Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Sat Apr 08 2023

Valid 00Z Sun Apr 09 2023 - 00Z Wed Apr 12 2023

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

A slowly deepening trough migrating southward from the Gulf of
Alaska will amplify into a closed low and begin to move onshore
the WA state coast by the end of the forecast period. Through this
evolution, mid-level flow will back from zonal Sunday to SWly
Monday, and then become more intense from the SW on Tuesday.
Within this flow, spokes of vorticity will shed away from the
aforementioned closed feature to spread waves of enhanced ascent
through PVA into the Pacific Northwest, with more robust PVA and
height falls spreading onshore Tuesday. This evolution will be
accompanied by increasing Pacific jet energy, and while the core
of this jet is progged to angle northeast into British Columbia
through Monday, it should arc back into the Pacific Northwest D3
bringing increased diffluence within its favorable RRQ. The
overlap of PVA, height falls, and this upper diffluence will
increase synoptic ascent, while dual cold fronts moving eastward
will drive low-level convergence and slowly falling snow levels.
This deep layer lift will act upon a moistening column as the
confluent mid-level flow and overlapping jet streak drive an
atmospheric river with high probabilities for IVT exceeding 500
kg/m-s onshore, especially beginning D2. While D1-D2 snowfall will
generally feature a minimal footprint due to lesser moisture and
snow levels of 5000-7000 ft. However, the approaching trough and
accompanying cold front will produce more widespread heavy snow D3
with snow levels falling to 1500-3000 ft, and some impactful
accumulations are possible at the Cascade passes. WPC
probabilities D1 and D2 do reach as high as 50% for 6 inches each
day, but only across the highest elevations of WA state. By D3 as
the snow levels crash, WPC probabilities of 30% or more for 6+
inches extends along the length of the Cascade crest, as well as
in the Olympics. Additionally, with snow levels falling below pass
levels, there is an increasing chance for at least 6 inches of
snow at Snoqualmie and Stevens Passes on Tuesday.

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


Weiss