Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
218 PM EST Sun Dec 25 2022

Valid 00Z Mon Dec 26 2022 - 00Z Thu Dec 29 2022

...Great Lakes through the Interior Northeast...
Days 1-2...

Remaining lake effect focus will be off Erie and especially
Ontario Day 1 as the bands have shifted back south from yesterday.
Stronger single band of Lake Ontario will favor the Tug Hill
tonight and early Monday before lifting back north into Tuesday
then finally stopping by Day 3. Two-day totals over 10 inches are
likely just south of Buffalo and also near Watertown/western Tug
Hill with local additional snowfall over two feet still possible.


...Corn Belt to the Mid-Mississippi Valley...
Day 1...

Guidance has trended weaker with this clipper, with amounts
forecast to be generally under 2.5" with <10% chance of exceeding
4" as it moves through the Corn Belt overnight tonight. Some light
icing is possible on the southwest side of the precip footprint
over eastern KS and western MO into northern AR.


...Pacific Northwest and Northern Rockies...
Days 1-3...

Stubborn arctic air remains in place over much of the Interior
PacNW east of the Cascades as onshore flow overrunning the trapped
cold air across the Columbia Basin promotes another round of ice
Monday and early Tuesday. Day 1-1.5 WPC probabilities for at least
0.10" of additional icing are generally 20-50% over east-central
Washington. By Day 3, enough scouring of the cold air should occur
to diminish the icing threat over all by the most sheltered
locations.

Snow levels will rise from 5000 to 7000ft through Monday (as
precip rates peak) before decreasing to around 4000ft Tuesday (as
precip rates also decrease) to around 3000ft Tuesday night as the
fairly cold core low approaches Monday and crosses WA Tuesday
night. Most of the snowfall will be on Days 2-3 with the
relatively lower snow levels and plentiful moisture, thanks to
successive systems and PW anomalies of +1 to +3 sigma. Two-day WPC
probabilities of at least 12 inches of snow are high over much of
the Cascades from WA through OR, as well as central Idaho. Snow
will start to impact some pass levels (e.g., Stevens and
Snoqualmie) starting mid-day Tuesday into Wednesday where the
probability of at least 8 inches is high there.


California/Great Basin/Southwest...
Days 2-3...

The strong, but progressive atmospheric river will be shunted
south down the entire length of CA Monday through Tuesday night as
the low pressure system shifts across the PacNW. Snow levels will
be quite high, 8000-10,000ft in the core of the moisture plume,
limiting snow to the High Sierra where there is a high probability
for a foot or more (two feet in the most favored areas). As the
system moves eastward, snow levels will still be generally high
(above 6000ft). The favored terrain of the Wasatch/Uintas into the
Colorado Rockies will see modest amounts of snow over 10 inches.


Fracasso