FOUS11 KWBC 070818
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
318 AM EST Fri Feb 7 2025

Valid 12Z Fri Feb 07 2025 - 12Z Mon Feb 10 2025

...Pacific Coast through the Northern/Central Rockies... Days 1-2...

Active period of winter weather continues over the next few days
throughout the West as a potent area of low pressure progresses
across the Intermountain West and Rockies through early Saturday, 
while onshore flow continues throughout the Pacific Northwest in 
concert with a cold airmass in place.

Low pressure currently analyzed entering the OR High Desert early
this morning is expected to track into ID today and drag along IVT
up to the 99th climatological percentile within areas of snowfall
per the 12z NAEFS. The northern precipitation shield extending
across OR, ID, and the northern Rockies will be mainly associated
with strong mid-level WAA through tonight before the upper trough
begins to shear within fast-zonal flow across the north-central
United States. Snow levels are expected to be around 3,000-5,000ft
throughout central ID, western WY and far southwestern MT, with
much lower snow levels just to the north within the arctic airmass
intruding from western Canada. Precipitation is also ongoing across
the Sierra Nevada this morning, but should come to an end by this
afternoon. WPC probabilities for at least 6 inches are high (>70%)
on D1 across the Sawtooths of ID and from the Wasatch Mts of UT 
through the western ID and southwest MT ranges. These high probs 
also extend into the northern CO ranges on D2 as upslope flow
enhances with the approaching cold front.

Elsewhere, weak area of low pressure moving onshore the PacNW early
D2 is expected to spread light snow and snow showers throughout the
region where deep cold air remains in place. This results in snow
levels outside of the immediate coast under a few hundred feet. WPC
probabilities D1-D2 for at least 4 inches of snow are high 
(60-90%) across the Cascades.

...Northern Plains through the Great Lakes and Ohio Valley... Days 1-2...

As the potent shortwave exits the Northern Rockies Friday evening 
and begins to race east within flat/progressive flow entrenched 
across the CONUS, a west-east oriented band of snowfall is forecast
to spread across the north-central United States. This shortwave 
trough will remain positively tilted through its evolution across 
the region and has trended flatter/weaker over the last few
forecast cycles, but still falls within a favorable jet streak
environment and associated PVA to produce impressive omega. This deep
layer lift as well as enhanced IVT (progged to exceed the 90th 
percentile of the CFSR climatology into the Plains/Upper Midwest).

This moisture will be additionally enhanced by a surge of
WAA/isentropic ascent, especially along the 285-295K surfaces,
downstream of the parent shortwave. At the same time, the DGZ is 
progged to deepen considerably in response to this WAA, which is 
reflected by SREF DGZ 100mb depth probabilities reaching 50% from 
the SD-ND border through central MN and into central WI. With the 
column being quite cold, this could result in fluffy and efficient 
dendrites that can accumulate rapidly. However, it should be noted 
that the best fgen lies at the top of this DGZ, so may not really 
manifest as an efficient "cross-hatch" signature for snow growth, 
and with the column so cold large aggregates will be difficult to 
achieve. Still, an above-climo SLR seems likely, and snowfall rates
of 1-2"/hr should be supported within this translating swath of snow.

The progressive nature of the system combined with the slightly
less than ideal snow growth, will somewhat limit total snowfall, 
but a swath of 4-8 inches still appears likely. The weaker/flatter
upper shortwave has lead to a slight northward trend with recent
guidance and will need to be monitored for short term mesoscale
trends. Current WPC probabilities are 50-70% for 6+ inches across 
central MT just to the north of Billings. These probs then lower
over the High Plains before increasing again near in the northern
Plains/Upper Midwest. D1-D2 WPC probabilities for at least 6 inches
are low (10-20%) along the ND-SD border and increase to medium
(30-60%) across central MN and central WI as better jet dynamics
increase lift over this region. This stripe of snow will also
extend into the L.P. of MI Saturday afternoon, with 30-50% probs of 6+ inches.

Farther to the southeast, the WAA will lift a warm nose more 
robustly immediately downstream of a developing surface low into 
the Upper Midwest and Ohio Valley, primarily after 12Z Saturday. This
will cause a stripe of mixed precipitation to develop from 
Illinois through eastern Ohio, resulting in another round of
accumulating sleet/freezing rain across these areas. WPC
probabilities for 0.01" of ice are moderate (40-70%) from eastern 
IL through eastern OH, with locally more than 0.1" possible 
(10-30%) in IN/OH.

...Mid-Atlantic and Northeast... Days 1-3...

Behind a strong low deepening in far eastern Canada, a potent cold
front will race quickly southeast in its wake, leading to 
pronounced CAA and briefly impressive low- level convergence. This 
will have a two- pronged impact to winter weather across the 
Northeast. The first will be an increased threat for convective 
snow showers and snow squalls Friday morning, especially across 
Upstate NY and into Northern New England. Although the timing of 
the front is not ideal for impressive squalls, there is forecast to
be enough overlap of low level RH, fgen, and instability to 
support scattered to widespread convective snow showers and 
isolated squalls. This is supported by simulated reflectivity in 
the high-res CAMs as well as HREF probabilities showing low-end 
chances for 1+"/hr rates. Additionally, current radar observations
and a recent Snow Squall Warning issued across western NY confirms
the ingredients for convective pockets of snow are in 
place. Accumulations with any squalls will be minor, but briefly 
heavy rates and lowered visibility could cause hazardous travel.

Then behind this front, CAA causes a drop in 850mb temperatures to
support sfc-850mb delta T nearing 20C across Lake Ontario (Lake
Erie is now completely ice covered). EL's approaching 15,000 ft
over Lake Ontario will help support a single band of lake effect
snow (LES) with rates at times reaching 1-2"/hr beginning this 
aftn and persisting into Saturday aftn, with some upstream 
connection to Lake Huron aiding in the threat. This will likely 
lead to some heavy snowfall accumulations, especially across the 
Tug Hill Plateau, where WPC probabilities for 4+ inches of snow D1 
are above 70%, leading to LES total snowfall as much as 12 inches 
in isolated areas.

A more significant system begins to take shape on D2. Despite the 
generally flat and progressive flow across the CONUS, a potent 
shortwave trough ejecting from the Plains will move into the Ohio 
Valley by 00Z/Sun, and then continue rapidly east across New 
England on Sunday. This will be accompanied by an impressive upper 
jet streak progged to reach 170kts as it arcs across the northern 
tier of the CONUS, with New England anticipated to be located in
the left-exit region for favorable upper divergence and increased
ascent. Downstream and to the south of this overlapping synoptic 
ascent, a wave of low pressure will begin to develop along a warm 
front as it lifts northward in response to intensifying WAA, 
leading to an expansion of precipitation on D2 from the Ohio Valley
northeast through the Mid- Atlantic and into New England.

High pressure ahead of this developing low will retreat to the east
Sunday, with the resultant isentropic ascent maximizing along the
290-300K surfaces. Mixing ratios within this lift are quite
impressive, 4+g/kg, which will push a plume of PWs exceeding the 
90th percentile according to the NAEFS climatology into the Mid- 
Atlantic and Southern New England. As this low skirts east off the 
NJ coast, it will likely intensify south of New England, and the 
resultant ageostrophic flow and CAA in its wake will halt the 
progression of the accompanying warm nose, keeping mixed 
precipitation confined to the Mid-Atlantic states. There remains
some uncertainty along the northern and southern fringes of the
snowfall forecast, with QPF the question for northern New England,
and ptype the question along southern New England. However, in
between there is increasing confidence in a swath of heavy 
snowfall (around 5-8 inches on average) from Upstate New York 
through much of New England. Despite the fast moving system, WPC 
probabilities for at least 6 inches of snow are high (70-80%) from 
the southern Adirondacks eastward to the Boston metro area and 
neighboring portions of southern VT/NH.

Additionally, a swath of moderate to heavy icing is expected
across the Central Appalachians and Mid-Atlantic states as a warm
nose quickly leads to sleet/freezing rain as the predominant ptype
within the climatologically favorable CAD regions. WPC probs for 
more than 0.1" of ice are above 80% (20-40% for 0.25" in the Laurel
Highlands and MD Panhandle), with 50-70% 0.1" probs extending
across northern MD and southern PA.

Snell/Weiss

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

https://www.wpc.ncep.noaa.gov/key_messages/La...

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