Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion
To   : wx-storm@lists.illinois.edu
From : COD Weather Processor
Date : Thu May 04 2023 20:03:11

FOUS11 KWBC 042003
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Thu May 04 2023

Valid 00Z Fri May 05 2023 - 00Z Mon May 08 2023

....Western U.S....
Days 1-3...

Former long-lived upper low came ashore CA today and has weakened
into an open trough that is forecast to turn northward on Friday
through the Great Basin and into the Northern Rockies. Trailing
troughiness along the CA coast will move through the Desert
Southwest into the Four Corners region and weaken tomorrow. The
former omega block pattern across the CONUS will morph into a Rex
block pattern over central Canada, further promoting additional
troughing into the Western CONUS over the next few days as the
overall ridge axis remains near 100W. This will maintain an
unsettled period for the mountains of the West toward the Rockies
with light to modest snowfall amounts. The below normal heights
and source region from the northwest (Gulf of Alaska) will favor
lower than normal snow levels across the region, but still
generally above many pass levels.

D1 snow will focus over the Great Basin with low to moderate
(<70%) probabilities for at least 4 inches over the NV ranges and
into central ID, along the track of the decaying short wave. By
D2, a new upper low is forecast to drop southeastward toward the
OR/NorCal coast, but will elongate in a NW-SE manner as additional
vorticity slides in from the NW. Cold pool aloft will still push
ashore and, coupled with LFQ upper jet divergence, will promote an
expansion of light to modest snow over the northern Sierra and
NorCal ranges into the southern Cascades. There, WPC probabilities
for at least 4 inches rise to more than 50% over the Sierra above
about 5000-7000ft from north to south. On D3, height falls will
lift northeastward as yet another Pacific upper low approaches the
West Coast (likely into D4), with decreased snowfall amounts of a
few inches mainly over the northern Sierra.

The probability of ice accumulations greater than 0.25 inch is
less than 10 percent.

Fracasso

$$



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