Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
315 PM EDT Tue Apr 11 2023

Valid 00Z Wed Apr 12 2023 - 00Z Sat Apr 15 2023

...Pacific Northwest...
Day 1...

An upper trough moving through the Pacific Northwest this
afternoon is followed by a closed low that will reach the WA/OR
border late tonight. Snow levels are around 2,000 feet this
afternoon, lowering to 1000ft or locally less late tonight into
Wednesday. Snowfall totals will be highest in the higher
elevations with Wly flow aiding orographic lift. The Oregon and
southern Washington Cascades for Day 1 feature high WPC
probabilities for >6" of snowfall and moderate probabilities
(40-60%) in the Clearwater and Salmon River Mountains of ID and
the highest OR Coast Ranges.

The upper low late tonight will contain exceptionally cold
temperatures for mid-April and 500 mb heights around -1.5 standard
deviations below average compared to climatology per the 12z
ECENS, enough to support minor snowfall accumulations down to
around 500ft. Even along the I-5 corridor south of Seattle and
north of Portland could see a coating of snow, but should any of
the snowfall accumulate, it would be more of a nuisance snowfall
event for these locations with accumulations generally confined to
grassy surfaces.


...Northern Rockies onto the Northern High Plains...
Days 2 and 3...

The upper low swings from the Pacific Northwest to Idaho
Wednesday/Wednesday night before pivoting northeast over
south-central/eastern MT Thursday/Thursday night, while also
taking on a subtle negative-tilt within the larger-scale trough.
Ample Pacific-sourced moisture will focus over the northern
Rockies Wednesday night/Thursday with lee-side cyclogenesis
focusing NEly flow over mainly MT ranges along with additional
moisture from the western Gulf via the Plains. High Day 2
probabilities for >6" are over the Clearwater Mtns of ID and most
SW/south-central MT and around Yellowstone in WY, with moderate
probs of at least 12" of snow over parts of MT above 6000ft. Day 3
snow probs are less expansive (with low 6" probs remaining over
the greater Yellowstone region and the Big Horn Mts of WY) as the
main precipitation shield exits the region as the low tracks over
the north-central Plains Thursday night. Bands of rain/snow become
all snow in the heavier/more dynamic activity. As of now Day 3
snow probabilities are low for >2" over central and northeastern
MT into western ND.


The probability for icing over 0.1" is less than 5% Days 1-3.


Snell