Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 AM EST Mon Dec 26 2022

Valid 12Z Mon Dec 26 2022 - 12Z Thu Dec 29 2022

...Great Lakes...
Days 1/2...

Continued cyclonic flow over the northern and eastern Great Lakes
from the low that caused the blizzard which is now over Hudson
Bay, and high pressure over the central Great Plains will allow
lake effect snow bands to continue over the U.P. and east of lakes
Erie and Ontario where there are local Day 1 probabilities for 6
or more inches with low probabilities for an additional foot near
Watertown, NY off the Lake Ontario single band.

A clipper grazes the northern Great Lakes Tuesday afternoon into
the night. Southerly flow ahead of the wave looks to bring a warm
nose above 0C, but ample surface cold air looks to refreeze most
precip that melts and bring sleet. Day 2 snow probabilities for 4
or more inches cover a larger area of the eastern U.P. with
moderately probabilities again just north of the Tug Hill off Lake
Ontario.


...Mid-South to TN Valley...
Day 1...

Clipper currently over IA gets sheared apart as it reaches the
Mid-South later this afternoon. Light snow and freezing rain is
possible with this wave which would be unremarkable, but it does
get into southern areas that do not typically see much
accumulating wintry precip. Day 1 snow probabilities for over an
inch are low over northeast MO to the St Louis metro then over the
lower OH valley with light snow potential through the TN Valley
through the southern Appalachians. Overrunning ahead of the wave
continues to bring a light ice glaze potential this morning to
northeast AR where temps are currently in the mid to upper 20s.


...Interior Pacific Northwest Ice Potential
Day 1...

Arctic air is slowly eroding over the Interior PacNW east of the
Cascades as onshore flow overrunning the trapped cold air across
the Columbia Basin promotes another round of ice through tonight.
Day 1 WPC probabilities for at least 0.10" of additional icing are
generally 20-60% over east-central Washington with low
probabilities for additional icing for the Columbia Gorge.
However, the cold air drainage that occurs in that narrow area is
not handled well by guidance and temperatures in the upper 20s are
noted just east of the Gorge this morning, so there is potential
for at least pockets of additional icing.


...The West...
Days 1-3...

A digging upper trough with a 150kt+ NWly jet will reach the WA/OR
coast late tonight with a potent atmospheric river surging into
OR/CA later today/tonight then shifting south down the length of
CA through Tuesday night as a full-latitude trough forms with the
base crossing the Desert Southwest Tuesday night/Wednesday.
Exceptionally high snow levels of 8000-11000ft will occur in the
core of the atmospheric river over OR/CA through tonight with NW
WA snow levels dipping to around 4000ft today. Snow levels then
drops Tuesday through Wednesday from north to south behind the
atmospheric river which will coincide with decreasing precip
rates. Snow probs tonight/Tuesday (Day 1.5) for 8 or more inches
are high for the higher Cascades, the Salmon River/Sawtooth of ID,
and the entire length of the High Sierra Nevada.

Pacific moisture shifts inland over the Great Basin Tuesday with
continued onshore flow for the West Coast. Tuesday night/Wednesday
(Day 2.5) snow probs for 8 or more inches are moderately high for
the WA/OR Cascades, the Sierra Nevada, and the terrain of UT and
western CO.

The trough axis reaches the southern Rockies Wednesday night with
lee-side cyclogenesis forming a Colorado Low that night with
central High Plains snow breaking out by early Thursday.


Jackson