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

Valid 12Z Fri Dec 17 2021 - 12Z Mon Dec 20 2021

...Northwest U.S....
Days 2 and 3...

Upper shortwave diving southeastward out of the Gulf of Alaska
will briefly close off near Haida Gwaii and slide eastward across
Vancouver Island at the start of the day 2 period (12Z Sat-12Z
Sun). 100kt jet streak 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. 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. Trailing upper shortwave will act to slow/stall
the cold front on Sunday, 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. Day 2 WPC probabilities are moderately
high for 6 or more inches of snow over the higher Olympics and
northern WA Cascades, and even around 10-50% around the higher
passes. Probabilities lower to the east over the northern tip of
ID/northwest MT ranges and into the OR Cascades.


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

Shortwave exiting Montana later today will combine 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
later today into the evening hours. Though progressive, models
indicate a small area of around a quarter inch or so of QPF which,
with temperatures in the teens to low 20s, will support a fluffy
few inches of snow. WPC probabilities of at least 4 inches of snow
nudged a bit higher to around 20-55 percent centered over
southeastern ND.


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

Upper shortwave moving out of the northern Plains tonight will
shift east through the Great Lakes Saturday, supporting surface
low development along a boundary over the Ohio Valley at the
tail-end of a 130kt jet. By the start of day 2, weak low pressure
will ride along the upper Ohio River Valley across the central
Appalachians with northward bagginess in the pressure pattern
toward the Southern Tier of NY. Surface temperatures will be
marginal, with an elevated warm nose lifting northward through the
day. 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 trend has been for a
bit flatter/weaker wave and background WSE nudged up the snowfall
across central/northern New England with the latest guidance. Day
2 WPC probabilities for at least 0.1" ice are 20 to 60 percent
along the PA/NY border and are just over 10% for at least 0.25"
ice in the Catskills to the upper Delaware River. Snow
probabilities for 4 or more inches are moderately high (60 to 90
percent) from the Adirondacks eastward through VT/NH through into
western Maine (and eventually eastern Maine by very early Sunday).


Fracasso