley continues east across UT and then CO, enhance moisture
pools in advance of the 700 mb front. Combined synoptic ascent
with the front and orographic enhancement in windward terrain of
the UT Wasatch/Uinta Mountains and then into the CO San Juan
Mountains should lead to enhanced snow totals in these ranges. The
highest probability of 4 and 8 inches of snow on Day 3/Thu is in
the CO San Juan mountains. The highest Days 1-3 snow totals are
forecast across the San Juans of CO/NM and Sangre DeCristo
Mountains of NM to adjacent southeast CO, where 1 to 2 feet are
expected over the next 3 days.


...Pacific Northwest through the Northern Rockies...
Days 1-3...
The upper trough moving east across MT and WY today will result in
a cold front moving south through the northern Rockies.
Post-frontal, low-to-mid level, northeasterly winds will promote
snow showers east of the Divide, with some locally heavy
accumulations possible along the Absaroka Range, Beartooth
Mountains and Bighorn Mountains. Another shortwave trough diving
south into the Pacific Northwest will bring mountain snows back
into the WA/OR Cascades on Wednesday, continuing inland to the OR
Blue Mountains and then the ranges of central to northern ID and
northwest MT. On Day 3/Thu, the upper trough drifts east across
MT and WY. Sporadic 300 mb divergence maxima occur over the ranges
of south central MT to northern WY and southeast ID, bringing
periods of snow, with several inches expected in the Absaroka
Mountains to the Beartooth and Bighorn Ranges. The vertical
motions are modest but the slow movement of the broad 700 mb
circulation should lead to extended snow showers in these ranges.

...Upper Mississippi Valley/Minnesota...
Day 2...
An upper trough/low currently is expected to produce mainly rain
as it tracks farther northeast from the central Plains on Tuesday
and into the upper Mississippi Valley Tuesday night. In the mid
level deformation zone, dynamic cooling should produce a brief
period of snow across central NE, and then eastern SD. As the
circulation crosses MN, a northern stream trough helps to bring
colder air in behind the system, increasing the potential for
accumulating snow across northeastern Minnesota Wednesday morning,
with at least a few inches likely across portions of the
Arrowhead. Up to half an inch liquid equivalent in the form of
snow is shown in the NAM and SREF, indicating potential for 4 to 6
inches of snow. The event winds down Thu as the circulation moves
across the border into Canada.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen


$$
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FOUS11 KWBC 240820
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Wed Mar 24 2021

Valid 12Z Wed Mar 24 2021 - 12Z Sat Mar 27 2021


...Southwest and Southern Great Basin to the Southern Rockies...
Days 1-3 Wed-Fri...

Deepening moisture and large-scale ascent near the track of a
low-to-mid level cyclone and accompanying front moving east across
Arizona into New Mexico is expected to support some several
additional inches of snow across the ranges of central to northern
New Mexico and southwestern Colorado. Meanwhile farther east,
post-frontal upslope flow will help produce heavier amounts along
the Sangre de Cristo and Sandia Manzano mountains. Probabilities
remain high for snowfall accumulations of 4 inches or more and low
for 8 inches for the central and southern Sangre de Cristo
mountains and for areas further east along the Raton Mesa. The
event winds down as the 700 mb low shears and lifts northeast onto
the southern Plains this evening.

On Thu., the next 700 mb shortwave trough amplifies as it moves
south across Nevada into Utah. This will bring high elevation snow
back into the ranges of Nevada, Utah and northern Arizona, with
potential for several inches of snow late Thursday into Friday. A
greater threat for heavy accumulations is farther east, where a
period of southwesterly inflow ahead of the approaching trough is
expected to support heavy snows occurring across the San Juan
Mountains. The probability of 8 inches is moderate 12z Thu to 12z
Fri in the CO portion of the San Juans.

...Pacific Northwest to the Northern Rockies and Northern Great
Basin...
Days 1-3 Wed-Fri...
A shortwave trough diving south into the northwestern U.S. is
expected to bring mountain snows across the WA/OR cascades, OR
Blue Mountains, and ranges of central to northern ID and northwest
MT. The highest probabilities for 8 inches of snow are across
the WA Cascades, where favored windward terrain should see amounts
of a foot in several locations. On Thu., the upper trough drifts
east across MT, southeast ID and WY. The best overlap of 700 mb
high relative humidity and pockets of ascent is across southeast
ID into western WY and adjacent southwest MT, where several inches
of snow is expected across favored terrain. Probabilities for 8
inches of snow are low. On Fri, the 700 mb trough moves east from
WY onto the Plains. Several inches of snow are possible in the
Bighorns and Laramie Range where confluent moist northwest flow
turns upslope. Elsewhere in eastern WY, accumulations are forecast
to be light.

...Northern Maine...
Day 3/Fri...
The models show potential for snow across northern Maine late Fri
through Fri night as a wave of low pressure moves east across
Maine, bringing widespread precipitation. Current forecasts show
it being too warm for snow at the onset. When the low pressure
passes, cold advection commences, with rain changing to snow
across northern Maine. The longer duration snow appears to be
near the northern most portion of Maine near the New Brunswick and
Quebec border, where there is potential for several inches of
snow. There is still latitudinal differences in the low track and
thus how far south the band of snow gets. The PWPF shows a 40-50
percent chance of 4 inches of snow along Maine's northern border
with Canada.

...Northern Minnesota...
Day 1/Wed...
An 850 mb low is forecast to track northeast from southeast MN to
Lake Superior. West of the low, rain currently in place across
northern MN will change to snow within the stronger lift aligned
with the mid level frontal band.
The precipitation winds down tonight as the low moves across the
border up into Canada. The WPC PWPF has a 40-50 percent
probability of snow accumulations of 4 inches or more across
parts of the northeast MN.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 250830
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Thu Mar 25 2021

Valid 12Z Thu Mar 25 2021 - 12Z Sun Mar 28 2021

...Northern, Central, and Southern Rockies...
Days 1-2...

A broad mid-level trough will sharpen and drift slowly southeast
from the Pacific Northwest, through the Great Basin, into the Four
Corners, and eventually towards the Southern Plains by Sunday.
Above this trough, Pacific jet energy will push onshore leaving
periodic divergence maxima across the Western CONUS, shifting
southeast with time as the trough digs eastward. The combination
of height falls and divergence will spread precipitation in the
form of rain and snow showers across much of the inter-mountain
west during the forecast period. The heaviest snow will be
associated with the strongest height falls, but also coincident
with the most robust 700-500mb WAA on SW flow ahead of the trough
axis. Additionally, wave of surface low pressure are likely to
develop and move across the West beneath the trough, enhancing
lift at times.

WPC probabilities for snowfall greater than 6" are high on D1
across many of the ranges in MT/ID southward through the Great
Basin and Four Corners, with the heaviest snow likely in the San
Juans where locally more than 12" is possible. By D2 the total
forcing and moisture begins to wane, but residual moderate
probabilities for 6" of snowfall continues across the ranges of WY
and CO Rockies/San Juans. By late Saturday, the forcing has
shunted off to the east and a respite to the moderate snow should
occur across the region.


...Great Lakes to Northern New England...
Days 1-3...

A negatively tilted shortwave trough ejecting northeast through
the Ohio Valley Friday morning is progged to close off and lift
quickly through Northern New England by Saturday morning. This
feature will be accompanied by a coupled jet structure and strong
upper divergence maxima to drive surface cyclogenesis near the
Missouri Valley, with rapid strengthening likely as the low shifts
towards New England and then into Atlantic Canada this weekend.
The environment is generally too warm for snowfall across much of
this area, however, a cold front sinking southward from Canada
combined with some dynamic cooling of the column should allow for
rain to change to freezing rain briefly, and then snow, across
parts of WI and MI, and then Northern New England. Freezing rain
accretions could reach up to 0.1" in far northern MI and eastern
WI, and WPC probabilities for 0.1" are as high as 20%. The
heaviest snow from this low is expected across northern Maine
which will have the longest overlap of precipitation with
sufficiently cold thermals for snowfall. WPC probabilities are as
high as 40% for 4 inches on D2 across far northern ME.

A secondary shortwave will follow quickly behind this first
impulse, moving across the Northern Plains and towards the Great
Lakes Saturday night /D3/. This shortwave is also likely to spawn
weak cyclogenesis, but the antecedent column is expected to be
sufficiently cold enough for an area of light to moderate snow
from Minnesota into the U.P. of Michigan. WPC probabilities on D3
are less than 20% for 4 inches of accumulation.

Weiss


$$
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FOUS11 KWBC 260810
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Fri Mar 26 2021

Valid 12Z Fri Mar 26 2021 - 12Z Mon Mar 29 2021


...Western U.S....
Days 1 & 3...

Through tonight, areas of high elevation snow will persist across
the terrain of the Four Corners as modest height falls continue in
response to a longwave trough axis shifting southeast across the
region. Weak to moderate ascent through these height falls and WAA
on SW flow ahead of the trough axis will promise periods of heavy
snow, generally above 5000-6000 ft. The weak surface low
associated with this ascent will gradually fill tonight, which
when combined with the eastward progression of the deeper ascent
will bring an end to the snowfall. WPC probabilities for 6 inches
are low to moderate and confined to the terrain of the CO Rockies.

On Day 3, a potent shortwave embedded within moist confluent flow
will race eastward accompanied by intensifying upper divergence
ahead of a SE diving jet streak. Pronounced moisture advection
ahead of the jet streak and within the confluent mid-level pattern
will spread precipitation across the Pacific Northwest Sunday,
while height falls, divergence, and low-level convergence along a
frontal band will produce ascent for snowfall across the region.
Snow levels ahead of this front will be quite high as tropically
sourced moisture spreads inland, but should fall dramatically as
the front races SE with time late on D3. WPC probabilities for 6
inches are quite high in the Olympics and Cascades, spreading into
the northern Rockies as well. Locally more than 12" is likely in
the Cascades.


...Great Lakes to the Northeast...
Days 2-3...

A shortwave dropping out of Alberta, Canada Saturday will move
eastward towards the Great Lakes while strengthening. As this
feature shifts across the Upper Midwest Saturday evening, a weak
wave of low pressure may develop beneath it with a cold front
dragging in its wake. Warm advection ahead of the trough axis will
spread precipitation northward, and as the column cools behind the
front rain changing to snow is likely, with some modest
accumulations possible across MN and into the Western Great Lakes.
WPC probabilities for 4 inches are generally less than 20%,
highest in the Minnesota Arrowhead on D2.

As this shortwave continues to progress eastward, it will interact
and potentially phase with southern stream energy coming out of
the Plains on Sunday. This will drive a more intense surface low
through the Ohio Valley and into southeast Canada late Sunday into
Monday. Impressive moisture transport from the Gulf of Mexico will
spread northward ahead of this system spreading precipitation
northward into New England. Much of this precipitation is likely
to be rain as WAA overwhelms any modest cold air in place.
However, for parts of far Northern New England moderate to heavy
snow is possible, and WPC probabilities are high for 4 inches,
primarily north of I-95 and U.S. 201 in Maine.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 270814
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 AM EDT Sat Mar 27 2021

Valid 12Z Sat Mar 27 2021 - 12Z Tue Mar 30 2021


...Pacific Northwest to the Northern Rockies...
Days 2-3...
Heavy snow is expected to return to the mountains of the Pacific
Northwest with the approach of an intensifying 700 mb wave,
bringing a surge in integrated water vapor transport and ascent
across the WA Cascades Sunday. Accumulations around a foot are
likely by Sunday evening across the higher elevations of the
Olympics and locally as much as 18-24 inches in favored windward
locations in the northern Cascades. Drying aloft eases snowfall
coverage and intensity Monday in the Cascades.

The surge in moisture advection and lift that crosses the Pacific
Northwest Sunday moves inland across the northern Rockies Sunday
night and early Monday. Although the 700 mb wave moves quickly
east across the northern Plains Monday, residual moisture remains
in place in the ranges of western MT Monday, with confluent flow
and upper level jet max supporting additional periods of ascent,
with several additional inches of snow expected. The low-mid level
flow advects moisture downstream into the Beartooth and Teton
Ranges Sunday, where several inches of snow are expected in
favored upslope areas.

Widespread accumulations of 8 inches or more are likely across the
mountains of northwestern Montana. Locally heavier amounts on the
order of 12-18 inches are most likely in the northwestern Montana
ranges for Sunday and Monday combined. WPC shows high
probabilities for accumulations exceeding a foot over the northern
Cascades and ranges of northwest MT.


...Northeast...
Day 2...

The models show a deep layer of warm/moisture advection moving
across northern New England Sunday. This occurs underneath a jet
streak aloft, with the combination leading to steady
precipitation, including an extended period of snow across
northern Maine.
The warm advection results in a change over to rain in most of
southern Maine. PWPF shows high probabilities for 4 inches or
greater across a good portion of northern Maine. It also
indicates 30 percent or greater probabilities for amounts of
8-inches or more across northern Aroostook County. The event
winds down as the upper trough passes early Sunday with drying
aloft sharply reducing snow coverage/intensity. The limited
duration of the event keeps probabilities for heavy snow limited.

...Northern Minnesota...
Day 1...
A 700 mb shortwave moving east across MN today will lead to a
period of enhanced moisture and lift within a couple upper jet
pattern. The 850-500 mb ascent leads to a period of snow across
northeast MN, with The trough progresses east overnight As the 700
mb trough passes, drying aloft brings the the Southwest, is
forecast to lift northeast from the central Plains into the Upper
Midwest ahead of a northern stream trough moving through the
Dakotas on Saturday. Models show precipitation developing as or
changing to snow along and behind the northern stream boundary as
it drifts across northern Minnesota on Saturday. This will likely
produce two to four inches of snow across the Arrowhead, with a
low probability of 4 inches.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen


$$
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FOUS11 KWBC 272040
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
440 PM EDT Sat Mar 27 2021

Valid 00Z Sun Mar 28 2021 - 00Z Wed Mar 31 2021

...Pacific Northwest and Northern Rockies...
Days 1-2...
A deep and cold low currently over the Gulf of Alaska opens into a
neutrally tilted trough tonight and shifts southeast, reaching WA
late Sunday and crosses the northern Rockies through Monday
morning. A surge in tropically sourced moisture precedes the
associated cold front bringing 0.75 inch PW air (one standard
deviation above normal) into western WA Sunday morning. Rapid
height falls Sunday afternoon will lower the snow level from
around 3000ft to about 1800ft by 00Z Monday. Heavy snow is
expected higher up with high Day 1.5 snow probabilities of a foot
or more for the highest Olympics and higher Cascades while
moderate probabilities for 4 or more inches extend south to
central OR Cascades and below pass level in the WA Cascades.
Accumulations around a foot are likely by Sunday evening across
the higher elevations of the Olympics and locally as much as 18-24
inches in favored windward locations in the northern Cascades.
Ridging quickly cuts most Pacific inflow for the Cascades late by
Sunday night.

The surge in moisture advection and lift that crosses the Pacific
Northwest Sunday moves inland across the northern Rockies Sunday
night and early Monday. Although the northern stream quickly east
across the northern Plains Monday, residual moisture remains in
place in the ranges of western MT Monday where moderate Day 2 snow probabilities exist for 8 or more inches.


...Colorado Rockies...
Day 3...

Reinforcing energy around the trough moving into the Pacific
Northwest Sunday shifts to NV/OR by late Monday before shifting
east across the CO Rockies late Tuesday. This may allow a cold
front to stall over central CO Monday evening with upper level
lift shifting across the same area Tuesday, making for a
potentially prolonged snow over a narrow corridor of
central/southern CO. This is highlighted by both the 12z ECMWF and
CMC and will need to be monitored. As of now, Day 3 snow
probabilities (ending 12Z Tuesday) have 20 to 40 percent
probabilities for 6 or more inches near Pikes Peak.


...Northern Great Lakes...
Day 1...
Developing surface low pressure ahead of a potent mid-level trough
tracks east-northeast from Chicago across lower MI tonight.
Enhanced northeasterly flow around this low brings Lake Superior
enhancement to the UP into northern WI where Day 1 snow
probabilities or 4 or more inches are moderate near Marquette and
up to 20 percent extending most of the UP to along the northern WI
border.


...Northern New England and Interior Northeast...
Days 1-2...

The warm conveyor belt east of the developing low pushing
northeast from MI Sunday brings a strong surge of moisture up the
Northeast Sunday into Sunday evening. Cold enough antecedent air
allows some snow to fall across higher elevations of interior
Maine and well as northern Aroostook Co where Day 1.5 snow
probabilities are moderate for 4 or more inches. Wrap around snow
has some upslope enhancement on the northern Adirondacks, Greens,
Whites, and along the western Maine border with Quebec where there
are moderate Day 2 snow probabilities of 2 or more inches.

The warm advection results in a change over to rain with some
pockets of freezing rain anticipated overnight into Sunday
particularly for the White Mtns of NH into Maine where there are
Day 1 10 to 20 percent probabilities or a tenth inch or more of
ice accretion.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Jackson


$$
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FOUS11 KWBC 280850
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
449 AM EDT Sun Mar 28 2021

Valid 12Z Sun Mar 28 2021 - 12Z Wed Mar 31 2021

...Pacific Northwest and Northern Rockies...
...Days 1-2...
A 700 mb trough moves east southeast from the northeast Pacific
and onshore across British Columbia, driving a 50-60 kt 700 mb jet
across the WA Olympics and Cascades, and then northern ID to
northwest MT. A surge in tropically sourced moisture precedes the
associated cold front bringing 0.75 inch PW air (one standard
deviation above normal) into western WA this morning.
Heavy snow is expected with high Day 1 snow probabilities of a
foot or more for the WA Cascades. Locally as much as 18-24 inches
is forecast in favored windward locations in the northern
Cascades.

The surge in moisture advection and lift that crosses the Pacific
Northwest today moves inland across the northern Rockies tonight
and early Monday. Although the northern stream trough moves
quickly east across the northern Plains Monday, residual moisture
remains in place in the ranges of western MT Monday where Day 2
snow probabilities are moderate to high for 4 inches and low for 8
or more inches along the Rocky Mountain front.

On Day 3/Tue., the slow approach of a 700 mb ridge from the
eastern Pacific advects drier air aloft across the northwest,
limiting snow potential.

...Colorado Rockies...
...Day 3...

A 700 mb wave with an associated pool of enhanced moisture moves
across CO in association with a cold front. After the frontal
passage, return flow results in boundary layer moisture
convergence in the mountains west of the CO Springs, CO area,
including locations around Pikes Peak. The ECMWF, UKMET, and
Canadian regional GEM show more QPF and resultant snow than the
00z NAM and GFE, and the former were given more weighting to
retain continuity. Several inches of snow are possible in the
foothills and mountains of central to southeast CO . The event
should wind down as the 700 mb wave/boundary layer moisture
convergence maxima move south of out CO.

Day 3 snow probabilities ending 12Z Tuesday have 30 to 50 percent
probabilities for 4 inches of snow in the foothills tot he front
range of central to southeast CO.


...Northern New York/New England...
Days 1-2...

The forecast closed low moving east from the northern Great Lakes
across southern Quebec advects both warmer air aloft and moisture
up across Maine today into this. Cold enough antecedent air allows
some snow to fall across higher elevations of interior Maine and
well as northern Aroostook Co where Day 1 snow probabilities are
moderate for 4 or more inches.

After the cyclone departs, cold advection develops across western
to northern NY, with snows occurring in the Tug Hill and western
Adirondacks, where a westerly component of low level flow turns
upslope in the windward terrain. Low probabilities exist for 4
inches of snow. The probabilities are limited by the duration of
snow in each area, given initial temps are too warm for snow.
On Mon, snow coverage wanes as a low level ridge crosses from the
Great Lakes, with drying aloft causing snow to taper.

The warm advection results in a change over to rain with some
pockets of freezing rain anticipated early today, particularly for
the mountains of western to northern Maine where there are Day 1
probabilities up to 30 percent for measurable ice accretion.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen


$$
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FOUS11 KWBC 282041
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
440 PM EDT Sun Mar 28 2021

Valid 00Z Mon Mar 29 2021 - 00Z Thu Apr 01 2021

...Pacific Northwest and Northern Rockies...
...Day 1...

A deep and anomalously cold trough for late March pushes
east-southeast across WA this evening with the main energy
shearing east along the Canadian border through Monday night and a
reinforcing trough amplifying the trough, digging it to CO through
Tuesday. 0.75 inch PWs (one standard deviation above normal) is
pushing into WA this afternoon with snow levels crashing from
3000ft to 1500ft by this evening as moisture and precip rates
taper off. The surge in moisture advection and lift reaches the
northern ID/MT Rockies this evening and NW Wyoming late tonight.
Day 1 snow probabilities are high for 6 or more inches for the
WA/OR Cascades, Bitterroots and ridges south of Glacier NP as well
as moderate for the Wallowa in northeast OR and the Tetons.


...Colorado Rockies...
...Day 2...

The amplifying wave from the Pacific Northwest tonight shifts down
the northern Rockies Monday into Tuesday, settling over CO.
Easterly upslope flow turns colder with rain changing to snow
along the Front Range after a cold frontal passage Monday evening
with the upper trough axis only slowly shifting east through
Tuesday, extending the snow in central/southern CO, particularly
from the Palmer Divide to the Pikes Peak area and down the Sangre
de Christos. The 12Z ECMWF, UKMET, and Canadian GDPS continue to
produce more QPF and resultant snow than the 12Z NAM and GFS with
preference remaining with the non-NCEP grouping. Several inches of
snow are possible in the foothills and mountains of central to
southern CO with Day 2 snow probabilities of 10 to 20 percent for
6 or more inches on central CO ridges.


...Far Northern New York/New England...
Day 1...

Low pressure now over southeastern Ontario will track across far
northern Maine tonight. Warm sector snow will change to rain or
sleet by this evening with pockets of light freezing rain in far
interior Maine. The upper trough will swing east across Maine
early Monday with wrap around snow on a westerly component of low
level flow makes for upslope snow in the windward terrain of the
Tug Hill, northwestern Adirondacks and northern Greens where there
are 10 to 40 percent Day 1 snow probabilities for 4 or more inches.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Jackson


$$
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FOUS11 KWBC 290743
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EDT Mon Mar 29 2021

Valid 12Z Mon Mar 29 2021 - 12Z Thu Apr 01 2021

...Northern to Central Rockies...
Days 1-2...

An anomalously deep closed 500mb low moving across southern Canada
will amplify a longwave trough across the West, with a surface
cold front dropping southeast beneath it. Height falls, modest
upper diffluence within the RRQ of the accompanying upper jet
streak, and 700-500mb warm and moist advection will spread snow
showers from the Northern Rockies this morning to the Central
Rockies tonight, with enhancement likely in the CO Rockies late D1
into D2. This enhancement is likely due to secondary shortwave
energy rotating atop the region combined with upslope flow behind
the cold front. WPC probabilities for 6 inches are high in the
Northern Rockies near Glacier NP D1, and low in the CO Rockies.
While probabilities on D2 remain low in the CO Rockies, the 2-day
totals may eclipse 8 inches in isolated locations along the Front
Range.


...Northeast...
Day 3...

An area of low pressure developing across the Mid-Atlantic will
lift northeast Thursday along a slowing cold front. This low will
strengthen in response to rapid height falls as an anomalous
mid-level trough advects east from the Great Lakes, and an
intensifying divergence maxima within the RRQ of an upper jet
streak. Large scale ascent will spread precipitation across the
Mid-Atlantic and Northeast, but WAA ahead of the front suggests
all precip will be rain initially. However, as the low deepens and
the cold front drops slowly southeast, the column will cool
rapidly to allow a p-type transition from rain to snow. This is
likely to be additionally enhanced by robust fgen/deformation NW
of the low. While there remains uncertainty into how much precip
will occur as the column cools enough for snow, this enhanced
ascent through the fgen should provide at least a burst of
snowfall, enough to overcome the antecedent rain and warm
conditions, especially in the terrain. WPC probabilities for 4
inches are as high as 30-40% in the Adirondacks and Tug Hill where
post frontal upslope flow will enhance snowfall, and 10-20% across
much of Upstate New York and into Vermont.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Weiss


$$
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FOUS11 KWBC 292028
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Mon Mar 29 2021

Valid 00Z Tue Mar 30 2021 - 00Z Fri Apr 02 2021

...Colorado Rockies...
Day 1...

Positively-tilted upper trough will move through the central
Rockies overnight into Tuesday morning as a deep upper low skirts
the U.S./Canadian border toward Hudson Bay. Surface cold front
will push through Colorado 00-12Z dropping temperatures below
freezing from northwest to southeast as 700mb temperatures drop to
-12 to -15C. Upper jet will sink southeast of the region quickly
late Tuesday as any moisture in the column dissipates by early
Wednesday. Limited moisture will be in place but a band of
0.2-0.3" precipitable water values will be able to squeeze out
0.25-0.50" liquid as upslope enhancement maximizes southwest of
Denver. General 4-6" amounts are expected with higher maxima 6-8"
possible in favored areas. Local maxima may also slide along the
Front Range west and northwest of Colorado Springs around 3-4" but
generally 1-2" below 7000 ft.

Fracasso


...Northeast...
Day 3...

A reinforcing shortwave trough currently over the WA/OR border is
rounding a parent trough which currently has its axis over
northern Rockies. This shortwave trough amplifies as it digs to UT
tonight, making a positively tilted trough axis connected to the
parent trough that will reach northern Ontario Tuesday. The
shortwave ejects east from the CO/NM border and across the
southern Plains Wednesday. Then Wednesday night the trough becomes
negatively tilted over the Midwest as rapid surface cyclogenesis
occurs along a slowing cold front and under the right entrance
region of the southerly jet over the Mid-Atlantic. Surface low
development continues Thursday it lifts over New England around a
now closed upper low over the eastern Great Lakes.

The combination of the surface low development and nocturnal
effects allows a rapid expansion of precip on the cold side of the
low with widespread snow developing over the eastern Midwest and
interior northern Mid-Atlantic (including the Appalachians)
Wednesday evening. This area of snow lifts north with the low
Thursday, mainly affecting northern PA, Upstate NY to VT. Ample
cold air and the deep cold core low do raise the prospect of snow
showers east of the Appalachians and perhaps all the way to the
Mid-Atlantic coast in the peak heating of Thursday afternoon. Day
3 snow probabilities for 4 or more inches are 10 to 30 percent
from the Allegheny Highlands of WV to northern PA, and 30 to 60
percent over interior Upstate NY (including the Finger Lakes
region) to the northern Greens of VT with 80% confidence for the
higher Adirondacks.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Jackson


$$
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FOUS11 KWBC 310832
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
432 AM EDT Wed Mar 31 2021

Valid 12Z Wed Mar 31 2021 - 12Z Sat Apr 03 2021

...Northeast...
Days 1-2...

...Late season significant winter storm likely for parts of
Upstate New York and Northern New England...

A shortwave moving across the Great Lakes will amplify in response
to a vorticity lobe swinging through its base atop the Central
Appalachians Thursday morning. This will cause the trough to take
on a negative tilt and close off, while at the same time a
poleward extending jet streak intensifies leaving a strengthening
divergence maxima over the region. The subsequent combination of
height falls, mid-level divergence, and upper ventilation will
drive surface cyclogenesis across the Mid-Atlantic, and this
surface low will strengthen as it lifts into Maine and then Canada
by Friday.

An impressive moisture plume noted by PW anomalies of +2 standard
deviations surging northward from the Gulf of Mexico will be wrung
out by the robust deep layer ascent across the Mid-Atlantic and
Northeast. Initially, this will be all rain for the region.
However, a cold front dropping southeast out of Canada will begin
to cool the column, while persisting SW flow aloft will maintain
moisture lifting atop the front. This column cooling will then
become enhanced by what is likely to be an intense deformation
axis overlapping strengthening fgen through the ageostrophic
response of the upper jet streak and the sharpening low-level
baroclinic gradient. This is favorable for a strong band of
snowfall, and both the HREF snow rate probabilities and the WPC
snowband prototype page indicate the potential for 1"/hr snowfall
as the column cools both through both dynamic and advective
processes. With the exception of the GFS which has become a
progressive outlier with its 500-700mb trough axis, the guidance
has come into better agreement tonight in depicting this
impressive band of snowfall developing late tonight across PA/NY
and shifting northeast into Thursday. Despite the hostile
antecedent conditions due to warmth and rain, these snow rates
should quickly begin to accumulate, first in the terrain and later
into the lower elevations, and as such the heaviest snowfall is
likely in the Adirondacks, Tug Hill Plateau, and northern
Catskills where WPC probabilities are above 50% for 6 inches.
Lighter accumulations are expected from the Laurel Highlands
northeast through much of Upstate New York except the Hudson River
Valley, and into much of northern and central Vermont.

As the low pulls away Thursday night and Friday morning, NW flow
should produce some upslope snow showers as well as periods of LES
downwind of Lake Erie and Ontario, with light additional snowfall
accumulations likely.

Additionally, there is likely to be a period of freezing rain
across parts of northern NH and ME where rain transitions before
turning over to snow. Heavy rates, warm antecedent conditions, and
freezing rain occurring during the April afternoon hours should
limit accretions. However, WPC probabilities are as high as 40%
for 0.1" of accretion across northern ME.


Weiss


$$
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FOUS11 KWBC 010826
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Thu Apr 01 2021

Valid 12Z Thu Apr 01 2021 - 12Z Sun Apr 04 2021


...Central Appalachians to Northeast...
Day 1...
An amplifying upper trough moves east over the eastern Great Lakes
and central Appalachians today, closing into an upper low over
Lake Ontario by this evening, elongating north to south from
Quebec to the Mid-Atlantic overnight. Surface low pressure
currently centered over southern New England and will track
north-northeast across eastern Maine through midday. Northwest of
the low track will continue to be a thump of moderate snow, moving
from far northeast PA and upstate NY and up well interior New
England through midday. Under the upper low increasing lapse rates
and some instability will allow scattered snow showers from
upslope areas of WV and up the Appalachians into New England
through this evening.

Day 1 snow probabilities are moderate for 4 or more inches from
the northern Adirondacks the northern Greens, and along the
NH/Maine and Quebec borders. A wintry mix is expected across
portions of far northern Maine. While widespread significant
icing is not expected, Day 1 ice probabilities are 20 to 30
percent in upper northeast Maine.

Jackson

$$
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FOUS11 KWBC 020822
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Fri Apr 02 2021

Valid 12Z Fri Apr 02 2021 - 12Z Mon Apr 05 2021


Days 1 into 2...
The probability of significant icing and/or snow is less than 10
percent.

Pacific Northwest...
Days 2/3...

Low pressure over the eastern Gulf of Alaska this morning ejects
southeast down the AK/BC coast through Saturday night before
shifting inland over WA on Sunday. Uncertainty with this low track
over the weekend continues to be rather high with the 00Z GFS the
greatest outlier and west of the 00Z GEFSmean along with the 00Z NAM/ECMWF/UKMET/CMC which together make a cluster of decent
confidence. Pacific moisture spreading inland over WA on Saturday
ahead of the trough allows some potential for moderate to locally
heavy snow, particularly in the northern Rockies Sunday night as
currently indicated by the 00Z ECMWF. This upper low track
warrants further monitoring.


New England
Day 3...

A reinforcing shortwave trough rounding the main low over the
Northeast will swing off the NC coast this morning and eventually
break off into is own low well off the New England coast on
Saturday as the parent low ejects north. This breakaway low then
likely retrogrades west toward eastern Maine Sunday night. There
is considerable uncertainty with this motion, but a solution like
the 00Z ECMWF/GFS/UKMET would put the low close enough to eastern
Maine to allow wrap around snow to cover much of Maine by Sunday
night. While heavy snow is not anticipated at this time, it is
worth monitoring the progress of this low.


The probability of significant icing is less than 10 percent.


Jackson

$$
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FOUS11 KWBC 030832
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
432 AM EDT Sat Apr 03 2021

Valid 12Z Sat Apr 03 2021 - 12Z Tue Apr 06 2021

Pacific Northwest...
Days 1-3...

An upper low over the southern Alaska Panhandle will move
south-southeastward along the British Columbia coast through
tonight before crossing the Pacific Northwest through Monday
morning then slowing and turning east, crossing Wyoming through
Tuesday. Its attendant cold front reaches northwestern Washington
this evening and Wyoming by Monday. Heights will fall over the
Cascades this evening as a narrowing corridor of PW values between
0.50-0.75" bisect the Olympics, spreading light snow to the higher
elevations of the Cascades. Snow levels will lower from about
5000ft this afternoon to about 2000ft tonight as colder air
filters in with moderate precip rates decreasing as the moisture
plume shifts south ahead of the front. Day 1 snow probabilities
are moderate for 6 or more inches in the far northern WA Cascades.

The GFS remains more positively tilted and farther west with the
resultant low, particularly on Monday. There is decent agreement
by Monday night among the 00Z NAM/ECMWF/CMC/UKMET. The slowed pace
of the system allows for a decent swath of snow over the northern
Rockies of ID/MT/WY Monday into Tuesday. So Day 2 snow
probabilities are limited to moderate for 4 or more inches in
Glacier NP before they blossom on Day 3 with low to moderate
probabilities of 8 or more inches across north-central ID,
southwest MT and northwest WY.

The probability of significant ice is less than 10 percent.


Northern New England...
Days 2/3...

A stretched upper low will split off its southern extent off
southeastern New England today and become negatively tilted as it
pivots south of Nova Scotia 60W. Late Sunday it will loop back to
the west near the southern tip Nova Scotia and wrap back some
light snowfall (or mixed rain/snow as temperatures moderate) to
portions of Maine and into far northern VT/NH Sunday night through
Monday night. The UKMET remains the farthest west while the CMC is
farthest east with decent agreement among the 00Z ECMWF/GFS which
are between the other two and are preferred. Continued with
conservative snow amounts given the uncertain evolution of the
cutoff system and marginal thermal conditions to produce only
light snow amounts. Due to the varied ensemble spread,
probabilities of at least 4 inches of snow are only about 5
percent on Day 2.

Jackson

$$
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FOUS11 KWBC 032023
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 PM EDT Sat Apr 03 2021

Valid 00Z Sun Apr 04 2021 - 00Z Wed Apr 07 2021


...Northern Rockies...
Days 1-3...
A low-amplitude ridge is expected to give way to a
positively-tilted upper trough dropping south across western
Canada and the Pacific Northwest on Sunday. Models show a split
in the upper pattern developing -- with a progressive northern
stream trough moving into central Canada on Monday, while in the
southern stream a closed low develops over the Pacific Northwest
before dropping southeast and tracking near the southern Idaho
border Monday night, to the southern Wyoming border on Tuesday.
Compared with the overnight runs, the 12Z guidance showed better
agreement -- with the GFS now notably faster than its previous
runs. This is expected to be a rather cold system, bringing snow
into the lower elevations of Idaho, western Wyoming and western
Montana. While precipitation is expected to be widespread,
amounts are expected to be generally light to moderate across the
region. This is reflected in the WPC PWPF. While it shows
widespread high probabilities for three-day total snow
accumulations of 4-inches or more across the central Idaho,
western and central Montana, and western Wyoming ranges, it also
little potential for accumulations exceeding 8-inches across the
region.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 040838
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Sun Apr 04 2021

Valid 12Z Sun Apr 04 2021 - 12Z Wed Apr 07 2021


...Northern Rockies to the North-Central High Plains...
Days 1-3...

A positively-tilted upper trough shifts southeast over western WA
today and through eastern OR tonight before the southern stream
spins off as a low over ID Monday which then tracks to the
CO/NE/KS border through Tuesday night. Decent agreement remains
with non-GFS 00Z guidance as the GFS remains more progressive with
the low starting Monday night over southwest WY. This system looks
to have a moderately cold core with snow generally contained to
higher elevations of the northern Rockies with snow levels
generally 5000 to 6000ft though they do drop to 4000ft on the
north side before the precip ends, bringing snow into the lower
elevations of ID, western WY and western MT. Widespread moderate
precipitation is expected with Day 2 snow probabilities of 4 or
more inches 40 to 70 percent from north-central ID across
southwest MT and northwest WY while Day 2 snow probabilities of 8
or more inches are generally under 30 percent.

The storm shifts east of the WY Rockies Monday night and onto the
north-central Plains through Tuesday night. Notable Day 3 snow
accumulations remain in the higher elevations though the potential
for a comma head band that brings accumulating snow to the
north-central High Plains will need to continue to be monitored,
particularly for Tuesday night. As of now Day 3 snow probabilities
for 4 or more inches is moderate for the Wind River and Bighorn
Ranges as well as the Black Hills with 10 to 20 percent probs of 2
or more inches on the Pine Ridge of northeast Neb.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 050844
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
443 AM EDT Mon Apr 05 2021

Valid 12Z Mon Apr 05 2021 - 12Z Thu Apr 08 2021

...Northern Rockies onto the North-Central High Plains...
Days 1-3...

A positively tilted upper trough shifting southeast from the
Pacific Northwest this morning will split into its own southern
stream closed low later today. This low will then track to
northern UT through tonight and north-central CO through Tuesday
before ejecting east across the central Plains/KS Tuesday night
through Wednesday. Moderate precip, snow levels 4000 to 5000ft
over southwest MT will be enhanced tonight as the closed low slows
and is able to draw in moisture of Gulf origin west across WY as
lee-side low pressure develops near the CO/KS border. It's on the
eastern slopes of the Big Horn, Wind River, and Absarokas that the
moisture, topographical lift and cold combine to produce the
heaviest snow of the system late tonight into Tuesday with these
areas with 70 to 80 percent likelihood for 6 or more inches in Day
1.5 snow probabilities. Snow levels drop to ground level tonight
in the Big Horn Basin which is beneficial for the area of
north-central WY which is currently under extreme drought
conditions.

As the low spills onto the plains and the lee-side surface low
becomes dominant, TROWAL formation northwest of the low looks to
line up well with the Black Hills and south through the Pine Ridge
of Northwest Neb. Day 2 snow probabilities for the Black Hills are
40 to 50 percent for 6 or more inches with 10 to 20 percent values
along the Pine Ridge. That much of this comma head banded snow
will occur during the daylight hours of Tuesday does not bode well
for accumulations and elevation should factor heavily for
accumulating snow. A lack of cold air farther east should being a
fairly quick end to snow on the northwest side of the low early
Wednesday as the low lifts toward the Upper Midwest.


...Pacific Northwest...
Day 3...

The next trough shifts down the Alaskan Panhandle Tuesday night
and the BC Coast Wednesday before shifting southeast across WA
Wednesday night. A moderate surge of Pacific moisture ahead of the
trough/cold front brings moderate precip rates and snow levels
around 2500ft. Day 3 snow probabilities are moderate to high for 8
or more inches in the Olympics and WA Cascades.



For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson


$$
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FOUS11 KWBC 060831
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 AM EDT Tue Apr 06 2021

Valid 12Z Tue Apr 06 2021 - 12Z Fri Apr 09 2021

...Northern Rockies onto the North-Central High Plains...
Day 1...

A closed southern stream low over northern UT will shift ESE to KS
through tonight. The lee-side surface cyclone currently near the
CO/KS border is directing Gulf moisture west over western SD/Neb
and into WY which will continue until the 700mb low shifts to
central Neb this evening, with snow rates dropping this evening as
the system moves into warmer, lower elevations.

00Z CAMs/regional QPF guidance notably higher than 00Z non-NCEP
global guidance in the TROWAL region from the Black Hills to Pine
Ridge west to the Big Horns, particularly this afternoon. This led
to an increase in confidence for higher snow potential
particularly in eastern WY to the Pine Ridge. However, the
majority of the QPF falls from 18Z-00Z which is peak diurnal
heating and it is April, so SLRs should be limited and elevation
should play a role in accums (though snow accums in fgen bands are
often fairly independent of elevation).

Day 1 snow probabilities are 50 to 70 percent for 6 or more inches
for the Black Hills and from Pine Ridge in far northwest Neb west
into WY with 30 to 40 percent in the Powder River/Thunder Basin.


...Pacific Northwest and Northern Rockies...
Days 2-3...

A shortwave diving down the coast of British Columbia Wednesday
will shift into WA Wednesday night and reach the WY Rockies by
late Thursday night. Briefly backed mid-level flow will bring warm
moist air into the Pacific Northwest, aided by a modest Pacific
jet streak arcing southward from the Gulf of Alaska. The trough
axis and associated surface front will drive the ascent to produce
fairly progressive precipitation across the Olympics and Cascades
D2, and the into the Northern Rockies D2.5/3. Day 2 WPC
probabilities for 6 or more inches are moderate to high in the
Olympics and WA Cascades with snow levels around 3000ft. Day 2.5
snow probabilities are moderate for 4 or more inches in the
northern ID/Bitterroots and around Glacier NP with low Day 3
probabilities for 4 or more inches in the northern Absarokas and
Big Horns.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson


$$
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FOUS11 KWBC 081954
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EDT Thu Apr 08 2021

Valid 00Z Fri Apr 09 2021 - 00Z Mon Apr 12 2021


...Pacific Northwest through Northern High Plains...
Days 1-3...

A potent shortwave digging through the Northern Rockies this
evening will drop southeast into the Central Plains producing
sharp height falls and PVA coincident with an upper divergence
maxima moving across the Rockies. Beneath this trough, a surface
cold front will drop southeast producing strong CAA, while a wave
of low pressure moves from Montana to Minnesota ahead of that
front. These features together will spread precipitation across
WY, SD, and NE, in the form of snow across the higher terrain.
Moderate to heavy snow is likely in the Big Horns on D1, with
heavy snow also expected in the Black Hills of SD. The
probabilities for 6" are highest in the Black Hills where N/NW
flow will drive intense upslope ascent into a saturated DGZ, and
snowfall rates of 1"/hr are possible at times. Locally more than
8" is possible in the highest terrain.

After a brief respite across the region, wet weather is forecast
to return to the Northwest by late Friday as the next system digs
along the coast of British Columbia into the region. This will
bring a round of moderate to heavy snow into the Olympics and
northern Cascades late Friday into early Saturday, with the WPC
probabilities showing a high risk for accumulations of 6-inches or
more. Like the previous system, this second wave is expected to
move progressively to the east, generating mainly light snows as
it moves into the northern and central Rockies on D3 where WPC
probabilities for 6 inches are confined to the highest terrain of
the Absarokas in NW WY.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 100756
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EDT Sat Apr 10 2021

Valid 12Z Sat Apr 10 2021 - 12Z Tue Apr 13 2021


...Pacific Northwest to the Northern Plains...
Days 1-3...
An amplifying upper level shortwave trough and its associated
frontal band will move progressively east from the Pacific
Northwest to the northern Rockies on Saturday. This fast-moving
system is expected to bring windy conditions and periods of snow
to the region. While snows may be briefly intense, accumulations
are expected to be generally light. For Day 1 (ending 12Z
Sunday), WPC PWPF high probabilities for accumulations of 4-inches
or more are largely confined to portions of the northern Cascades,
the northern Idaho and northwestern Montana ranges.

As the system moves into the northern Plains on Sunday it is
expected to assume a negative tilt. A rain-snow mix is expected
across western to central North Dakota on Sunday, changing over to
mostly snow Sunday night. While widespread heavy accumulations
are not expected, still monitoring the potential for at least an
inch or two, with some potential for heavier accumulations,
setting up along an axis of strong forcing supported by an
elongated low-to-mid level low centered over western North Dakota.

On Monday, this system is expected to phase with a low centered
farther east over the Upper Great Lakes. Cold air advection on
the northwest side of the consolidating low will support light to
moderate snows spreading across northwestern Minnesota and North
Dakota on Monday. WPC PWPF shows some 10-40 percent probabilities
for accumulations of 4-inches or more centered over north-central
to northeastern North Dakota on Day 3 (ending 12Z Tuesday).

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Pereira


$$
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FOUS11 KWBC 102023
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 PM EDT Sat Apr 10 2021

Valid 00Z Sun Apr 11 2021 - 00Z Wed Apr 14 2021


...Northern and Central Rockies to the Northern Plains...

Days 1-3...

An upper level shortwave trough and its associated frontal band
will move out of the northern Rockies and east across the northern
Plains Sunday. This fast-moving system is expected to bring windy
conditions and periods of snow to the higher elevations of central
Montana. Several inches of snow are expected in windward areas
tonight as the low level are near saturation in the forecast
soundings.
Tomorrow, as the upper trough departs, the drying aloft should cut
down on snow shower coverage/intensity despite steep lapse rates.

As the system moves across the northern Plains on Sunday and upper
MS Valley Sunday night it is expected to assume a negative tilt.
The post-front cold advection advects colder air east and changes
rain to snow starting in North Dakota and then northern Minnesota.
While widespread heavy accumulations are not expected, a general
2-4 inches is forecast in a band across northern North Dakota,
with lighter amounts elsewhere in the northern Plains.
Persistent moist conditions from the surface to 700 mb and return
northeast flow sets up the likelihood of several inches of snow in
the Absaroka and Wind River Ranges late Sunday night into Monday
night.

On Monday night-Tuesday, the northern Plains system shears and
moves slowly. the deformation band north of the circulation sets
up the possibility of a long duration light to moderate snow over
northern North Dakota and northwest Minnesota. The uncertainty is
how far south the precip band and colder air can progress across
North Dakota and northern into central MN. Phasing differences
lead to spread among solutions and more uncertainty.

WPC PWPF shows 10-40 percent probabilities for accumulations of 4
inches centered over northern North Dakota on Day 3.

In the Rockies, the jet stream axis and zone of moist confluent
flow drops south form Wyoming into Colorado.
Downstream from the closed 700 mb low over the Great Basin,
moisture increases in CO and 700 mb ascent begins. Low low level
flow has an easterly component to it, favoring the foothills and
front range getting snow developing Tuesday. Several inches are
possible in the CO ranges on Wed. The GFS is wetter/whiter, so
while the version 16 has improved over the old version 15, it
still has a bit of a high bias. Consequently, other models were
weighted more in CO. The PWPF has a 50-60 probability of 4 inches
in the CO front range Tue.


For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Petersen


$$
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FOUS11 KWBC 110917
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
517 AM EDT Sun Apr 11 2021

Valid 12Z Sun Apr 11 2021 - 12Z Wed Apr 14 2021


....Northern Plains...
Days 1-3...
An amplifying shortwave trough and its associated frontal band
will continue to move east of the northern Rockies into the Plains
this morning. Models continue to show an area of enhanced
precipitation centered along an elongated low-to-mid level center
sliding across North Dakota on Sunday. Thermal profiles suggest a
rain/snow mix during the day on Sunday, changing to mostly snow
Sunday night. This is expected to produce a stripe of mostly
light accumulations from northwestern to central North Dakota.
Heaviest accumulations through early Monday are expected to center
over northwestern North Dakota, where the Day 1 WPC PWPF (ending
12Z Monday) shows a small area of 30-50 percent probabilities for
accumulations of 4-inches or more.

Models show an elongating upper low developing over the northern
Plains by early Monday, with snow spreading across northern
Minnesota and North Dakota. Periods of snow are expected to
continue through Tuesday and into early Wednesday as the low
drifts slowly east, resulting in widespread but generally light
accumulations across the region. For the two-day period ending
12Z Wednesday, WPC PWPF shows high probabilities for accumulations
of 4-inches or greater extending across much of northern North
Dakota into the the northwestern corner of Minnesota.

...Great Basin to the Central Rockies...
Day 3...
A shortwave trough dropping south from western Canada on Monday,
is expected to develop a closed low over the Pacific Northwest and
northern Intermountain West on Tuesday. Divergence aloft will
support snow developing across northern Nevada and southern Idaho,
northern Utah, and western Wyoming by late Tuesday and then
continuing into early Wednesday. In addition to favorable upper
dynamics, increasing upslope flow will begin to support snow
developing along the High Plains into the eastern slopes of the
central Rockies, with some potential for significant accumulations
beginning to develop along the southeastern Wyoming and central
Colorado ranges by early Wednesday.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira


$$
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FOUS11 KWBC 112050
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
450 PM EDT Sun Apr 11 2021

Valid 00Z Mon Apr 12 2021 - 00Z Thu Apr 15 2021


....Northern Plains...
Days 1-3...
A deep layer trough and its associated frontal band will continue
to drift east across the northern Plains
tonight. Post-frontal cold advection in northern North Dakota
leads to a precip type change over from rain to
snow Sunday night. This is expected to produce a stripe of mostly
light accumulations from northwestern to central North Dakota.
Heaviest accumulations through early Monday are expected to center
over northwestern North Dakota, where the Day 1 WPC PWPF shows an
area of 60-80 percent probabilities for accumulations of 4 inches
or more. With the initial ground and air temperatures above
freezing, more accumulations is expected over grassy surface than
roads.

As the upper low/trough elongate and move east from North Dakota
to Minnesota Monday night-early Tuesday,
mid level deformation and frontogenesis persists over northeast
North Dakota and northwest Minnesota.
Periods of snow are expected to continue through Tuesday as the
low drifts slowly east, resulting in widespread accumulations
across the region. For the two-day period ending 12Z Wednesday,
WPC PWPF shows high probabilities for accumulations of 4 inches or
greater extending across much of northern North Dakota into
northwestern Minnesota.

...Great Basin to the Central Rockies...
Day 2/3...

On Day 2, the models show confluent flow aloft leading to a jet
streak over the CO Rockies to central high Plains. Low level
convergence and modest upslope flow leads to light snow developing
int he foothills and front range of the CO Rockies Tue. Several
inches of snow are possible.

The snow expands in coverage Tue night into Wed as a closed 700 mb
low is forecast to develop
over Nevada and move across northern Utah to near the Wyoming
border by 0z Thu.
Coupled divergence aloft/low level convergence will support snow
developing across northern Nevada and southern Idaho, northern
Utah, and western Wyoming Wednesday. Several inches of snow are
expected in favored upslope areas of elevated terrain. Continuing
upslope flow will begin to support more snow into the eastern
slopes of the central Rockies, with some potential for significant accumulations along the southeastern Wyoming and central Colorado
ranges.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen



$$
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FOUS11 KWBC 130827
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Tue Apr 13 2021

Valid 12Z Tue Apr 13 2021 - 12Z Fri Apr 16 2021

...Northern Plains...
Day 1...

A closed low positioned over northern MN will spin nearly in place
on D1 before finally ejecting to the E/SE D2. Beneath this
feature, a surface low will retrograde slightly near the Arrowhead
of MN before kicking to the east by Wednesday morning. Spokes of
vorticity rotating westward around the upper low will drop south
across the Northern Plains, coincident with a surface trough and
aligned with an axis of mid-level deformation and fgen. Together,
these features will drive deep layer ascent and periods of
moderate snowfall will continue from MT through ND and into
western MN. The overlap of moisture and forcing is expected to be
modest, so much of the snowfall intensity should be of the light
to moderate variety, and this is reflected by WPC snowband
probabilities of 0.5"/hr or less. However, some subtly stronger
banding is possible beneath the S/SW moving deformation axis.
Should this occur locally higher accumulations are possible
through this evening, but otherwise WPC probabilities for 4 inches
are generally 20-50% in pockets across the region. As the low
pulls away and weakens this evening and tonight, forcing will wane
and snow should shut off late D1.

...Great Basin to the Central High Plains...
Days 1-3...

Complex and long-lasting snow event is likely to bring periods of
heavy snow from the Sierra to the Central High Plains much of the
week.

A sharpening trough digging through the Pacific Northwest this
morning will drop southward and amplify into a closed low over the
Great Basin Wednesday morning and then drift nearly in place
through Thursday as the mid-level pattern across the CONUS gets
blocked. This feature will eventually fill and eject eastward
towards the Southern plains on D3. As the closed low pivots across
the Great Basin, a subtropical jet streak arcing west to east will
strengthen from Southern CA into the Texas, placing favorable LFQ
diffluence across the area. This will combine with persistent WAA
and mid-level divergence to drive deep layer ascent. Meanwhile
700mb flow will orographically ascend some of the terrain,
enhancing lift already impressive through height falls. While
moisture on D1 may be somewhat limited outside of the CO Rockies,
by D2 Gulf of Mexico moisture begins to lift northwestward into
the region, driving PW anomalies of +1.5 standard deviations into
CO/WY. As this occurs 700mb omega becomes intense across parts of
WY through both mesoscale and synoptic ascent, and while guidance
continues to feature quite a bit of spread in position of heaviest
snow, there has been some trend towards consensus this morning for
the heaviest snow focused in WY D2. By D3 the closed low begins to
open and shift eastward, but continued warm/moist advection,
height falls, upslope, and jet diffluence will maintain periods of
heavy snow on D3.

The heaviest snow D1 is expected in the CO Rockies including the
Front Range where height falls and upslope flow will help wring
out slowly increasing column moisture. WPC probabilities for 6
inches are high across this area, with locally more than 8 inches
possible. The heaviest snow develops D2 across central and
northern WY where persistent low-level convergence in a region of
high column moisture will produce heavy snowfall, and rates are
likely to exceed 1"/hr. The heaviest accumulations are likely on
the upslope side of the Wind Rivers, Absarokas, Tetons, Big Horns,
and south into the Uintas, where WPC probabilities for 6 inches
are high, and locally more than 12 inches is likely. Further
south, a pronounced dry slot will likely limit accumulations
towards Cheyenne and into Colorado. By D3, forcing becomes more
spread out to the south and east. This should allow snowfall to
spread east into the High Plains of NE and SD, as well as the
Front Range of CO and back into the Laramie/Snowy ranges of WY,
while continuing in the western WY terrain. WPC probabilities on
D3 are moderate for 6 inches in these areas. 3-day snowfall may
reach 18 inches in parts of the highest terrain of WY and CO.


...Northeast...
Day 3...

A deep closed low of -2 standard deviations at 500mb and in the
bottom 10th percentile for mid-April according to NAEFS ensemble
tables will move from Michigan southeast to be positioned over
Long Island by the end of D3. Rapid height falls accompanying this
feature will combine with LFQ diffluence of a strengthening jet
streak to the south to drive pressure falls and a surface low is
likely to develop off the VA coast Thursday morning. This low is
then progged to lift northeast to off Southern New England while
strengthening. WAA ahead of the low will spread precipitation
northward, some of which could be quite heavy, but initially will
be all rain due to warm low-level thermal structure. However, as
the low deepens and the core of the upper cold shifts southeast,
the combination of CAA and intense deformation will rapidly cool
the column causing a p-type transition from rain to snow. Guidance
still features a wide spread in placement of the best forcing and
subsequent accumulations, but WPC ensemble means have trended
upward this morning. Despite it being mid-April, overnight
snowfall Thursday into Friday combined with what could be intense
snow rates of >1"/hr should accumulate, especially in the terrain
of the Catskills, Berkshires, and Greens. WPC probabilities
currently indicate a 20-30% for 4 inches in these areas.


For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Weiss


$$
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FOUS11 KWBC 140834
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
434 AM EDT Wed Apr 14 2021

Valid 12Z Wed Apr 14 2021 - 12Z Sat Apr 17 2021

...Northeast...
Days 2-3...

A late season nor'easter will bring rain changing to heavy snow
across primarily interior portions of the Northeast and New
England through Friday.

A 500mb closed low progged to reach -2 standard deviations below
the climo mean and within the bottom 10th percentile for mid-level
temperatures according to NAEFS ensemble tables will drop
southeast from Michigan Thursday morning to near Long Island
Friday morning to finally east of Cape Cod Saturday morning. This
slow moving feature will maintain intensity through the period as
it is reinforced by waves of vorticity rotating around it. At the
same time, a zonally oriented jet streak south of the primary
trough axis will intensify, and the resultant LFQ diffluence
aligned with significant height falls will promote surface
cyclogenesis off the VA coast Thursday morning. This low will then
deepen as it lifts slowly northeast to a position near Cape Cod
Friday morning and then stalls briefly as it becomes stacked
beneath the upper low.

Robust moist advection will precede this mid-level trough as
moisture from both the Gulf of Mexico and the Atlantic are drawn
northward into the system. Rich theta-e advection wrapping around
the deepening low as a WCB/TROWAL will help spread heavy
precipitation northward into New England, and heavy QPF is likely.
Initially, all of the precipitation should be rain. However, as
cold advection commences behind the system and within the upper
low, rain will begin to change to snow. This cooling will likely
be intensified as well by dynamic effects through a pivoting
deformation axis overlapped with mid-level fgen, and some elevated
instability on the periphery of a westward advancing dry slot into
New England. As rain changes to snow, it will likely be heavy at
times where any banding can occur, and also where low-level
upslope flow can enhance the already intense dynamics. Heavy snow
is most likely in the terrain of the Catskills, Adirondacks, and
Whites, but most impressively in the Berkshires and Greens where
the longest duration of heavy precip and cold temperatures are
predicted.

SLRs during this event are likely to be quite low both due to
marginal thermal profiles and the fact that it is April, with
daytime snow across eastern new England struggling to accumulate.
In fact, many of the SLR ensemble plumes depict ratios as low as
4:1, suggesting outside of the terrain snowfall will struggle to
accumulate. Still, periods of moderate to heavy snow are likely,
and WPC probabilities for 4 inches are high on D2 in the
Adirondacks, and especially Greens and Berkshires generally above
1500 ft. Locally more than 8 inches is possible. On D3, the
heaviest snow should shift northeast to include the Whites of NH
and ME, where WPC probabilities are again high for 4 inches with
locally more than 8 possible. Additionally, some light snow is
possible as far as the NH/ME coast as cold air funnels into the
systems despite mid-April sun angle.


...Great Basin to Central High Plains...
Days 1-3...

Long duration snowfall begins today across the Rockies as a deep
closed low moves slowly form the Great Basin to the Central Plains
while gradually weakening. This closed low is positions over
Nevada this morning and will maintain intensity through Thursday
evening will drifting eastward in an amplified flow pattern across
the CONUS. Height falls, PVA, and mid-level divergence all
combining across the Great Basin and Rockies will drive deep layer
ascent, aided by persistent diffluence within the LFQ of a
subtropical jet streak south of the trough axis. While guidance
still features some discrepancy into the placement of both the
mid-level wave and resultant surface low, there is better
agreement this morning overall.

Embedded within this jet streak and due to long duration fetch
from the Pacific south of the 500mb low, moisture will begin to
increase across the region. However, the most robust moist
advection will stream out of the Gulf of Mexico Thursday into
Friday as 850-700mb winds back to the S/SW driving PWs to +1.5
standard deviations above the climo mean. The prolonged synoptic
ascent within this moist airmass will spread precipitation across
the region, and by Thursday night there is likely to be an
expansive plume of snowfall covering much of the Central Rockies
and High Plains.

The heaviest snowfall is likely to be in the upslope favored
terrain of the Uintas, Wind Rivers, southern Absarokas, Tetons,
and Front Range, where WPC probabilities on D1 and D2 are moderate
to high for 6 inches. While widespread intense snowfall (rates

1"/hr) are not expected, the enhancement due to upslope in the

colder high elevation areas could produce locally well in excess
of 12", highest in the Uintas and Wind Rivers. Additional heavy
snow is likely in the Front Range. Late D2 and into D3, the
heaviest snowfall is progged to spread eastward into the High
Plains before the primary wave opens and shunts southward. While
prolonged moderate snow is likely during this time frame, marginal
thermal structure outside of the terrain (generally below 4000 ft)
will limit total accumulations. Still, WPC probabilities are
moderate for 6 inches in the Black Hills, Pine Ridge, and NE CO
Plains. As the forcing shunts southward D3, heavy snow should
become confined to the Front Range once again.


For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Weiss


$$
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FOUS11 KWBC 162001
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Fri Apr 16 2021

Valid 00Z Sat Apr 17 2021 - 00Z Tue Apr 20 2021

...Northeast...
Day 1...

A potent late season nor'easter will pivot slowly away from the
New England coast tonight. Banded snowfall NW of the low which has
been prevalent this aftn will begin to wane as the most robust
ascent weakens and the low pulls away. The intense closed low
aloft will continue to work in tandem with upper diffluence on the
LFQ of a slowly departing jet streak to drive omega across the
region, with a slow exit from west to east through Saturday
morning. Additional accumulations should be light outside of the
terrain, but could reach all the way to the coast of Maine and New
Hampshire overnight. WPC probabilities for significant snow (>4")
are confined to the highest peaks of the White Mountains and into
northern Maine where the chance is as a high as 40%.


...CO/NM Rockies...
Days 1-2...

A sharp upper trough digging over CO today will shift east, but
persistent longwave troughing will be reinforced by a secondary
shortwave shifting across the Great Basin and into the Four
Corners on Saturday, while persistent but weakening LFQ diffluence
from a subtropical jet streak aids in deep layer ascent. In the
low levels, a cold front will be well south of the region into
Texas, but continued cold low-level flow from the N/NE will drive
upslope precipitation into the Rockies and Sangre De Cristos, with
additional snowfall spreading into the San Juans as overrunning
and moist mid-level flow persists. WPC probabilities on D1 are
high for 6 inches in the San Juans and Sangre De Cristos, with
locally 12 inches likely in the best upslope regions. By D2, the
best forcing shunts southward, but moderate probabilities for 6
inches continue in the San Juans and New Mexico portion of the
Sangre De Cristos.


...Northern/Central Rockies...
Day 3...

An amplified mid-level flow pattern across the CONUS will be
characterized by a short-wavelength but impressive ridge along the
west coast, with large cyclonic troughing across the eastern 2/3
of the CONUS. Embedded within this trough, a shortwave is progged
to dig out of the Alberta/Saskatchewan Sunday evening. Beneath
this trough, a potent cold front will drop southward across the
Northern Plains and then into the Central Plains, while banking
against the Central Rockies by the end of the forecast period.
Height falls and upslope flow will be the primary mechanisms for
ascent, however a modest but strengthening polar jet streak is
likely to provide some enhancement through upper diffluence as it
digs southward in tandem with the other features. Mid-level
moisture profiles indicate that Pacific moisture will be limited
and blocked by the ridge to the west, and PW anomalies weaken with
southward gain during D3. However, periods of robust ascent in at
least a modestly moist column will provide areas of heavy snow,
especially in the higher terrain. WPC probabilities on D3 are high
for 6 inches in the Absarokas, Big Horns, Wind Rivers, Black
Hills, and into the Front Range of CO. The heaviest snowfall is
likely in the Big Horns where more than 12 inches is possible.


For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Weiss


$$
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FOUS11 KWBC 170831
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Sat Apr 17 2021

Valid 12Z Sat Apr 17 2021 - 12Z Tue Apr 20 2021


...CO/NM Rockies...
Days 1-2...

Water vapor imagery this morning shows an upper low over southwest
Utah that is adverting high level moisture towards southern CO and
northern NM.
The NAM forecasts a 700 mb low to drift east across southeast UT
into southern CO today, and drift south into NM tonight.
Bands of 700 mb convergence rotate around the low and combine with
difluent flow aloft to produce ascent.
QPF and snow maxima re expected in the San Juans of southwest
CO/northern NM, and also further east in the Sangre De Cristos.
WPC probabilities on D1 are high for 6 inches in the San Juans and
Sangre De Cristos, with locally 12 inches likely in the best
upslope regions.
On Sunday, the weak 700 mb low tracks across eastern AZ southward
to the Mexican border, with modest convergence downstream from the
low supporting snow showers as the convergence maxima cross the
mountains of eastern AZ and western NM. Several inches are
possible before the wave departs and ascent decays Sun night.

...MT/WY Rockies Day 2 and WY/CO Rockies and Central Plains Day
3...

A potent cold front will drop southward across the Northern Plains
and then into the Central Plains. Frontal convergence/pooling of
moisture and upslope flow will be the primary mechanisms for
ascent, combined with divergence maxima aloft as the jet moves
south across Montana into Wyoming Day 2. WPC probabilities on D2
are high for 6 inches in the Rocky Mountain Front/Glacier National
Park, Absarokas, and Big Horn Mountains. The heaviest snowfall is
likely in the Big Horns where more than 12 inches is possible.

On Day 3, Tue., the upper jet continues to build south, crossing
southern Wyoming and northern CO before moving east out on the
central Plains. Upper divergence maxima combine with 700 mb
convergence to produce ascent, starting in the WY Wind River,
Snowy and Laramie ranges and then building south across the
foothills and front range of CO Monday to Monday evening. Several
inches are likely in these areas, with isolated totals up to a
foot possible. The highest 8 inch probabilities are moderate in
the front range of northern CO 12z Mon-12z Tue.

As the front moves south steadily across the central Plains,
precipitation occurs in association with the front and enhanced by
300 mb divergence maxima in the right entrance of the 300 mb jet
crossing Nebraska. This favors post-frontal snow for several
hours centered on southern Nebraska and northern KS.
Probabilities are low for 4 inches of snow in this area.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen


$$
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FOUS11 KWBC 172011
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
410 PM EDT Sat Apr 17 2021

Valid 00Z Sun Apr 18 2021 - 00Z Wed Apr 21 2021

...Rockies and Central High Plains...
Days 1-3...

Two distinct systems will bring periods of heavy snow during the
next 3 days.

The first is an upper low which will continue to gradually sink
S/SW over the Four Corners through Sunday. Height falls will be
minimal, but weak upper divergence combined with modest warm/moist
advection around the upper low will continue to produce periods of
moderate to heavy snow in the Southern Rockies until forcing shuts
off late D1. WPC probabilities for 6 inches are moderate, and
confined to the higher terrain of Sangre De Cristos in NM and
White Mountains of AZ.

A more widespread and significant precipitation event will begin
D2 as an amplified mid-level pattern takes shape across the CONUS.
A potent shortwave digging through broad longwave troughing from
Canada will spill southward Monday and Tuesday, driving a string
surface cold front southward as well. Aloft a jet streak will
develop in its wake, and guidance has trended stronger with this
feature today, producing better upper level diffluence to drive
ascent. While on D2 moisture will be somewhat limited by lack of
Pacific or Gulf advection, by D3 return flow out of the Gulf of
Mexico will increase moisture for precipitation. Despite a true
ocean connection, PWs as high as +1 standard deviations will move
across the Northern and Central Rockies D1, with enhanced ascent
through the low-level convergence and upper divergence couplet
producing heavy snowfall. On D2, WPC probabilities for 6 inches
are high from the Northern Rockies through the Wind Rivers, Big
Horns, Black Hills, and into the Front Range of CO. Locally more
than 8 inches is likely in the Black Hills which is close to the
WSE mean, with more than 12 inches possible in the highest terrain
of the Big Horns which will have the longest duration of robust
ascent, including upslope enhancement, and moisture. As the upper
features continue to dig southward in conjunction with the surface
front, moderate WPC probabilities for 6 inches shift into the CO
Rockies, including the Front Range, on D3.

...Central Plains to Midwest...
Day 3...

The same shortwave responsible for the snowfall across the Rockies
D2 will begin to sharpen and amplify into a neutrally tilted
feature on D3 as it shifts into the Central Plains. This feature
will be accompanied by an intensifying jet streak dropping
southeastward with enhanced upper diffluence. The combination of
robust upper diffluence, height falls, and PVA will lead to an
intensifying area of low pressure developing along the southward
advancing cold front/baroclinic gradient. Guidance has trended a
bit further southeast/faster with these features today, and with
increasing moist advection from the Gulf of Mexico there is likely
to be a swath of moderate precipitation, some of which will occur
as snow. While accumulating snow in mid-April is difficult to
achieve, and SLRs are forecast to be generally 7-8:1, less than
the Baxter climatological mean, an strengthening region of
850-700mb fgen will help promote locally enhanced ascent to
dynamically cool the column and lead to snowfall rates which could
overwhelm the warmer soils and lead to accumulations. There
remains considerable uncertainty at this time, but WPC
probabilities on D3 show a 10-20% chance for 4" of snow from
central KS into northern MO, with a few inches of accumulation
possible as far northeast as Chicago, IL.


For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Weiss


$$
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FOUS11 KWBC 180842
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
442 AM EDT Sun Apr 18 2021

Valid 12Z Sun Apr 18 2021 - 12Z Wed Apr 21 2021

...Northern to Central Rockies and Central Plains...
Days 1-2...

A potent shortwave digging south from Canada will drive a couplet
of upper divergence/lower level convergence in conjunction with a
cold front as it sinks south across Montana this afternoon and
overnight, continuing across Wyoming and northern CO on Monday,
before the jet and upper divergence move east across the central
Plains. In the favored windward terrain over the Northern and
Central Rockies WPC probabilities for 6 inches are high from the
front range of western MT, the Absarokas, Wind Rivers, Big Horns,
Black Hills, and into the Front Range of CO. Isolated maxima of
12 inches is possible in these ranges due to upslope enhancement,
and moisture. As the upper trough and jet maxima/associated 300 mb
divergence maxima/700 mb convergence maxima move south into
northern Co in conjunction with the surface front, moderate WPC
probabilities for 6 inches shift into the CO Rockies, including
the Front Range, on D2. The peak highest Day 1 plus Day 2 totals
are over the WY Bighorns, as northwest flow leads to well defined
and persistent, long lasting upslope flow. The models cluster well
in the distribution of QPF and snow, so the models were equally
weighted.

...Mid MS Valley to Midwest/Great Lakes...
Days 2/3...

The same shortwave responsible for the snowfall across the Rockies
and central Plains D2 will begin to sharpen and amplify on D3/Tue
as it moves east across the mid MS Valley to the Midwest and Great
lakes. This feature will be accompanied by an intensifying jet
streak crossing WI and MI. The combination of divergence aloft in
the right jet entrance region and convergence along the 700 mb
front leads to ascent and mixed precipitation, with rain changing
to snow across southeast Iowa to northern IL, southern lower MI,
and northern IN. An intensifying area of low pressure developing
along the cold front leads to a possibility of an area of several
inches of snow across northern IN and lower MI.

A strengthening region of 850-700 mb frontogenesis as the 850 mb
low develops in IN will help promote enhanced ascent to
dynamically cool the column and lead to snowfall rates that result
in snow accumulations. There remains considerable uncertainty as
the models still have spread on the low track, intensity, and
corresponding QPF/snow amounts, as the SREF Mean, NAM and ECMWF
have an axis of snow that is further north than the 0z GFS,
Canadian global and UKMET. WPC probabilities on D3 show a low
chance for 4" of snow from central KS into northern MO, southeast
IA, and northern IL. The probabilities start increasing to
moderate in southern Lower MI as more models/members show
potential for snow as the cyclone starts to develop, and increased
low level frontogenesis leads to higher QPF and snow amounts.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen



$$
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FOUS11 KWBC 182047
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
446 PM EDT Sun Apr 18 2021

Valid 00Z Mon Apr 19 2021 - 00Z Thu Apr 22 2021

...Northern to Central Rockies and Central Plains...
Days 1/2...

A vigorous shortwave digging south from Canada along with a strong
cold front will dive south across the Northern Rockies tonight and
reach the Central Rockies/Plains on Monday. Meanwhile, upper level
divergence in conjunction with the left exit region of a 300 mb
jet streak will promote enhanced lift between tonight and Monday
morning from western Montana to eastern Wyoming. Upslope
enhancement will also add to snowfall accumulations across the
windward terrain over the Northern and Central Rockies from
western MT, the Absarokas, Wind Rivers, Big Horns, Black Hills,
and into the Front Range of CO. This is also where the highest WPC probabilities for greater than 6 inches of snow exist. Total
maximum snowfall amounts may exceed 12 inches across the higher
peaks in these ranges. By Monday evening the jet and upper
divergence move east across the Central Plains, which will remove
necessary forcing and leave only light snow across the central
High Plains by Tuesday morning. Nonetheless, strong 700 mb
frontogenesis associated with the aforementioned cold front will
promote snowfall across the central High Plains Monday night. The
timing is important given it is now the second half of April, as
much of the snow across eastern Colorado and Kansas will occur at
night. WPC probabilities for over 4 inches are fairly low (between
10 and 20 percent), likely because the precipitation will be fast
moving and snowfall rates will struggle to top 1 inch/hour. The
peak highest Day 1 plus Day 2 totals are over the WY Bighorns and
north-central Colorado Rockies, as northwest flow leads to well
defined and persistent, long lasting upslope flow. The models
cluster well in the distribution of QPF and snow, so the models
were equally weighted.

...Mid MS Valley to Midwest/Lower Great Lakes...
Days 2/3...

The same shortwave responsible for snowfall across the Rockies and
Central Plains on D1/Monday will race eastward and begin to
amplify on D2/Tue as it crosses the mid MS Valley toward the
Midwest and Lower Great lakes. This feature will be accompanied by
an intensifying jet streak crossing IA, WI, and MI. The
combination of divergence aloft in the right jet entrance region
and convergence along the 700 mb front leads to ascent and snow
across northern Missouri to northern IL, southern lower MI, and
northern IN. An intensifying area of low pressure developing along
the cold front leads to a possibility of an area of several inches
of snow across northern IN and lower MI. Given surface
temperatures will start out above freezing, ground temperatures
well above freezing, and an increasing sun angle, it will take
intense snowfall rates occurring during nighttime hours to produce
hazardous snowfall.

By Tuesday night, a potent band of 850-700 mb frontogenesis will
help promote enhanced ascent and dynamically cool the column and
lead to snowfall rates that result in snow accumulations between
northern Indiana and lower Michigan. The strengthening area of low
pressure and associated precipitation is then forecast to move
northeast and into the Interior Northeast by D3/Wednesday. Snow
will remain possible along the axis of strongest frontogenesis and
to the northwest of the low pressure system, but there remains
high uncertainty regarding the position of the highest snowfall on
D3. Recent model trends have shifted this axis of potentially
heavy snow southeastward, while lowering amounts slightly as well.
The 12z GFS remains the most progressive solution and thus the
farthest southeast with the heavy snow axis, while the 12z ECMWF,
NAM, and other guidance continues to be highlighting an area
toward the northwest. WPC probabilities have low chances for
greater than 4 inches of snow from northern Indiana to lower
Michigan between Tuesday night and Wednesday morning, which
extends into far western New York State by the end of D3.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Snell



$$
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FOUS11 KWBC 192010
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
410 PM EDT Mon Apr 19 2021

Valid 00Z Tue Apr 20 2021 - 00Z Fri Apr 23 2021

...Central Rockies/Central Plains/Mid Mississippi Valley/Great Lakes/Northeast...
Days 1-3...
An amplifying shortwave trough will usher in well-below normal
temperatures across the central and eastern U.S., with
accumulating snows likely from the central Rockies to the
Northeast. Heavy amounts are possible across portions of the
central Rockies, as well as parts of northern New York and New
England.

A shortwave trough currently digging south across the northern
Rockies is expected to drop into the central Rockies Monday
evening. Right-entrance region upper jet forcing along with the
low-to-mid level frontogenesis, and low level upslope flow are
expected to help support moderate to heavy snows developing late
Monday and continuing into the overnight. Latest WPC PWPF shows
high probabilities for snowfall accumulations of 4-inches or more
extending from the Colorado Front Range eastward into the I-25
Corridor, including the Denver Metro. Heaviest amounts are
expected to fall along the Front Range, where the PWPF indicates
that local amounts of 8-inches or more are likely.

This is expected to be a fast-moving system, with the better upper
forcing moving quickly east into the central Plains overnight.
Here too, favorable forcing aloft, along with low-to-mid level
frontogenesis, are expected to contribute to a period of
potentially heavy snowfall. Accumulating snows appear likely
across a large portion of southern Nebraska, and northern and
central Kansas into northwestern Missouri overnight. Strong
low-to-mid level frontogenesis centered over central to
northeastern Kansas overnight is helping to support an increase in
probabilties for accumulations of 4-inches or more across the
region.

This system will continue to move progressively east on Tuesday,
with accumulating snow possible from the lower Missouri and mid
Missippi valleys to Lower Michigan. However, the mid April sun
angle should help limit the threat for widespread heavy
accumulations.

Probabilties for heavier amounts increase some as the system lifts
into the lower Great Lakes region Tuesday night. Then by late
Wednesday, the system is expected to begin to slow, with a closed
low developing over the Northeast by early Thursday. This is
expected to bring a more prolonged period of snow to Upstate New
York and northern New England Wednesday and Thursday.

Accumulations of 4-inches or more are likely across portions of
western New York, the Finger Lakes, and the Tug Hill -- where lake
effect and upslope snow showers following the synoptic snows will
likely bolster amounts. Probabilities for accumulations of
4-inches or more are also high across the Adirondacks and the
northern New England mountains. Locally heavier amounts are
likely within this area, with WPC PWPF showing high probabilies
for storm total amounts of 8-inches more for portions of the
northern Adirondacks and far northern Maine.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Pereira


$$
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FOUS11 KWBC 202017
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
417 PM EDT Tue Apr 20 2021

Valid 00Z Wed Apr 21 2021 - 00Z Sat Apr 24 2021

...Ohio Valley/Great Lakes/Northeast...
Days 1-3...
An amplified shortwave will continue to drive the leading edge of
an unseasonably cold airmass from the Midwest into the eastern
U.S., while supporting late-season accumulating snows from the
Ohio Valley northeastward into the interior Northeast. Heavy
accumulations are possible across portions of Upstate New York and
northern New England.

The upper level shortwave associated with the significant snowfall
that occurred across portions of the central Rockies and Plains
overnight and earlier today is forecast to lift from the Ohio
Valley into the Lower Lakes region overnight. Anomalously cold
air along with low-to-mid level frontogenesis west of a deepening
surface low will support a stripe of light to moderate snows from
the Ohio Valley into the Lower Lakes overnight, with the WPC PWPF
showing that at least an inch or two of accumulating snow is
likely from central Indiana to the shores of lakes Erie and
Ontario. As the shortwave continues to lift north, with a closed
low developing along the U.S./Canada border, snow will shift east
across the interior Northeast on Wednesday. The low is expected
to continue to deepen as it moves tracks into Atlantic Canada on
Thursday, with windy conditions and snow showers expected across
the Northeast on the backside of the departing system. Three day
snowfall totals of 6-inches or more are likely across portion of
western New York, the Finger Lakes and Tug Hill -- where lake
effect following the synoptic snows are expected to bolster
totals. Overall, the heaviest snowfall totals are expected to
occur across the Adirondacks and the New York North Country, as
well as far northern Vermont to Maine, where accumulations of
8-inches or more are likely.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 210805
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Wed Apr 21 2021

Valid 12Z Wed Apr 21 2021 - 12Z Sat Apr 24 2021

...Great Lakes/Northeast...
Days 1-2...

An area of low pressure developing along the cold front in the mid
Appalachians is forecast to move into eastern New York and New
England, intensifying along the way.
Snow is occurring in northwest PA and western NY and this snow
area should move northeast today in the low-to-mid level
frontogenesis maxima west of the deepening surface low.
Locally heavy snow is possible in the NY Adirondacks, northern
Green Mountains, White Mountains, and ranges of western Maine.
East of the low track, snow accumulations get reduced due to warm
advection and a short duration of snow. Lake effect snow follows
the system as northwest winds cross Lake Ontario with less shore
convergence leading to snow showers from Rochester to Syracuse and
along the southeast shore of the lake. Several inches of snow are
expected in these areas.

Low pressure exists Maine on Thursday, with continuing mid level deformation/frontogenesis supporting ascent and potential for
several more inches of snow in northwest Maine.
Overall, the heaviest snowfall totals are expected to occur across
the Adirondacks and the New York North Country, as well as far
northern Vermont to Maine, where day 1 plus day 2 accumulations of
8 inches or more are likely.

...Northern Rockies and Northern High Plains Day 2/3...

The next low-mid level frontogenesis maxima and enhanced pooling
of moisture occurs with the next front moving southeast out of
Alberta across Montana/Idaho on Day 2. Post-front northeast winds
turn upslope in windward locations of the front range/Glacier
National Park south to the Beartooth Mountains of MT. Several
inches of snow are expected in favored windward terrain, with a
low probability of 8 inches forecast. Slightly weaker
frontogenesis maxima and more progressive 700 mb convergence
maxima cross the ID Clearwater Mountains, so most solutions don't
have as much snow potential in ID.
On Day 3, the weak front progresses steadily across the Bighorn
Mountains, with enhanced relative humidity near 90 percent Fri as
the front passes. Lift is provide from the front and also upslope
flow in windward terrain. A modest 4-6 additional inches of snow
are possible there before the front passes to the south and drying
aloft occurs.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 220831
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Thu Apr 22 2021

Valid 12Z Thu Apr 22 2021 - 12Z Sun Apr 25 2021

...Northern Maine...
Day 1...

An area of low pressure will continue to lift northeast away from
New England and into the Canadian Maritimes through this evening.
The potent mid-level closed low associated with this feature will
drift across Maine through the evening, and the combination of
height falls, cold advection, and NW upslope flow will lead to
periods of moderate to heavy snowfall until the column dries out
tonight. While snow showers may accumulate a few inches in
portions of upstate NY and VT/NH, any significant accumulations
should be confined to northern Maine where WPC probabilities for 4
inches are as high as 80%.


...Northern Rockies...
Day 1...

Modest shortwave energy rotating around a larger mid-level arctic
gyre will drop down out of Alberta this morning, plunging a cold
front southward into Wyoming on Friday. Modest height falls will
accompany the shortwave, but briefly intense low-level convergence
aided by a steepening baroclinic gradient/frontogenesis and
intensifying upslope flow behind the front will produce a round of
moderate to heavy snow across parts of ID, MT, and WY on D1. The
heaviest snow is likely where upslope enhancement can occur, and
this is likely on the eastern side of the Northern Rockies,
Absarokas, and Beartooth ranges on D1, shifting into the Big Horns
on D1.5. WPC probabilities for 6 inches are as high as 50% in
these areas.


...Central Rockies...
Day 1...

A weak shortwave traversing the flow from the Four Corners will
race eastward while a lee cyclone in eastern CO drops to the south
through this evening. Modest ascent through height falls, broad
LFQ jet level diffluence, and some upslope ascent on easterly flow
north of the surface wave will produce periods of snowfall in the
CO Rockies and San Juans, with a mid-level omega max depicted in
guidance suggesting the heaviest snowfall will occur in the higher
terrain of the Sawatch and Elk ranges of CO. There, WPC
probabilities are high for 6 inches, with lower accumulations of
less than 4 inches expected elsewhere.


...Pacific Northwest...
Day 3...

A deepening mid-level impulse will amplify into a closed low off
the coast of OR Saturday night as it drops southeast from the Gulf
of Alaska. As this trough sharpens, a downstream jet streak will
intensify, stretching from Northern CA to the Northern Rockies,
with favorable LFQ and RRQ diffluence providing ascent across the
region. Within this jet streak and south of the mid-level closed
low, moist advection will enhance from the Pacific, providing
ample moisture evidenced by PWs climbing to +1 standard deviations
above the climo mean to be wrung out by the deep layer ascent, and
the result will be an increasing area of precipitation from CA to
WA, eastward into MT. Snow levels are expected to rise initially
on D3 as the warm moist air floods inland, but a cold front ahead
of the mid-level wave will help bring down snow levels by the end
of the period. Much of the heaviest precip may hold off until just
beyond this forecast period, but WPC probabilities are moderate
for 6 inches on D3 in portions of the Olympics, Northern Rockies,
Northern Sierra, and Siskiyous, primarily above 4000 ft.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 5 percent.

Weiss

$$
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FOUS11 KWBC 232048
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
447 PM EDT Fri Apr 23 2021

Valid 00Z Sat Apr 24 2021 - 00Z Tue Apr 27 2021

...Pacific Northwest, Northern Rockies, and California...
Days 2-3..

A closed mid-level low dropping southeast across the northeastern
Pacific will swing towards the Pacific Northwest by Sunday morning
before opening and moving inland across northern California Sunday
evening. Ahead of this low and to its south, a direct Pacific
moisture tap will lead to increasing warm and moist advection.
Additionally, a strengthening jet streak will drive moisture
farther onshore. PWs are progged to reach as high as +1 to +1.5
standard deviations above the climo mean in response to the
increased 700-500mb moist flow. IVT within this saturating column
is not forecast to be as robust, with values around 250 kg/m/s
according to the GFS and ECMWF, but a long duration of modest IVT
will still support heavy precipitation expanding across much of
northern California and the Sierra Nevada through the broadening
deep layer ascent. Additionally, orographic lift will add to the
enhanced precipitation rates throughout the Sierra Nevada.

The prolonged WAA will initially drive snow levels up to around
6000ft across much of the West as precipitation overspreads the
region Saturday, but a cold front moving eastward through the
weekend should allow levels to fall to around 4000ft on Sunday.
WPC probabilities on D2 are high for 6 inches in the Sierra Nevada
and some of the higher terrain around the Siskiyous/Trinities of
Northern CA, with maximum amounts over 12 inches possible. By D3,
high elevation snowfall coverage is likely to expand throughout
the Intermountain West as the best forcing spreads across the
region. WPC probabilities for 6 inches are moderate across the
Northern Rockies, as well as the Uinta and Wasatch Mountains of
northern Utah.

...Northern Plains...
Day 2...

A small and quick-hitting stripe of moderate snow is increasingly
likely across portions of the Northern Plains on Sunday morning
from northeast South Dakota to west-central Minnesota. Robust WAA
will drive a band of increasing 700 mb frontogenesis and dynamic
cooling. Uncertainty is still relatively high, as much of the
accumulation will be determined if snowfall rates can overcome
warm surface temperatures. Some guidance, including the GFS, is
hinting at the possiblitity of at least 1"/hr rates on Sunday
morning. The best chances for accumulating snow will be across
northeast South Dakota, where WPC probabilities for 2 inches of
snow have increased to over 60%.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Snell

$$
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FOUS11 KWBC 240822
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Sat Apr 24 2021

Valid 12Z Sat Apr 24 2021 - 12Z Tue Apr 27 2021

...Pacific Northwest, Northern Rockies, and California...
Days 1-3..

A closed 500mb low dropping along the coast of British Columbia
will advect onshore the OR coast Sunday morning while gradually
filling. This trough will then continue to move eastward into the
Great Basin and eventually the Four Corners by the end of the
forecast period. Despite a slow filling of the primary low, the
overall trough will deepen through time becoming a full latitude
trough across the West by Tuesday. As this trough shifts eastward,
a downstream 250mb jet streak is progged to intensify near the
base of the trough. This will act to both increase moisture into
the region as Pacific air floods eastward within the trough, while
also strengthening deep layer ascent through LFQ upper diffluence
and associated divergence maxima. W/SW mid-level flow will also
promote warm and moist advection across the West, and IVT is
forecast to reach 250 kg/m/s according to GEFS and ECENS members.

The combination of increasing column moisture, deep layer ascent,
both of which will be of long duration, is likely to spread rounds
of precipitation across much of the region through Tuesday. While
snow levels will initially be around 6000 ft on the warm
advection, they are expected to lower to around 4000 ft as an
associated cold front works eastward beginning late Sunday. For
D1, the heaviest snow is likely in the higher terrain from the OR
Cascades into the WA Cascades, and eastward to the Northern
Rockies and Absarokas where the best overlap of high mid-level RH
and mid-level divergence align. WPC probabilities on D1 are high
for 6 inches in these regions. By D2, precipitation snowfall
becomes more widespread but sinks slightly SE in response to the
shifting mid-level low. WPC probabilities for 6 inches are high
for the ranges of SW MT, NW WY, central ID, and into OR/northern
CA, with snow amounts in excess of 12 inches likely in the Sierra
where upslope enhancement will occur. By D3, forcing for ascent
shifts eastward, leaving residual high probabilities for 6 inches
confined to the Uintas, Big Horns, and areas around Yellowstone NP.


...Northern Plains...
Days 1-2...

A wave of low pressure moving eastward across the High Plains of
WY will drape a warm front eastward into the Dakotas. As this
front lifts northward Sunday morning, it will be accompanied by
increasing moist advection as low-level southerly flow taps the
Gulf of Mexico, but impressive warm advection along the frontal
boundary. Robust WAA will sharpen the baroclinic gradient
producing a stripe of intense mid-level fgen, aided by an upper
divergence maxima on the LFQ of a modest jet streak. These
features together will produce a narrow band of precipitation
which will likely being as rain, but transition to snow as it
lifts northeast, especially early on D2. A cooling column, both
through wet-bulb and dynamic effects will cause rain to change to
snow, and there is potential for snowfall rates to reach 0.5 to 1
inch per hour as shown by the HREF and WPC snowband probabilities.
These intense rates are likely to cause modest accumulations as
the band shifts northeast. While WPC probabilities for 4" at any
location are less than 5%, briefly heavy snow rates and gusty
winds could produce a few inches of accumulations, including
roadways, early Sunday morning across parts of central ND, eastern
SD, and southern MN. Lesser accumulations are likely further
northeast as the band weakens and April sun increases through the
afternoon.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 242055
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
454 PM EDT Sat Apr 24 2021

Valid 00Z Sun Apr 25 2021 - 00Z Wed Apr 28 2021

...The West...
Days 1-3..

A closed 500mb low shifting southeast off the Pacific Northwest
this afternoon will reach the OR/CA coast Sunday morning and stall
over northern NV through Monday before a reinforcing shortwave
trough shifts the focus to southern CA Tuesday. As this trough
shifts eastward, a downstream 250mb jet streak will intensify near
the base of the trough increasing Pacific moisture and deep layer
ascent from north to south over CA. The combination of increasing
column moisture, deep layer ascent, both of which will be of long
duration, is likely to spread rounds of precipitation across much
of the region through Tuesday. Snow levels in northern CA will
initially be around 5000 ft on the warm advection, but lower to
around 4000 ft tonight as an associated cold front pushes
eastward. Day 1 snow probabilities are moderate for 8 or more
inches along the CA Cascades and northern/central Sierra Nevada
(increasing to high for Day 1.5 on the Sierra Nevada) and low for
additional amounts over the northern Rockies of ID/MT/eastern WA.
The focus shifts inland over the Great Basin and north-central
Rockies on Day 2 with moderate probabilities for 8 or more inches
for the eastern NY ranges, northern UT ranges and northwest WY.
The southeast movement continues with Day 3 probabilities for 8 or
more inches for much of the CO Rockies.


...Northern Plains to Lake Superior...
Days 1-2...

Lee side cyclogenesis downstream of the low approaching the CA/OR
border forms in eastern WY tonight. As this low/front lifts
northward Sunday morning, it will be accompanied by increasing
moist advection as low-level southerly flow taps the Gulf of
Mexico. Strong WAA will sharpen the baroclinic gradient producing
a stripe of low to mid-level fgen that will produce a narrow band
of precipitation. Rain will transition to snow on the northern
side of the precip stripe through wet-bulb and dynamic effects.
There is potential for snowfall rates to reach 0.5 to 1 inch per
hour over northeastern SD and into or through southern MN as shown
by the 12Z HREF and WPC snowband probabilities. These rates are
likely to cause modest accumulations as the band shifts northeast.
Day 1 WPC snow probabilities for 4 or more inches have risen to
30-50 percent along the eastern SD/ND border and over the Coteau
des Prairies in eastern SD down to the Buffalo Ridge in southwest
MN. Briefly heavy snow rates and gusty winds could produce a few
inches of accumulations on roadways in these areas Sunday morning
with lesser accumulations are likely farther east as the band
weakens and during peak heating from the strong April Sunday
afternoon.

A secondary impulse travels along this frontal boundary with
precip blooming again over northern MN late Sunday night. Day 2
snow probabilities are around 10 percent for 4 or more inches in
the Arrowhead.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 250823
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Sun Apr 25 2021

Valid 12Z Sun Apr 25 2021 - 12Z Wed Apr 28 2021

...Ranges of California to the Great Basin and WY/CO Rockies...
Days 1-3..

A slow moving upper low drifting east into OR today and tonight
supports the trough reaching the CA coast and moving onshore
tonight.
As this trough shifts eastward, a downstream 250mb jet streak will
intensify near the base of the trough increasing Pacific moisture
and deep layer ascent from north to south over CA. The combination
of increasing column moisture and deep layer ascent of long
duration, is likely to spread snow across higher elevations of CA
through Monday. Snow levels in northern CA will initially be
around 5000 ft on the warm advection, but lower to around 4000 ft
tonight as an associated cold front pushes eastward. Day 1 snow
probabilities are moderate to high for 8 or more inches along the
CA Cascades and northern/central Sierra Nevada.

Tonight, as the upper level jet moves across NV into UT and
western WY, pre-frontal convergence picks up and induces ascent in
the ranges of central to northeast NV to western WY. As Monday
progresses, the upper jet continues an east drift across the
ranges of northern UT and western WY, with several additional
inches likely as 700 mb convergence maxima combine with the
difulent jet flow to produce ascent. Day 2 has moderate
probabilities for 8 or more inches for the Bighorn range of WY,
northern UT ranges and northwest WY ranges.

As the upper low forms over the southwest Tuesday, the low level
front drifts across southeast Wyoming and CO. The ECMWF has a
slower frontal progression and longer period of return flow to
produce upslope conditions across the northeast CO Plains and
foothills/front range, resulting in heavier QPF and snow potential
than other models.
Given the ECMWF ensmebles were lighter than the operational run,
plus only modest 850 mb moisture fluxes and anomalies, the
operational ECMWF QPF and resultant snow potential appears
overdone. Given much better agreement among the 21z SREF Mean, 12z
ECMWF Ensemble Mean, and 0z GEFS Mean QPF and resultant snow,
these solutions were given more weight in the forecast. Day 3
probabilities for 8 or more inches of snow are low to moderate for
much of the CO Rockies front range.


...Northern Plains to Lake Superior...
Days 1-2...

As a low and associated front lifts northward Sunday morning, it
will be accompanied by increasing moist advection. Strong WAA will
sharpen the baroclinic gradient, producing a stripe of low to
mid-level frontogenesis that will produce a narrow band of
precipitation. Rain will transition to snow on the northern side
of the precip stripe through wet-bulb and dynamic cooling in
northeast SD and adjacent southwest MN. There is potential for
snowfall rates to reach 0.5 to 1 inch per hour over northeastern
SD and southern MN as shown by the HREF probabilities. Briefly
heavy snow rates could produce a few inches of accumulations on
roadways in these areas today with the warm frontal passage
possibly resulting in a change to rain. This keeps the probability
for 4 inches of snow as low.

As the warm front lifts northeast, a cross-lake Superior fetch
picks up moisture fluxes from the lake. Lee shore convergence in
the arrowhead of MN combined with an upslope component of flow
leads to a period of snow late tonight into Monday over northeast
MN. Day 1 snow probabilities are around 10 percent for 4 or more
inches in the Arrowhead of MN.
Day 2 snow probabilities are low, as the warm frontal passage may
allow precip to change over to rain, capping the event potential.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 252042
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
442 PM EDT Sun Apr 25 2021

Valid 00Z Mon Apr 26 2021 - 00Z Thu Apr 29 2021

...Sierra Nevada and Great Basin to the Rockies...
Days 1-3..

An upper low drifts east into the Great Basin tonight as a
reinforcing shortwave trough dives south off the West Coast.
Existing troughing south of the low center continues to pump
Pacific moisture over the Sierra Nevada tonight where heavy snow
will continue to occur above the snow level with drops from 5000ft
to 4000ft overnight and into the Great Basin. As the upper
low/trough drifts east, pre-frontal convergence picks up and
enhances ascent in the ranges of central to northeast NV to
western WY tonight, spreading across northern UT on Monday.
Several additional inches are likely in the Great Basin areas
above the 5000 to 6000ft snow levels as 700 mb convergence maxima
combine with the difulent jet flow to enhance ascent. Day 1 snow
probabilities are moderate for 6 or more inches from south-central
OR terrain south down the CA Cascades and Sierra Nevada across
central and eastern NV ranges, the northern Wasatch in UT, over
southeast ID ranges and northwest WY ranges. These shift east for
Day 1.5 to including southern UT ranges, the Uinta of UT and the
Big Horns of WY.

The reinforcing trough shifts inland over southern CA Tuesday,
promoting a southward shift of the upper low center to southern AZ
by Tuesday night. Snow levels of 6000 to 7000ft over the southern
Intermountain ranges keep Day 2 snow probabilities for 6 or more
inches to the highest terrain of northern AZ, the western San
Juans of CO and the Big Horns of WY. Lee side surface cyclogenesis
over southeast CO on Tuesday allows moisture increases from the
Plains to the CO Rockies by Tuesday night where upper level
difluence promotes heavy snow in north-central CO ranges where Day
2.5 snow probabilities for 6 or more inches are moderate to high.


...Northern Great Lakes...
Days 1-2...

Low level convergence of moist return flow up the Plains and a
surface ridge axis near the MN/Ontario border will promote precip
enhancement late tonight through Monday over northern MN/WI and
the UP of MI. A frontogenetic band of snow is expected to develop
near Lake Superior late tonight where a cross-lake Superior fetch
picks up additional moisture fluxes from the lake. Lee shore
convergence in the arrowhead of MN combined with an upslope
component of flow leads to a period of snow late tonight through
Monday over northeast MN as well as snow over mostly interior
portions of the western UP. Continued low level warm air advection
quickly brings about a warm nose with a switch to sleet and
freezing rain over these areas through the day Monday. Despite
midday April conditions, most guidance maintains wet bulb
temperatures around 30F for the North Shore/Arrowhead and interior
UP meaning freezing rain could continue to accrete. Day 1 snow
probabilities are moderate for 2 or more inches along the North
Shore escarpment and moderate along the central WI/MI border. Day
1 ice probabilities for a tenth or more freezing rain are moderate
over interior sections of the Arrowhead where light freezing
rain/drizzle could continue into Monday evening.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 262020
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Mon Apr 26 2021

Valid 00Z Tue Apr 27 2021 - 00Z Fri Apr 30 2021

...Intermountain West to the Rockies...
Days 1-2..

A closed 500mb low over northern NV will stall there through
Tuesday as a reinforcing shortwave trough currently off the CA/OR
border closes into a low center as it reaches the northern Baja
peninsula late Tuesday and becomes the dominant low center with
this system/trough as it drifts east over the southern tier of the
CONUS through the next several days. The downstream southwesterly
trough reaches its peak intensity (a little over 130kt) over NM
Tuesday night and with persistent mid-level confluence/warm and
moist advection will produce high mid-level RH and PW anomalies
reaching more than +1 standard deviation above the climo mean.
This moisture will continue to be wrung out as it drifts east as
periods of rain and mountain snows (snow level generally 6000 to
7000ft) over northern AZ, southern and eastern UT and into western
CO and up across WY tonight. Lee-size low pressure developing over
eastern CO tonight will put a precip focus on north-central CO for
Tuesday afternoon through Tuesday night before drifting south to
northern NM through Wednesday evening.

Day 1 snow probabilities are moderate to high for 6 or more inches
for the highest peaks of AZ (along with the Kaibab Plateau), the
southern UT ranges, the Uintas of northeast UT, and the Wind
River, southern Absarokas, and Big Horn of WY along with the
western San Juans of CO. Day 1.5 snow probs center on the Front
Range of CO with moderately high probabilities for a foot or more
along the entirety of the CO Front Range. Then on Day 2 moderate
probabilities for 4 or more inches expand south down the Sangre de
Christos into NM.


...Northern Great Lakes...
Day 1...

Surface temperatures will remain around in the Arrowhead of MN and
interior sections of the western UP while southerly 850mb flow
will maintain a warm nose and enough moisture to keep a threat for
continued light freezing rain in pockets of these areas of MN/MI
through this evening where a few hundredths additional ice
accretion is possible after 00Z.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 140832
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
432 AM EDT Sun Apr 14 2024

Valid 12Z Sun Apr 14 2024 - 12Z Wed Apr 17 2024

...Northern California across Great Basin and Colorado Rockies...
Days 1-3...

Occluding cutoff low on the northern CA coast continues to fill
today as it drifts east to Nevada. A shortwave rounding the low
currently lifting up the Sierra Nevada maintains a surface low over
the northern Great Basin today, shifting the heavy snow focus from
the Sierra Nevada north over the Shasta/Siskiyou east through the
Warner Mtns in the far northeast corner of CA today with snow
levels remaining around 5000'. Rates within snowbands here peak
early this morning around 1"/hr with Day 1 PWPF for >6" after 12Z
generally 40-70%.

The low will continue an eastward progression through the Great
Basin into Monday with large scale ascent focused over Utah tonight
into Monday before shifting to the CO Rockies Monday night.
Sufficient Pacific moisture allows for peak 1"/hr rates under the
upper low Monday afternoon over the Wasatch and Uinta mountains of
Utah where Day 2 PWPF for >6" is 40-70% with snow levels rising
from 6500' to 7500'.

As the low reaches western CO Monday afternoon, lee-side
cyclogenesis in northeast CO helps direct Gulf-sourced moisture
streaming up the Plains to be drawn over the central Rockies,
enhancing snow for CO Ranges generally west from the Continental
Divide. Snowfall rates peak late Monday night with Day 2.5 PWPF
for >6" in the 50-80% range with snow levels generally 7500'.
Probabilities for >12" are around 50% for the CO Front Range,
Medicine Bow, and Park Range for Day 2.5.


...Northern Rockies...
Day 3...

A northern stream trough currently over the Gulf of Alaska shifts
southeast to southeastern BC by early Tuesday which promoted an
inverted trough up the northern Plains from ND well into
Saskatchewan. By Tuesday night, the upper trough overspreads MT and
enough moisture from the Plains/Prairies rounds the inverted trough
to allow some upslope snow over the eastern side of the northern
Rockies of MT into northern WY. This air will be cold with snow
levels dropping generally from 3500' to 1500' over MT. Day 3 PWPF
for >6" is 40-60% around Glacier NP as well as the Absarokas north
and east of Yellowstone and the Bighorn Range.

For days 1-3, the probability of significant icing is less than 10
percent.

Jackson

$$
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FOUS11 KWBC 150837
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
437 AM EDT Mon Apr 15 2024

Valid 12Z Mon Apr 15 2024 - 12Z Thu Apr 18 2024

...Utah and Colorado Mountains...
Days 1/2...

Upper low over Utah early this morning will shift east to Colorado
today with 1"/hr snow rates over the length of the Wasatch and
Uinta where Day 1 PWPF for >8" are 50-80% above the snow level that
rises from 6500' to 7500'.

By this evening, the heavy snow focus shifts east to the CO
Rockies thanks to a combination of the favored upper ascent and
aid from lee- side cyclogenesis over northeast CO. Moisture from
the Gulf will funnel into the northern and western CO Rockies
around this low with a primed upslope pattern within the terrain
over northern CO. PWPF for >8" encompasses much of this terrain
above 7500' with the probabilities between 50-80% in the Medicine
Bow and Park Ranges. Probs for >12" are within the aforementioned
area with probabilities of exceedance running between 30-60% with
best chances in the Front Range. Heavy snow in CO tapers to light
through Tuesday morning.

...Northern Rockies...
Days 2-3...

A positively-tilted trough will continue to shift southeast over
BC with downstream/lee-side flow allowing an inverted trough to
persist into Wednesday. The combination of strong surface ridging
providing prominent cold air advection for the season along with
the inverted trough will create a primed upslope enhancement over
the eastern portions of the northern Rockies late Tuesday night
into Wednesday, persisting over northern WY into Thursday. PWPF for

6" is highest on Day 2.5 within the Absarokas over to the

Bighorns with values of 70-90%. In the Lewis Range/Glacier National
snow levels plummet (Falling 3500' to 1500' by Wednesday AM) due
to cold air advection regime and falling heights aloft.
Probabilities for at least 6" are between 40-60% for Glacier
National Park, and generally between 50-90% across those areas over
into Yellowstone over to the Bighorn Range by Tuesday evening.


The probability of significant icing is less than 10 percent.


Jackson


$$
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FOUS11 KWBC 160829
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Tue Apr 16 2024

Valid 12Z Tue Apr 16 2024 - 12Z Fri Apr 19 2024


...Colorado Rockies...
Day 1...

Upper low over eastern CO this morning will shift northeast to
eastern Nebraska today. Heavy snow continues over the northern CO
Rockies this morning with snow levels around 8000ft as lagging
vorticity rotates through, aided by NNWly winds into the terrain.
Expect a few more inches in the highest elevations. Snow will
diminish this afternoon as the upper low pulls away and heights
rise in its wake.



...Northern Rockies...
Days 1-2...

Trough over southern British Columbia digs through Montana this
afternoon through Wednesday. Post cold-frontal snow aided by
northerly upslope flow beneath the left exit of a 120kt Wly jet.
High pressure nosing down the Canadian Rockies tonight will help
increase snowfall and rates up around 1"/hr over favored areas,
like in the Lewis Range/Glacier NP but especially into NW Wyoming
(Yellowstone, Absarokas) and southern MT and the Bighorns. WPC snow probabilities are currently offline, but several inches can be
expected through Wednesday above snow levels that drop tonight to
around 3000ft. Mainly light snow persists Wednesday night through
Thursday night.


The probability of significant icing is less than 10 percent.


Jackson



$$
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FOUS11 KWBC 170715
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
315 AM EDT Wed Apr 17 2024

Valid 12Z Wed Apr 17 2024 - 12Z Sat Apr 20 2024


...Northern Rockies...
Day 1...

A positively-tilted trough over Montana early this morning digs
southeastward into Wyoming into this evening. Post- cold frontal
upslope on surface northerly flow and the left- exit region of a
110kt NWly jet will further add lift over southwest MT and
northern WY. Snow, moderate at times, will continue over the
Absarokas around Yellowstone and the Bighorns through today with an
additional 6-12" at higher elevations. Snow levels in these areas
will drop to around 4000ft today. Later tonight the parent low over
the Canadian Prairies will shift east, moving the northern Rockies
into the left- entrance region of the jet allowing rapid weakening
snow in the Rockies.


...Central Rockies...
Day 3...

The combination of high pressure building down the northern Plains
providing an upsloping easterly component to cold low level flow,
moisture streaming up the southern Plains, and lift in the right
entrance region to a jet stream over the central Plains looks to
allow snow bands to develop over southern WY and/or northern CO
Friday night. As of now these bands may reach moderate intensity
and spill from the mountains onto the High Plains. This will need
further monitoring.


The probability of significant icing is less than 10 percent.


Jackson


$$
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FOUS11 KWBC 180728
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EDT Thu Apr 18 2024

Valid 12Z Thu Apr 18 2024 - 12Z Sun Apr 21 2024

...Central Rockies...
Days 1-2...

A dome of high pressure positioned over western Canada will
gradually inch southward and remain in place through the second
half of the week thanks to an upper level omega block over
northwestern North America. While there will be persistent
upsloping easterly flow into the CO/WY Front Range through Friday
morning, precipitation rates pick up in intensity by Friday
afternoon as an upper level trough tracks into the southwestern
U.S.. Broad 250-500mb PVA over the Central Rockies out ahead of the
upper trough and enhanced upper level divergence beneath the
right-entrance region of a 250mb jet streak aloft will coincide
with a surge in low level easterly flow due to the strengthening
surface pressure gradient. The end result is a burst of heavy snow
over the Front Range with some light-to-moderate snow in the
central High Plains Friday night into early Saturday morning. Snow
may linger into the late morning hours Saturday, but given the time
of year, snow accumulations beyond mid-morning Saturday will be
tough to come by. Any snowfall looks to conclude by Saturday
afternoon. WPC PWPF sports moderate-to-high chances (50-70%) for
snowfall totals >6" at elevations >9,000ft in the Front Range of
the Colorado Rockies. There are moderate chances (40-60%) for
snowfall totals >2" in parts of southeast Wyoming and western
Nebraska, but probabilities quickly drop down to lower chances
(10-30%) for >4" of snow in these same areas, indicating most
totals will be on the lighter side.


The probability of significant icing is less than 10 percent.


Mullinax

$$
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FOUS11 KWBC 190725
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 AM EDT Fri Apr 19 2024

Valid 12Z Fri Apr 19 2024 - 12Z Mon Apr 22 2024

...Central Rockies/High Plains...
Days 1-2...

An expansive dome of high pressure has enveloped much of the Great
Plains and extends as far north as the Canadian Prairies. This area
of high pressure is responsible for an air-mass that is unusually
chilly by mid-April standards and will remain locked in place
through Saturday thanks to an upper level omega block entrenched
over northwestern North America. Later today, an upper level trough
tracking into the southwestern U.S. will generate broad upper level
divergence over the Central Rockies through Saturday morning. In
addition, the region resides beneath the right-entrance region of a
250mb jet streak that will further enhance vertical ascent within
the atmosphere. To top it off, the dome of high pressure in place
coincides with a developing surface low over the Four Corners
region, prompting a strengthening pressure gradient to ensue and
easterly flow into Colorado's Front Range to increase. These
factors will lead to a period of heavy snow along the Front Range
of the Colorado Rockies and perhaps into the higher elevations of
the Palmer Divide Friday evening and into Saturday morning. Some
light snow accumulations may occur over southeast Wyoming,
northeast Colorado (including the Denver metro area) and the
Nebraska Panhandle. Latest WPC PWPF shows high chances (>70%) for

8" of snowfall along the Colorado Front Range at elevations
9,000ft. In the central High Plains (including the Denver metro

area) sport low-to moderate chances (20-50%) for snowfall totals

2" with the more elevated areas (Boulder metro, the Laramie

Range, near Cheyenne) closer to the 40-50% probabilistic range.
Most accumulating snowfall will conclude by midday Saturday as
snowfall rates diminish and the strong mid-April sun angle
aids in rapid melting on all surfaces through Saturday afternoon.

The probability of significant icing is less than 10 percent.

Mullinax


$$
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FOUS11 KWBC 230711
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
311 AM EDT Tue Apr 23 2024

Valid 12Z Tue Apr 23 2024 - 12Z Fri Apr 26 2024

...The West...
Day 3...

A pair of upper level troughs; one in the central Rockies and
another approaching the Pacific Northwest, will produce some high
elevation mountain snow in parts of these regions mountain ranges
Thursday night. The most notable ranges expecting moderate-to-
heavy snowfall include the Olympics, Cascade Range, Blue Mountains,
the Absaroka, Tetons, Wind River, and Colorado Rockies. Elevations
most likely to see the heaviest totals would be >6,000ft in the
Olympics and Cascade Range, >7,000ft in the Blue Mountains, and

9,000ft in the Absaroka, Tetons, Wind River, and Colorado Rockies.

WPC PWPF generally shows low chances (10-30%) for >6" of snowfall
at or above those listed elevations. Look for additional mountain
snow to take shape during the day on Friday. Little in the way of
snowfall in the valleys of the Intermountain West are expected.


The probability of significant icing is less than 10 percent.


Mullinax


$$
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FOUS11 KWBC 240746
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 AM EDT Wed Apr 24 2024

Valid 12Z Wed Apr 24 2024 - 12Z Sat Apr 27 2024

...Northern Maine...
Day 1...

A potent shortwave trough traversing the Northeast today will help
to spawn a wave of low pressure in eastern Quebec this afternoon
while rapid cooling on the western flank of the developing area of
low pressure results in a brief burst of moderate snowfall. Latest
forecast calls for generally 1-3" in northern Maine, but given WPC
PWPF shows low chances (10-30%) of snowfall totals >4" west of
Caribou, localized amounts topping 4" cannot be ruled out. Snow
will quickly end Wednesday evening with an unusually cold day late
April day to follow on Thursday.

...The West...
Days 1-3...

A series of upper level disturbances will carve out a broad
longwave trough across the western U.S. that will be responsible
for some late season mountain snowfall from the Cascades and Sierra
Nevada to as far east as the Colorado Rockies. The initial 500mb
disturbance approaching southern California will be responsible for
a burst of heavy snow over the White Mountains along the CA/NV
border today and into this evening. Mountain snow will then pick
up over the Four Corners region by Thursday afternoon as the same
upper trough moves in overhead. Farther north, the next upper
level trough looks to amplify over the Pacific Northwest Thursday
evening. While the lead disturbance over the Four Corners region
will generate some high elevation snow >9,000ft in the Colorado
Rockies through Thursday night, it is the trough along the West
Coast that will be the primary driver in mountain snow Friday and
into the upcoming weekend. By Friday morning, falling 700-500mb
heights over the Intermountain West and a surge in Pacific moisture
allows for more snow to breakout across many mountain ranges that
include the Blue Mountains, the Sawtooth, Absaroka, Tetons, Wind
River, Big Horns, central Great Basin, and the Uinta. By Friday
night, the heaviest snowfall rates are anticipated in the Absaroka
and Wind River Ranges where 500-700mb winds out of the southeast
will lead to additional upslope enhancement in these ranges. These
ranges will remain favored for the heaviest snowfall into Saturday
morning as the 500mb low tracks over the Colorado Rockies.

WPC PWPF shows high chances (>70%) for snowfall totals >8" in the Sawtooth
of central Idaho, the Absaroka, Wind River Range, and both the
Wasatch and Uinta ranges, all at elevations >9,000ft. The Wind
River Range in western Wyoming sports moderate chances (40-60%) for
snowfall totals >12" through Saturday morning. Note that
additional snowfall is still not over in the Absaroka and the Wind
River Range through Saturday afternoon as the upper trough deepens
over the Four Corners region and spawns a surface low in lee of the
Colorado Front Range on Saturday.


The probability of significant icing is less than 10 percent.

Mullinax

$$
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FOUS11 KWBC 250722
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 AM EDT Thu Apr 25 2024

Valid 12Z Thu Apr 25 2024 - 12Z Sun Apr 28 2024


...The West...
Days 1-3...

Dual amplifying shortwaves within an increasingly anomalous trough
developing across the Western CONUS will result in widespread
late-season heavy snow across much of the terrain.

The first of these will be a southern stream impulse lifting into
the southern CA coast to start the period /Thursday morning/. This
feature will lift quickly northeast through the Four Corners
before closing off over the Central Rockies/Central High Plains
Friday morning. This strengthening low will spawn a surface low in
the lee of the Rockies with precipitation spreading into the High
Plains, but the column appears too warm for any wintry
precipitation with this first wave. However, the synoptic evolution
of this first impulse will cause large scale height falls across
the West, noted by NAEFS 700-500mb height anomalies reaching -1
sigma across much of the area by Friday aftn.

As the longwave trough amplifies behind the first shortwave, a more
impressive feature will dig from the Pacific Northwest towards the
southern Great Basin Friday aftn, and this will also close off at
500mb over the Four Corners during Saturday. This will additional
lower heights within the trough over the West, while producing
enhanced synoptic lift through height falls, PVA as lobes of
vorticity shed around it, and increasing upper diffluence as the
subtropical jet arcs poleward leaving the favorable LFQ overhead.
As this feature lifts again into the Central Rockies/Central
Plains, following a similar path to the first, it will tap into
impressive moisture noted by PW anomalies reaching +1 to +2 sigma
across the Central Rockies, resulting in expanding precipitation,
with heavy snow likely shifting eastward from the Northern Rockies
and northern Great Basin into the Central Rockies.

At the same time, dual surface fronts will be digging through the
Northern High Plains and into the Central Rockies, providing
additional ascent through upslope flow and periods of enhanced
fgen, potentially overlapping a deformation axis in the Central
Rockies, to produce heavier snowfall rates that may reach 1"/hr at
times. Snow levels will remain generally elevated, primarily above
7000-8000 ft, but with steepened lapse rates beneath the cold
trough and some areas of enhanced ascent, snow is possible into
much lower elevations.

For D1, the focus of the heaviest snow will be in the OR Cascades
where WPC probabilities for more than 6 inches reach 60-80%.
However, much of the period will be dominated by snow across the
Intermountain West D2 and D3. During this time, WPC probabilities
reach above 80% for 6+ inches D2 in the Absarokas, Wind Rivers,
Uintas, Wasatch, and into the Front Range, then focusing in the
Front Range, San Juans, Park Range, and Mosquito Range D3. While
there is some uncertainty into how far west into the Front Range
the heaviest snow will fall, locally 2-3 feet appears likely in the
higher peaks.

The probability of significant icing is less than 10 percent.


Weiss


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