Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 PM EST Mon Dec 27 2021

Valid 00Z Tue Dec 28 2021 - 00Z Fri Dec 31 2021

...Northern Plains through Great Lakes...
Days 1-2...

After an all too brief hiatus, winter weather returns to the
northern Plains late tonight as the next wave crosses the
north-central Rockies late tonight with a sfc low shifting from KS
to Chicago Tuesday before lifting over the Great Lakes into
Wednesday. The more southerly low track allows accumulating snow
to reach northern IL with the bulk from central WI through all but
far SWrn MN to eastern ND then more lake enhanced snow for the
North Shore from fetch off Superior. Day 1/1.5 snow probabilities
for over 4 inches are moderate across these areas, moderately high
for the North Shore. Light icing is likely across eastern IA into
northern IL/southwest WI.

...Northern Mid-Atlantic to the Northeast...
Day 1...
Warm advection ahead of the wave crossing Lake Superior this
evening will allow continued freezing rain probabilities to expand
from central PA across central/southern NY and into
central/southern New England tonight. The greatest threat for 0.1"
ice is over the Catskills tonight.

...Western U.S...
Days 1-3...
An amplified positively-tilted shortwave trough pivots across the
Great Basin by this evening and across the north-central Rockies
tonight. Heavy snows will develop in the Transverse and and
northern Peninsular ranges by this evening. Bands of snow will
continue to shifts south over Great Basin, reaching northern
Arizona and the central to southern Rockies tonight. Day 1 snow
probabilities for 8 or more inches are focused on SW UT and
northern AZ terrain as well as the western/southwestern CO ranges.

The next in a long series of amplifying shortwave troughs shifts
off the Pac NW coast late Tuesday, spinning off into a closed low
off the central CA coast on Wednesday before slowing as it
continues south off SoCal Wednesday night/Thursday. This and a
leading weaker shortwave will allow some periods of moderate snow
for the OR and CA Cascades and Sierra Nevada where Day 1.5/2 snow
probabilities for 8 or more inches are low to moderate. The threat
for heavy precip then ramps up Wednesday night/Thursday for SoCal
with heavy snow for the Transverse and northern Peninsular ranges
with snow levels generally near 5000ft currently progged and Day 3
snow probabilities are moderate for over a foot.


~~~Key Messages for Western U.S. Heavy Snow and Cold Through The
Rest of This Week~~~
Mountain snow
Heavy mountain snow continues across the Southwest through the
Southern Rockies through tonight. Periodic moderate snows can be
expected down the Cascades and Sierra Nevada for the rest of this
week.

Lowland snow
Sea level snow levels persist across the Pacific Northwest through
Thursday, so any precipitation will be snow which could be
moderate at times tonight in western Oregon and on Thursday in
western Washington.

Widespread impacts to travel and infrastructure
Travel will remain discouraged especially along mountain passes
where long duration closures may be slow to end given the
continued cold and snow. Avalanche warnings remain in effect for
the Sierra Nevada, Northern through Central Rockies, and Wasatch.

Dangerous temperatures
Particularly cold air will continue across the Northwest through
midweek. Low temperatures below 0F remain common from the Cascades
eastward through the Northern Plains through Thursday.

Jackson