FOUS11 KWBC 190719
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 AM EDT Wed Mar 19 2025

Valid 00Z Wed Mar 19 2025 - 00Z Sat Mar 22 2025

...Central High Plains through the Great Lakes... Days 1-2...

...Significant storm system to bring blizzard conditions to
portions of the Central Plains and Upper Midwest. Key Messages are
being issued for this system...

The low-pressure which has been progged for several days now to
deepen in the Central High Plains is taking shape this morning
noted by pressure falls and increasing convective elements across
CO/KS/NE. The parent PV anomaly is clearly noted in satellite
imagery across far SE CO and into the OK Panhandle, which is
generally S/SW of all available guidance. This suggests that the
E/SE trend that began 24 hours ago is valid, and the event may end
up even farther SE than the 00z suite of models. While the exact
track will be critical to the placement of the most significant
impacts related to this blizzard, a large scale significant event
is still likely from the Central High Plains northeast into the Great Lakes.

As this low deepens today in response to favorable synoptic and
mesoscale ascent, precipitation will expand downstream. Current WV
imagery shows both the low and mid level moisture streaming into
the developing cyclone, and where isentropic ascent lifts into the
intensifying deformation axis, impressive omega will squeeze out
all available moisture in the form of heavy rain and snow. The
deformation axis on the NW side of this system will likely be quite
intense, and will be overlapped by a cyclonically surging TROWAL to
enhance both ascent and instability. Where this occurs, any rain
will quickly change to snow through dynamic cooling, and then come
down extremely heavily, reaching 2-3"/hr at times where CI
(thundersnow) can occur. This is supported by HREF 1"/hr and 2"/hr probabilities that reach 90% and 70%, respectively, and by the WPC
prototype snowband tool that suggests locally 3"/hr is possible (20%).

These snowfall rates will quickly overwhelm hostile antecedent
conditions (some places were near 80 degrees Tuesday and will get
heavy snow today!), and WPC probabilities have increased for
significant snowfall despite the SE shift. The heaviest
accumulations are likely from central NE through NW IA where the
environment supports a pivoting deformation band, and WPC
probabilities are high (>70%) for at least 8 inches. Here, locally
12-15 inches is possible (30%) in some locations. Farther
downstream, the band will begin to translate more quickly, limiting
the duration of these intense rates, but still heavy snowfall is
likely as reflected by WPC probabilities that are high (>70%) for
6+ inches from SE Minnesota through the central U.P. of Michigan.
Additionally, strong winds of 50 mph or more will accompany this
snowfall, so despite SLRs that will generally be below climo, and
will start as a heavy/wet snow, as SLRs climb and aggregates
fracture, blizzard conditions are expected which will make travel
almost impossible in some areas.

Another interesting development that has shown up tonight is for a
secondary deformation axis to potentially develop in the vicinity
of the primary PV anomaly behind the surface low, bringing some
heavy snow rates of around 1"/hr from SE Iowa to the Chicago metro
area. While this is not expected to be as impressive as the axis to
the NW, moderate snowfall accumulations for which WPC
probabilities indicate have low chance (10-30%) of exceeding 4
inches, which when combined with gusty winds could still produce
moderate travel impacts.

This entire system exits quickly on Thursday, with some residual
deformation snow pivoting across lower Michigan, and some northern
flow in its wake leading to lake enhanced snowfall across the U.P.
and southeast of Lake Michigan. Still, additional snowfall should
be modest as reflected by WPC probabilities that are just around
10% for an additional 2 inches of snow in these areas.

...Pacific and Interior Northwest... Days 1-3...

The seemingly never-ending training of shortwaves and moisture
continues this week across the Northwest CONUS.

The first of these will be the most potent, as a strong shortwave
trough pushes onshore this evening with a subtle negative tilt.
Impressive height falls ahead of this trough will drive ascent,
aided by at least weak LFQ diffluence as a streak pivots through
the base of this trough. Together, this will produce deep layer
lift from the northern Sierra through the Cascades and into the
Interior Northwest, supporting heavy precipitation as IVT surges
northeast on downstream S/SW flow. Snow levels will gradually rise
within the IVT core due to accompanying WAA, before falling again
with the passage of a cold front later D1. Despite this, snow
levels will generally be quite low through the period as the region
remains cold, rising from around 2500 ft to 3500 ft, before
crashing again to 2000-2500 ft behind the cold front. This suggests
that snow will impact many of the Cascades and northern CA Passes,
as well as into the interior NW causing hazardous travel. This
first system is progressive, however, and WPC probabilities peak
around 70% for 6+ inches in the highest peaks of the WA Cascades
and Olympics, with generally 4 inches or less forecast elsewhere.

As this first trough shifts eastward, forcing will weaken across
the Northwest, but flow across the Pacific becomes increasingly
zonal due to squeezing between broad ridging well west off Baja and
a modest trough across the Gulf of Alaska. This zonal flow will
maintain onshore moisture advection to support rounds of
precipitation into the Northwest, with periodic vorticity lobes
swinging eastward (although maintaining latitude generally north of
OR) to enhance ascent. Snow levels D2-3 will fluctuate generally
between 2500ft and 4500ft, with rounds of precipitation leading to
periods of heavy snowfall. WPC probabilities both D2 and D3 are
similar in magnitude and areal extent, reaching 70% or more for 4+
inches from the Olympics, to the WA/OR Cascades, eastward through
the Salmon River/Sawtooth Ranges, and into the Northern Rockies.
2-day snowfall in some of the higher elevations could reach 2-4
feet, with up to 1 foot at the higher passes of the Cascades.

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

Shortwave energy moving into the Mid-Atlantic will cause a split
jet stream (one rotating across the Carolinas while another pivots
towards Greenland) resulting in a modestly coupled structure to
help deepen a surface low south of New England beginning Thursday
night. The guidance has trended maybe just a bit weaker with this
low the past 24-hrs, but ensemble low points still agree in a
deepening system to around 990mb tracking from norther NJ to Cape
Cod and then into the Gulf of Maine by Saturday morning.
Uncertainty in the intensity and track remains, but spread is
relatively minimal for this time range, and deformation developing
on the NW side of this low is still likely to result in an axis of
rain changing to heavy snow. The GFS remains the deepest solution
and is somewhat discounted based on recent upstream performance,
but all the guidance indicates that an axis of sloped 850-700mb
fgen will drive omega into the deepening DGZ early Friday through
Friday night, which will dynamically cool the column to produce an
axis of heavy snowfall rates. The antecedent conditions are not
ideal for snow accumulations due to warm temperatures and rain, but
higher elevations of the interior Northeast from Upstate NY
through Maine could still see impactful accumulating snow.
Confidence is low, and trends for this event need to be monitored
over the next few model cycles, but WPC probabilities are modest at
just 10-30% for 4+ inches of snow in the higher elevations.

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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