Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EST Fri Dec 17 2021

Valid 00Z Sat Dec 18 2021 - 00Z Tue Dec 21 2021

...Northeast...
Days 1/2...

An upper shortwave crossing the Dakotas tonight will maintain a
positive tilts as it shifts east through the Great Lakes Saturday
and off the New England coast Sunday. Low development will occur
along a boundary over the Ohio Valley at the tail-end of a 130kt
jet tonight with a steady increase in intensity as it crosses the
central Mid-Atlantic Saturday and along the southern New England
coast Saturday evening. Surface temperatures will be somewhat
marginal in advance of the wave, with an elevated warm nose
lifting northward through Saturday. A wintry mix of snow to the
north, some sleet, and freezing rain (lingering in
colder/sheltered areas) will move northward as temperatures rise a
few degrees above freezing as the surface wave passes through the
region. Areas along and north of I-80 in PA through the Southern
Tier may see several hundredths of an inch of freezing rain while
farther north, within a deeper cold layer, snow is likely through
the event (northern NY eastward to Maine). QPF will maximize in
this region between 0.5-0.75" as a mid-level frontogenesis band
pushes through. Guidance continues to agree on a flatter/weaker
wave. Day 1/1.5 WPC probabilities for at least 0.1" ice are 20 to
60 percent along the PA/NY border, across the Catskills and to the
Berkshires and up to 10% for at least 0.25" ice in the Catskills.
Day 1.5 Snow probabilities for 6 or more inches are moderately
high (50 to 80 percent) from the Adirondacks eastward through the
Green and White Mtns through western Maine with notable lower
probs over southern/eastern Maine on Day 2.


...Northwest U.S....
Days 1-3...

An upper shortwave diving southeastward out of the Gulf of Alaska
today will briefly close off near Haida Gwaii tonight and slide
ESE across Vancouver Island Saturday. 100kt Wly jet streak
downstream of the trough axis will pass across the Washington
Cascades on Saturday, providing broad-scale ascent as a warm front
lifts through, raising snow levels to around 4000-5000ft. The
attendant cold front will be quick to push into the area, with a
modest plume of moisture nosing into the area (PW values around
0.75" or about +1 to +1.5 sigma). Moderate snowfall is expected
for mainly at and above pass level with some freezing rain
possible on the eastern side of the Cascades where cold air will
be loathe to dislodge Saturday morning. The trailing upper
shortwave closing into a low well off the OR/CA coast Sunday will
act to slow/stall the cold front, allowing a steady feed of
moisture into the Pac NW but focused into Oregon and stretched
northeastward into northern Idaho/northwestern Montana as the
surface boundary slows its southward push. This low pressure
system then merely drifts east off the OR/CA coast Monday which
pushes the frontal boundary north into WA while QPF ahead of the
low generally remains offshore. Day 1 WPC probabilities are high
for 6 or more inches of snow over the higher Olympics and the WA
Cascades, and even around 10-50% around the higher passes with
probabilities expanding to the east over the northern tip of
ID/northwest MT ranges and into the OR Cascades through Day 2 then
inching back north a mid and more moderate on Day 3.


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

A positively-tilted trough crossing the Dakotas tonight has
combined with a NW-SE surface trough to the northwest of a wave
along a front over the mid-Mississippi Valley to produce a
targeted area of snowfall along the eastern SD/ND border. This
area drifts east this evening with additional snowfall of 2" 40 to
60% likely after 00Z per the Day 1 WPC snow probabilities.


Jackson