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

Valid 00Z Sat Apr 24 2021 - 00Z Tue Apr 27 2021

...Pacific Northwest, Northern Rockies, and California...
Days 2-3..

A closed mid-level low dropping southeast across the northeastern
Pacific will swing towards the Pacific Northwest by Sunday morning
before opening and moving inland across northern California Sunday
evening. Ahead of this low and to its south, a direct Pacific
moisture tap will lead to increasing warm and moist advection.
Additionally, a strengthening jet streak will drive moisture
farther onshore. PWs are progged to reach as high as +1 to +1.5
standard deviations above the climo mean in response to the
increased 700-500mb moist flow. IVT within this saturating column
is not forecast to be as robust, with values around 250 kg/m/s
according to the GFS and ECMWF, but a long duration of modest IVT
will still support heavy precipitation expanding across much of
northern California and the Sierra Nevada through the broadening
deep layer ascent. Additionally, orographic lift will add to the
enhanced precipitation rates throughout the Sierra Nevada.

The prolonged WAA will initially drive snow levels up to around
6000ft across much of the West 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.
WPC probabilities on D2 are high for 6 inches in the Sierra Nevada
and some of the higher terrain around the Siskiyous/Trinities of
Northern CA, with maximum amounts over 12 inches possible. By D3,
high elevation snowfall coverage is likely to expand throughout
the Intermountain West as the best forcing spreads across the
region. WPC probabilities for 6 inches are moderate across the
Northern Rockies, as well as the Uinta and Wasatch Mountains of
northern Utah.

...Northern Plains...
Day 2...

A small and quick-hitting stripe of moderate snow is increasingly
likely across portions of the Northern Plains on Sunday morning
from northeast South Dakota to west-central Minnesota. Robust WAA
will drive a band of increasing 700 mb frontogenesis and dynamic
cooling. Uncertainty is still relatively high, as much of the
accumulation will be determined if snowfall rates can overcome
warm surface temperatures. Some guidance, including the GFS, is
hinting at the possiblitity of at least 1"/hr rates on Sunday
morning. The best chances for accumulating snow will be across
northeast South Dakota, where WPC probabilities for 2 inches of
snow have increased to over 60%.


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

Snell