Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EST Sun Jan 02 2022

Valid 12Z Sun Jan 02 2022 - 12Z Wed Jan 05 2022

...Pacific Northwest to the Northern Rockies...
Days 1-3...

Closed low in the Gulf of Alaska will shear out through Tuesday
with pieces of shortwave energy beginning to rotate through the
Pacific Northwest early Monday through late Tuesday. Ahead of the
main trough axis, increasing mid-level confluence beneath the
intensifying jet streak will push a plume of higher moisture
onshore. PWs of 0.5-1.0" will push onshore aided by 850 mb flow of
50-60 kts, and these values are +1 sigma above the climo mean.
This will spread moisture from the Pacific Northwest late Sunday
into Monday across the Olympics and WA Cascades initially then
southward through OR and eventually northern CA Monday into early
Tuesday. Snow levels will remain quite low east of the Cascades
through the period, likely peaking only around 1500 ft, but will
rise within each plume of moisture closer to the coast. Snow
levels with the first surge could rise above 3000 ft, and then
over 4000 ft, especially in northern CA day 2 into day 3.

The heaviest snowfall and greatest WPC probabilities ramp up late
in the day 1 period where the 8 inch probabilities are 95+ percent
for the Olympics and WA Cascades with even a 80+ percent
probability for the 18 inches for the highest peaks of the WA
Cascades. By Day 2, those high probabilities for 8 inches spread
southward across much of the OR Cascades and the northern CA
ranges while some of the moisture and lift work eastward across
the northern Rockies. Here, the WPC probabilities for 8 inches is
moderate for the day 2 period. The approach of upper level ridging
from the west will push the moisture plume back to the north on
Day 3, focusing the heaviest snowfall potential on the central OR
Cascades northward through WA.


...Central and Southern Appalachians and southern Mid-Atlantic...
Days 1-2...

Southern stream shortwave trough along with an impressive 250 mb
jet streak will help drive and intensify surface low pressure
across the Southeast late tonight into Monday morning. A swath of
heavy precipitation is expected north/northwest of the low track.
The 00Z guidance continues to show a stronger system and a
north/northwest shift. Initially, warmer air will lead to rain at
the onset but the increasingly higher rates will drive dynamic
cooling and a changeover to wet, heavy snow is increasingly likely
across portions of the southern/central Appalachians (as the
trough axis swings through) and then over the southern
Mid-Atlantic once the surface low deepens. For this forecast
iteration, there was a considerable increase in QPF which led to a
substantial increase in the deterministic snow amounts but there
continues to be a larger spread in the ensemble guidance making
this a tricky and challenging forecast. Warm ground temperatures
will also have an effect ton the potential accumulations. However,
becoming increasingly concerned for potential for narrow but
intense snowfall rates of 1-2"/hr Monday morning across the
southern DelMarVa where the 00Z HREF has high probabilities for
1"/hr rates. It also shows high probabilities for 3"/3-hr along
this corridor. The current WPC probabilities for 4 inches show
moderate 30-50 percent probabilities for the southern DelMarVa but
high probabilities for the terrain areas of the southern
Appalachians. 


...Northern New England...
Day 1...

Low pressure tracking along a wavy stationary boundary draped
across the Ohio Valley through the Northeast has been the focus
for widespread precipitation from the Great Lakes into northern NY
this morning. Aided by favorable right entrance region jet
dynamics, a stripe of heavier snowfall is forecast continue during
the day 1 period, focused primarily on the eastern Great Lakes
through northern/interior New England states. Behind the departing
sfc low and cold front, lake effect snow will increase with some
isolated/narrow bands of heavier snow possible. The highest WPC
probabilities for 4 inches are this morning across northern
VT/northern NH into northern ME before the system begins to depart
and move offshore.

...Northern Plains...

Day 3...

A fast moving shortwave trough coming out of the shearing Pacific
Northwest trough axis will push into the northern Plains during
the day 3 period. This will lead to an area of low pressure to
track through the Plains and Upper Mississippi Valley. Left exit
jet dynamics will help drive large scale forcing for ascent and a
corridor of snowfall is expected to develop. With upwards of
0.10-0.20" liquid possible in a higher snow ratio environment,
several inches of fluffy snow will be possible. A favorable storm
track will enhance possible totals downwind of Lake Superior where
isolated 4-6 inch totals are possible. The WPC probabilities for 4
inches are 50-70 percent for far northern WI and along Lake
Superior of the U.P. of Michigan.

Taylor