FOUS11 KWBC 052020
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 PM EST Sun Jan 5 2025

Valid 12Z Sun Jan 05 2025 - 12Z Wed Jan 08 2025

...Central Plains through the Mid-Atlantic... Days 1-2...

...Major winter storm continues from Central Plains and Ohio
Valley today through the Mid-Atlantic...

The major winter storm which has been advertised for several days
now is ongoing to start the forecast period. This system is being
forced by a closed 500mb low which will be positioned over Kansas
City, MO to start the period. This closed low will move
progressively east, reaching Indianapolis, IN by 12Z Monday, and
then northern VA at the start of D2 /00Z Tuesday/. During the
latter half of D1, however, this feature is progged to open into a
wave, while still potent, but with subtly less downstream ascent.
At the same time, however, a potent jet streak rotating around the
base of the trough will amplify towards 150 kts, helping to offset
some of the loss of deep layer lift resulting from the opening of
the trough. At the surface, the accompanying low pressure will
skirt east across the lower OH VLY/TN VLY, with secondary
development likely occurring across the Carolinas and then pressing
east off the Mid-Atlantic Coast by Tuesday morning.

Together, this will result in a large swath of heavy
precipitation, with all p-types likely. Downstream of the primary
surface low, strong isentropic ascent along the 290K-295K surface
will draw impressive moisture northward into the system (NAEFS PW
above the 97th climatological percentile) supporting an expansion
of widespread precipitation. At the same time, the accompanying
theta-e ridge rotating cyclonically into the system will lift into
a TROWAL, especially early in the period, before the upper low
opens and the low occludes to the secondary development. Despite
that, some enhanced elevated instability beneath this TROWAL will
support heavy precipitation rates, leading to widespread
significant precipitation amounts of snow, sleet, and freezing
rain. While confidence in heavy precipitation causing impacts is
high, the exactly placement of the transition zone and associated
ice/snow amounts remains uncertain.

There continues to be two primary camps with the track of this
low. THe GFS/NAM/high-res members remain a bit north of the
CMC/ECMWF. While latitudinal spread has decreased, it is still
significant in terms of where the transition zone will occur.
However, regardless of the exact track, a stripe of very heavy snow
(rates 1-2"/hr) is likely within the WAA, and the conceptual model
for a laterally translating band from west to east fits this
system. This suggests a narrow but intense band of snow, supported
by both NBM and DESI probabilities, as well as the WPC snowband
tool which indicates a high potential for 1-2"/hr snow rates from
northern MO eastward to MD and DE. Since this band will be
translating along its long/axis (more eastward motion than north)
some locally very high snow totals are possible as reflected by
10-30% chance for 12+ inches of snow from northern WV eastward
through southern DE, but in general WPC probabilities support a
high (>70%) risk for 6+ inches along this same axis with a sharp
N/S gradient on both sides.

Some of this snow will be additionally enhanced by the post-system
comma head/deformation which will pivot across the Mid-Atlantic
Monday night. The guidance continues to trend a bit stronger with
this feature, and while latitudinal placement of the most
significant forcing remains quite uncertain within this axis,
renewed moderate to heavy snow, with much fluffier SLRs than
earlier, will likely regenerate across the region adding up to a
few more inches before exiting by Tuesday morning, but WPC
probabilities after 00Z Tuesday for additional snow accumulations
exceeding 2" are just around 10%, highest around the Chesapeake Bay.

South of the heavy snow area, this system will also pose a
significant to damaging ice threat within the warm nose/p-type
transition zone. Intense WAA over-topping the cold surface layer
accompanied by persistent E/NE dry-bulb advection is a classic
setup for damaging freezing rain. Although there is some
uncertainty into how efficiently the ice can accrete at times due
to what should be intense precipitation rates, the threat for
damaging ice remains from central KY into the southern/central
Appalachians and Foothills where WPC probabilities for 0.25"+ are
50-90% across northern KY and until WV/VA. The greatest risk for
damaging ice however continues to focus across eastern KY where a
10-30% chance exists for 0.5" of ice. This could result in downed
limbs and power lines leading to widespread long-lasting power
outages and impossible travel.

Finally, between the heavy snow and heavy icing, the forecast
profiles indicate a narrow stripe where the p-type may remain sleet
for a considerable temporal duration. This is due both to a subtly
weaker surge of the warm nose as well as a deeper sub-warm- nose
cold layer which will allow refreezing of droplets. At the same
time, cold/dry wet-bulb advection on E/NE winds will maintain this
cold layer effectively to prevent a more rapid p-type transition.
Heavy accumulations of sleet are difficult to achieve, but in this
setup it is possible that up to 1" of sleet will accumulate causing
tremendous driving impacts. The greatest risk for the significant
sleet accumulations D1 extend west to east from northern KY through
southern VA.

This system has prompted the issuance of key messages which are
linked at the bottom of the discussion.

...Great Lakes and Northeast... Days 2-3...

Behind the large system moving off the Mid-Atlantic coast Tuesday
morning, a push of arctic air will dig into the Great Lakes and
Northeast, fueled initially by a potent shortwave dropping nearly
due south from Ontario Tuesday morning. 850mb temps are progged to
crash to -15C to -20C, steepening lapse rates considerably, and
resulting in a favorable setup for both lake effect snow (LES) and
upslope snow into the upwind terrain of the Northeast.

The greatest potential for upslope snow will occur D2 /Tuesday/ as
the potent shortwave dives south pulling a cold front with it.
This will mark the leading edge of the coldest air, and cause
impressive NW flow into a moistening column. The temperatures
during this time will be very cold, so the DGZ will be quite low,
resulting in effective upslope into the snow growth region. Light
and fluffy snow will efficient accumulations, so despite a short
duration, WPC probabilities for 4+ inches of snow are moderate
(50-70%), highest in the NW Adirondacks and N Greens.

Although some heavy LES may occur downstream of Lake Michigan
early D2, the greater coverage and intensity of LES is likely D3
behind this front. GLERL lake-surface temperatures are still
sampled to be +5C to +8C, so the very cold air moving overhead will
support lake-induced instability to drive LES. At this time the
LES appears less intense as the last round, but WPC probabilities
D3 feature a low risk (10-30%) for 4+ inches of snow in the favored
N to NW snow belts.

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

Pacific shortwave will enter Oregon later this evening with
another surge in moisture to the OR Cascades into the northern
Great Basin as it weakens and slips southeastward into Nevada
overnight. High initial snow levels around 6000-7000ft over the OR
Cascades only drop to around 5000-6000ft under the trough axis,
restricting significant snow to the highest OR Cascades well above
pass level. Snow levels will be closer to 5000ft over the Great
Basin and 4000ft over the northern Rockies, where several inches of
snow are likely D1 over northern NV, southeastern ID, western
WY/southwestern MT and then into the CO Rockies. WPC probabilities
for at least 4" of snow are >50% in these mountainous regions.

Into D2, height falls (and trailing height falls) will rotate
through the Southwest and sharpen, eventually closing off over the
lower CO River Valley late Tue. With high pressure nosing down out
of the northern Plains, upslope snow will expand across the CO
Rockies into the Front Range via an easterly low level flow. The
upper low will likely move into northwestern Mexico by the end of
the period, helping to drag the snowfall southward as well through
the Mogollon Rim, Sangre de Cristos, Sacramento Mountains, etc. For
the D2-3 period, WPC probabilities for at least 4 inches of snow
are 20-50% over the Front Range into the CO Rockies, San Juans, and
higher elevations in AZ/NM above 8000ft or so.

Weiss/Fracasso/Snell

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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FOUS11 KWBC 060759
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Mon Jan 6 2025

Valid 12Z Mon Jan 06 2025 - 12Z Thu Jan 09 2025

...Ohio Valley through the Mid-Atlantic... Day 1...

...Major winter storm continues from the Ohio Valley through the Mid-Atlantic...

The major ongoing winter storm which has already dumped over a foot
of snow from central KS to northern MO is forecast to rapidly slide
eastward today while producing additional areas of heavy snow and
freezing rain between the Ohio Valley and Mid-Atlantic. This
system is being forced by a closed 500mb low which will be
positioned over the IL-IN border to start the period. This closed
low will weaken into a potent open wave today while pushing due
east toward the Mid-Atlantic by this evening. The weakening is
being aided by a separate stronger upper low diving southward
towards New England and helping shear out the storm system as it
exits the East Coast. At the same time, however, a potent jet
streak rotating around the base of the trough will amplify towards
150 kts, helping to offset some of the loss of deep layer lift
resulting from the opening of the trough. At the surface, the
accompanying low pressure will skirt east across the lower OH
VLY/TN VLY, with secondary development likely occurring across the
Carolinas and then pressing east off the Mid- Atlantic Coast by
Tuesday morning.

Together, this will result in a large swath of heavy
precipitation, with all p-types likely (at least at the beginning
of the forecast period). Downstream of the primary surface low,
strong isentropic ascent along the 290K-295K surface will draw
impressive moisture northward into the system (NAEFS PW above the
90th climatological percentile) supporting the expansiveness of
widespread precipitation.

A stripe of very heavy snow (rates 1-2"/hr) is likely within the
WAA during the first half of D1, and the conceptual model for a
laterally translating band from west to east fits this system. This
suggests a narrow but intense band of snow, supported by both NBM
and DESI probabilities, as well as the WPC snowband tool which
indicates a high potential for 1-2"/hr snow rates from northern VA
and MD eastward to DE following the band weakening across southern
OH right before 12z. Since this band will be translating along its
long/axis (more eastward motion than north) some locally very high
snow totals are possible as reflected by 40-50% chance for an
additional 8+ inches of snow from northern VA eastward through
southern DE, but in general WPC probabilities support a high (>70%)
risk for 4+ inches along this same axis with a sharp N/S gradient
on both sides. These similar probabilities are also found in
western MD and WV where an upslope component following the passage
of the low pressure system.

Some of this snow will be additionally enhanced by the post-system
comma head/deformation which will pivot across the Mid-Atlantic
Monday night. The guidance continues to trend a bit stronger with
this feature, and while latitudinal placement of the most
significant forcing remains quite uncertain within this axis,
renewed moderate to heavy snow, with much fluffier SLRs than
earlier, will likely regenerate across the region adding up to a
few more inches before exiting by Tuesday morning.

South of the heavy snow area, this system will also pose a
continuing ice threat within the warm nose/p-type transition
zone. Damaging ice accretion of 0.5-0.75" has already been realized
along a stretch spanning from eastern KS to KY, but thankfully
precipitation will be coming to an end across this area as light
snow/freezing rain pushes eastward today on the backside of the
low. Intense WAA over- topping the cold surface layer will continue
this morning across parts of the southern Mid-Atlantic, however.
WPC probabilities for at least an additional 0.1" beginning 12z
today fall between 10-40% across southern VA.

This system has prompted the issuance of key messages which are
linked at the bottom of the discussion.

...Great Lakes and Northeast... Days 2-3...

Behind the large system moving off the Mid-Atlantic coast Tuesday
morning, a push of arctic air will dig into the Great Lakes and
Northeast, fueled initially by a potent shortwave dropping nearly
due south from Ontario Tuesday morning. 850mb temps are progged to
crash to -15C to -20C, steepening lapse rates considerably, and
resulting in a favorable setup for both lake effect snow (LES) and
upslope snow into the upwind terrain of the Northeast.

The greatest potential for upslope snow will occur D2 /Tuesday/ as
the potent shortwave dives south pulling a cold front with it.
This will mark the leading edge of the coldest air, and cause
impressive NW flow into a moistening column. The temperatures
during this time will be very cold, so the DGZ will be quite low,
resulting in effective upslope into the snow growth region. Light
and fluffy snow will efficient accumulations, so despite a short
duration, WPC probabilities for 4+ inches of snow are moderate
(50-70%), highest in the NW Adirondacks and N Greens.

Although some heavy LES may occur downstream of Lake Michigan
early D2, the greater coverage and intensity of LES is likely D3
behind this front. GLERL lake-surface temperatures are still
sampled to be +5C to +8C, so the very cold air moving overhead will
support lake-induced instability to drive LES. At this time the
LES appears less intense as the last round, but WPC probabilities
D3 feature a low risk (10-30%) for 4+ inches of snow in the favored
N to NW snow belts.

...Intermountain West, Rockies, and Central High Plains.... Days 1-3...

Pacific shortwave diving across the Intermountain West today will
surge moisture ahead of it and feature favorable divergence to
promote scattered high elevation upslope snow spreading southward
across the central Great Basin and Central Rockies today before
reaching the Southern Rockies late D2/early D3. Several inches of
snow are likely D1 over northern NV, southeastern ID, western
WY/southwestern MT and then into the CO Rockies. WPC probabilities
for at least 4" of snow are >50% in these mountainous regions.

Into D2, height falls (and trailing height falls) will rotate
through the Southwest and sharpen, eventually closing off over the
lower CO River Valley late Tue. With high pressure nosing down out
of the northern Plains, upslope snow will expand across the CO Front
Range via an easterly low level flow along with a relatively low
DGZ. The upper low will likely move into northwestern Mexico by the
end of the period, helping to drag the snowfall southward as well
through the Mogollon Rim, Sangre de Cristos, Sacramento Mountains,
etc. For the D2-3 period, WPC probabilities for at least 4 inches
of snow are 20-50% over the Front Range into the CO Rockies, San
Juans, and higher elevations in AZ/NM above 8000ft or so.

...Southern Plains... Day 3...

As the aforementioned upper low closes off over the Southwest late
D2 it continues to dig into northwestern Mexico D3 and become
anomalously deep (below the 0.5th climate percentile per the 12z
GEFS). Meanwhile, a separate shortwave rounding the far eastern
Pacific ridge is forecast to dive southward and interact with this
feature and spawn strong southwest flow containing rich moisture
and prominent upper divergence. Given the strong high situated to
the north over the Central Plains and cold the low-level airmass in
place, snow and mixed precipitation is expected over much of
western and north-central TX. Currently, WPC probabilities for at
least 2 inches are low (10-30%) through Thursday morning but these
may trend up if precipitation start time trends faster. This storm
will need to be monitored as 00z ensembles depict heavier snowfall
impacting parts of the Southern Plains on D4.

Snell

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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FOUS11 KWBC 021904
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
304 PM EDT Wed Apr 2 2025

Valid 00Z Thu Apr 03 2025 - 00Z Sun Apr 06 2025


...Northern Plains & Upper Midwest... Day 1...

...Major winter storm continues to bring heavy snow and gusty winds
through Thursday afternoon. Key Messages have been issued for this
system and are linked below...

The system which is expected to bring a late season major winter
storm to the Dakotas and Minnesota is organizing this morning as
reflected by an increase in reflectivity across the region in the
vicinity of increasing 925-700mb PVU. This system will deepen today
in response to a mid-level closed low deepening as it moves across
Minnesota and into Ontario by Thursday morning. This low will
become quite intense, reflected by NAEFS 850-700mb height
anomalies falling below the 1st percentile in the CFSR database,
with similar 500mb anomalies dropping below 2.5 percentile. This is
reflective of a strong system, with surface low deepening
encouraged by the robust height falls and concurrent/overlapping
upper diffluence as a strengthening jet streak pivots poleward
downstream of the mid-level low. The guidance has come into better
agreement today with the track of this surface low, and it is
expected to track from near Minneapolis this aftn through the
western U.P. of MI by Thursday morning.

This evolution will have a two-pronged impact on the winter weather
impacts. First, this slightly more northern track will help push a
dry slot and a warm nose northward leading to an axis of light
sleet and freezing rain, but accumulations are expected to be minimal.

More significantly, NW of the surface low, a potent deformation
axis is progged to develop and pivot from eastern ND/SD through
north-central MN. This deformation axis will occur concurrently
with an intensifying TROWAL as theta-e advection on moist
isentropic ascent increases and wraps cyclonically around the
system. The TROWAL will be most impressive from generally 18Z today
through 06Z Thursday across N-Central MN, and cross-sections within
this region suggest a threat for CSI to enhance snowfall rates.
This is additionally supported by both the WPC prototype snowband
tool and HREF snowfall rate probabilities which indicate a moderate
to high risk (50-80%) for 1+"/hr rates, with locally 2"/hr rates
possible. This will overwhelm antecedent warmth and the April sun,
leading to rapid snowfall accumulation beneath this deformation,
and WPC probabilities are moderate (30-50%) for an additional 4+
inches after 00Z, with storm total snowfall of 6-10" in some areas.
While there is still some uncertainty into the exact placement of
this deformation, and the gradient on the NW and SE side should be
pretty significant, this band of heavy snow will be impactful
before the entire system exits to the northeast by late Thursday
morning/early Thursday aftn.

...Northeast... Days 1 & 3...

The same system that will be producing heavy snow across the Upper
Midwest D1 will occlude to a secondary low and attendant warm
front farther east. This warm front will lift northeast into
Upstate NY and New England tonight into Thursday, bringing a period
of heavy precipitation through Thursday aftn. The front will lift
rapidly northward, so the duration of precipitation will be
limited, and p-types will vary widely as the warm nose causes a
change from snow, to sleet, to freezing rain, and eventually rain
before ending (likely ending as freezing rain the highest terrain
of NH and northern ME). Despite this, the multitude of
precipitation types will create hazardous conditions, especially in
the higher terrain from the Adirondacks through Vermont, New
Hampshire, and Maine where WSSI-P probabilities indicate a 10-30%
chance for moderate level impacts.

The impressive fgen along this front should result in periods of
heavy precipitation rates as well, regardless of the p-type. This
could result in a few inches of snow and sleet, before changing to
freezing rain. Total accumulations will be generally modest, but
WPC probabilities indicate a high risk (>70%) for at least 4 inches
of snow in the highest terrain of NH and ME, and a 50-70% chance
for at least 0.1" of ice in the Adirondacks and portions of
NH/VT/ME as well, ending by 00Z Friday.

Another round of mixed precipitation is likely as moisture funnels
northward on return flow through the Mississippi Valley and into
New England Saturday. At this time any significant wintry
precipitation accumulations are expected to be confined to the
highest terrain of northern New England, with a mix of snow and
freezing rain likely. Total accumulations are expected to be modest
however, as reflected by WPC probabilities for 0.01" of ice that
are generally just 10-30%.

...Intermountain West... Days 1-3...

Expansive mid-level trough will remain entrenched across much of
the West through the weekend, but evolution of embedded shortwaves
will lead to amplification and wavelength shortening by Friday.
Before this occurs, generally modest ascent and modest moisture
will lead to widespread but light snow across much of the terrain
from the Northern Rockies southward through the Four Corners
states. Within this broad ascent, there is likely to be two areas
of more consolidated ascent and heavier snowfall D1: the Northern
Rockies and from the Wasatch to the San Juans. In the Northern
Rockies, a cold front sagging southward will cause some enhanced
fgen and post-frontal upslope on E/NE winds. This will more
efficiently wring out available moisture leading to heavy snow in
the vicinity of Glacier NP where WPC probabilities are high (>70%)
for at least 6 inches of snow. Additionally on D1, a subtle
shortwave ejecting from the southern stream will work together with
downstream mid- level divergence and some upslope ascent to
increase snowfall from the Wasatch into the San Juans where WPC
probabilities are 70-90% for 4+ inches of snow.

More widespread, generally light, snow occurs across the
Intermountain West terrain on D2, but some focused heavier snowfall
is likely in the vicinity of the MT/WY border, including the
Absarokas, Wind Rivers, and in the vicinity of Yellowstone NP where
the sinking cold front helps to enhance snowfall. WPC probabilities
in this region reach as high as 30-50% for 6+ inches of snow.

Then late D2 into D3, the amplifying trough and concurrent closing
off of an upper low across the Four Corners will yield much more
significant snow across the Southern Rockies and into Colorado.
While there is still uncertainty as to how this system will evolve,
and trends have been for a slightly farther south track, confidence
is increasing that heavy snow will become widespread across
northern NM and into CO. This will be in response to increasing
synoptic ascent driven by both downstream divergence and increasing fgen/upslope flow as the cold front continues to sag southward.
This will additionally cause isentropic lift to surge moisture
northward, and as this pivots back to the west it will yield an
expanding area of heavy snowfall, especially in the terrain from
the Front Range, to the Palmer Divide, the Raton Mesa, and the
Sangre de Cristos. Additionally, as snow levels crash behind the
front, impactful snow may spread into the High Plains including the
urban I-25 corridor, with at least moderate snow potentially
pivoting into the TX/OK Panhandles late in the forecast period and
into D4. At this time, WPC probabilities begin to rise late D2,
reaching 30-50% for 4+ inches along the Front Range, before
expanding and increasing D3 to 70-90% from the San Juans to the
Raton Mesa and along the Sangre de Cristos. Additionally, with
snowfall expanding and snow levels falling, WPC probabilities
indicate a low risk (10-30%) for at least 4 inches as far east as
the western TX/OK Panhandles and across the northeast NM High Plains.

Weiss

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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