Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Sun Apr 23 2023

Valid 12Z Sun Apr 23 2023 - 12Z Wed Apr 26 2023

...Pacific Northwest and northern/central Rockies...
Days 1-3...

A sharp upper trough or compact/small upper low is forecast to
move onshore coastal Washington late tonight and continue
southeastward into southwestern ID by Monday afternoon. Modest
precipitable water values will combine with progressive height
falls and upslope flow to wring out several inches of snow D1 over
the Cascades and Olympics, but the system will certainly continue
east of the terrain. From the Blue Mountains eastward across
central ID to southwestern MT into D2, snow will expand as the LFQ
of the upper jet promotes broad divergence which will combine with
lower-level convergence/frontogenesis along/east of the Divide
where WPC probabilities of at least 6 inches are moderate (>40%).

Into D3, a preceding weak shortwave will maintain lighter snow
over northern UT into the CO Rockies, ahead of a developing area
of heavier QPF. Southerly to southeasterly flow in the lower
levels over the High Plains out of the Gulf will supply the CO
Rockies with sufficient moisture as the upper low moves into
southwestern CO late Tuesday. Column cooling should increase past
dark Tuesday night with snow levels lowering from over 7000ft to
under 6000ft as upslope into the Front Range will support heavier
rain below the freezing line and heavy snow into the higher
terrain. WPC probabilities of at least 6 inches of snow D3 are
high (>70%) above 9000ft, but are moderate (>40%) for at least 4
inches down to around 7500ft.


The probability of significant icing greater than 0.25" is less
than 10 percent for Days 1-3.


Fracasso