Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 PM EDT Fri May 06 2022

Valid 00Z Sat May 07 2022 - 00Z Tue May 10 2022


...Pacific Northwest...
Days 1-3...

A shortwave trough will approach the Oregon coast late tonight
through early Saturday followed by another shortwave Sunday that
then becomes absorbed into a broad longwave trough settling over
much of the Pacific Northwest later Sunday through early next
week. This will set the stage for a cooler regime with periodic
rounds of precipitation including elevation snow, some of which
could be heavy.

The system approach tonight/early Saturday will be associated with
a surface low opening up as it moves onshore while aloft, left
exit region dynamics from a 130 kt jet provides large scale
support. The best chances for heavy snow will be for the OR
Cascades but also some of the highest peaks of the WA Cascades and
eastward into the central Idaho ranges and northwest MT. Snow
levels initially 5-6 kft begin to fall toward 3-4 kft by tomorrow
late morning and afternoon. WPC probabilities for 4 inches are
high (80+ percent) for the OR Cascades as well as the peaks of the
central Idaho ranges with even a moderate signal (40-60 percent)
for totals during Day 1 to exceed 8 inches.

The second system arrives on Sunday within the cooler pattern and
overall lower heights. A stronger jet axis is focused on northern
California, positioning the favorable left exit region dynamics
over the Pacific Northwest. 500 to 700 mb height anomalies are
between 2 and 3 sigma below normal and the latest NBM snow levels
fall to below 3000 ft Sunday into Monday, reaching as low as
2000-2500 ft in places. With the core of the closed mid/upper
level low overhead, precipitation will be showery but with some
upslope flow component, the terrain areas should still a prolonged
period of light/moderate to occasionally heavy snowfall. For the
Day 2/3 period, the greatest totals will be found for the northern
California ranges, OR/WA Cascades but also eastward across
northern Nevada, central Idaho, northwest to southern MT, and
parts of Wyoming including the Wind River range. WPC probabilities
for 4 inches are moderate to high while 8-12 inches or more are
likely for the northwest CA ranges, OR Cascades, and southwest MT
ranges.

For days 1-3, the probability of significant icing is less than 10
percent.

Taylor