Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Fri Apr 23 2021

Valid 12Z Fri Apr 23 2021 - 12Z Mon Apr 26 2021

...Pacific Northwest, Great Basin, and California...
Days 2-3..

A closed mid-level low dropping out of the Gulf of Alaska today
will progress southeastward towards the Pacific Northwest, opening
and coming onshore the OR coast Sunday evening. This low will be
preceded by increasing warm and moist advection to its south in
response to intensifying confluence and a direct Pacific moisture
tap. Additionally, a downstream jet streak is likely to
strengthen, further driving moisture onshore, and PWs are progged
to reach as high as +1.5 standard deviations above the climo mean
in response to the increased 700-500mb moist flow. IVT within this
saturating column is only forecast to reach 250 kg/m/s according
to both ECENS and GEFS ensemble means, but a long duration of
modest IVT will support heavy precipitation expanding across much
of the West through the broadening deep layer ascent.
Additionally, W/SW mid-level flow will likely amplify ascent
through orographics, especially in the Sierra.

The prolonged WAA will initially drive snow levels up to around
6000ft as precipitation overspreads the region Saturday, but a
cold front moving eastward through the weekend should allow levels
to fall to around 4000ft on Sunday. Guidance overnight has eased
slightly on precipitation and snowfall, with both NBM
probabilities and WSE means lower than with previous runs.
However, the spread in the guidance remains significant, primarily
in the Sierra. This leads to a somewhat lower confidence forecast,
so although it is likely heavy snow will overspread much of the
region, exact amounts are still in question, especially by D3. WPC
probabilities on D2 are low to moderate for 6 inches in the
Northern Rockies, and some of the higher terrain around the
Siskiyous/Trinities of Northern CA. By D3, snowfall coverage is
likely to expand as the best forcing spreads across the region.
WPC probabilities for 6 inches are moderate from the OR Cascades
through the Absarokas and ranges of NW WY, as well as the Northern
CA ranges and Sierra. However, the heaviest snow is likely in the
Sierra where more than 12" of snow is possible in the higher
terrain on D3.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Weiss