Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 PM EST Mon Jan 20 2020

Valid 00Z Tue Jan 21 2020 - 00Z Fri Jan 24 2020

...Northwest U.S....

Days 1-3...
A pair of lows tracking east into BC/AK Panhandle over the next
few days brings a return to onshore flow and heavy mountain snows
to the Pacific Northwest through the rest of the work week. The
first low approaches the Queen Charlotte Sound tonight with the
associated occluded from reaching the WA/OR/northern CA coast
(along with one standard deviation above normal moisture) early
Tuesday. Snow levels will drop from around 4000ft tonight to
3000ft late Tuesday under the associated mid-level trough. Day 1
probabilities for 8 or more inches are moderate to high for the CA
Cascades/Klamath Mtns, up the OR/WA Cascades and the Olympics.
Some Pacific moisture makes it across to the northern Rockies with
Day 1 probs for 6 or more inches low to moderate for the Blue Mtns
of OR/ID.

The warm front associated with the second system arrives into the
WA/OR coast Wednesday with considerably more moisture (2 standard
deviations above normal) with snow levels quickly rising  to 5000
to 6000ft late Wednesday/Wednesday night. Day 2 snow probs for 8
or more inches are moderate to high for a larger area of the WA/OR
Cascades and Olympics than Day 1 given the heavier rates. With
snow levels generally above 6000ft on Day 3, snow probs for 8 or
more inches are limited to the highest WA Cascades.


...Ozarks/Mid Mississippi Valley/Upper Great Lakes...
Sprawling high pressure shifting east across the Midwest/Mid-South
through Tuesday night sets up a broad warm advection pattern from
the western Gulf of Mexico to the Upper Great Lakes on Wednesday.
A mid-level shortwave lifting out of the base of the broader scale
trough is expected to support organized, but generally light
wintry precipitation from the southern Plains, Ozarks, lower
Missouri and mid-Mississippi valleys northeastward to the Upper
Great Lakes Wednesday into early Thursday. A mix of light snow and
ice accumulations are expected from roughly central KS and eastern
OK northeast through the Upper Great Lakes through Thursday.
Overall snow accumulations are expected to be light, with 48hr
snow probs for 4 or more inches generally 5 to 10 percent in a
stripe from southeast NE across IA, southeast MN, and northern
WI/the UP of MI.

The risk of icing is best across the eastern Ozarks, with 10 to 20
percent chances for a tenth inch of ice from eastern OK into AR on
Day 2 and 20 to 30 percent chances for a tenth inch across the
AR/MO border on Day 3. That said, surface temperatures will be key
to ice accretion in this case, so uncertainty with that through
the Ozarks should make for adjustments to the forecast as the
event gets closer/more high resolution guidance is available.

Jackson