Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Mon Dec 20 2021

Valid 12Z Mon Dec 20 2021 - 12Z Thu Dec 23 2021

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

Strong/persistent upper ridging in the central Pacific will favor
continued/renewed troughing off the CONUS West coast for the next
few days. Lingering stationary boundary near the OR/WA border will
meander over the area on day 1 as a modest plume of moisture ahead
of a cold front offshore slowly shifts westward. Eastward
extension of the stationary boundary will lie across northern
Idaho and into western Montana where a developing 110kt jet streak
later today will promote transitory broad-scale lift before
slipping east by tomorrow. On Tuesday, moisture plume just
offshore will drift back to the east, but focused farther south
over northern California into the northern Sierra Nevada. By
Wednesday, a northern stream shortwave (or closed low) will dive
southeastward out of the Gulf of Alaska as the upper low offshore
California weakens but gets nudged eastward as the northern system
starts to draw closer. This will bring a more significant plume of
moisture to central California including much of the Sierra with
southwest flow aloft and PW values around 0.75" or +2 to +3 sigma.
Farther north, the northern streak system will be a bit limited in
moisture owing to its origin/trajectory, but will still yield a
modest and locally heavier area of snow for the Olympics and
Cascades. Snow levels will fall late Tuesday from near 5000ft down
to around 2000ft by late Wednesday/early Thursday.

WPC probabilities for at least 6 inches of snow are above 40
percent for the WA Cascades eastward across northern ID into
western MT on day 1. Day 2 probabilities begin to increase in
advance of the northern system over the northern WA Cascades and
also across northern CA as the Pacific moisture plume comes east.
By day 3, probabilities for at least 6 inches of snow increase
above 80 percent for the WA Cascades/Olympics southward along the
Oregon Cascades and into northern California through the Sierra
Nevada. Moderate probabilities (40-70%) of at least 6 inches of
snow lie over northern and central Idaho.

...Northern Tier/Great Lakes...
Days 1-2...

Sharp but progressive shortwave trough will move through the
northern Plains early Tuesday with a fast-moving surface low
tracking from central North Dakota to the U.P. of Michigan. Along
and north of the low, a band of snowfall is expected during a
period of brief warm air advection. Forecast soundings in the
region show a favorable setup for light, fluffy snow with deep
saturation within the preferred DGZ and coincident with the
maximum lift through the column, though for a limited time. This
should lead to a higher SLR event with the WPC SLR adjusted toward
18-20:1. With between 0.10-0.20" (locally 0.25") of QPF, this is
expected to produce a stripe of 3-4" of snow during the period
from eastern North Dakota to the North Shore of Minnesota. As the
wave progresses eastward across the Great Lakes region, westerly
to northwesterly flow will support enhanced snow totals downwind
the Great Lakes, particularly Lake Superior where WPC
probabilities for at least 4 inches during the two-day period
approach 80 percent across the eastern U.P. of Michigan and are
near 50-70 percent over the Keweenaw Peninsula.

...New England...
Day 3...
Coastal system will lift just offshore New England with some light
precipitation on its northwest side. However, it may wrap up and
deepen into Atlantic Canada which could enhance snowfall over
eastern Maine on on Wednesday depending on its evolution. Spread
remains rather high with the details of the system, how it
organizes off the Southeast/Mid-Atlantic coast, and how much
moisture it may be able to wrap around. WPC probabilities for at
least 4 inches of snow over northeastern Maine are above 40
percent.


For days 1-3, the probability of ice accumulation greater than
0.25" is less than 10 percent.

Fracasso