Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Fri May 05 2023

Valid 12Z Fri May 05 2023 - 12Z Mon May 08 2023

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

Former long-lived upper low turned open trough over the West is
forecast to slide northward on D1 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 ahead of the next approaching upper-level system to near
the OR/NorCal coastline. This will be followed by another stronger
system by D3. 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 through at
least this weekend 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
(heaviest located in the central Sierra). The below normal heights
and source region from the northwest (Gulf of Alaska) will favor
lower than normal snow levels across the region for early May, but
still generally above many pass levels. Generally, snow levels for
the D1-D3 period will range from about 6000-7000ft across the
Intermountain West and Northern Rockies, down to about 5000ft
along the Cascades and central/northern Sierra.

D1 snow will focus over the Great Basin with low (<40%)
probabilities for at least 4 inches over the northern NV ranges
and into central ID, along the track of the decaying short wave,
as well as the central Sierra as the next weak wave of moisture
moves onshore. Continuing into D2, moderate snow is expected to
continue across the Sierra as the previously mentioned approaching
upper low is forecast to weaken as it slides southeastward from
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 70% over the
Sierra above about 5000-7000ft from north to south. Mesoscale
banding and dynamic cooling aloft associated with rather chaotic
vorticity lobes and potential surface lows rotating northwestward
through MT could promote a brief burst of snow across the NW MT
ranges on Saturday, but confidence remains rather low. On D3,
height falls will lift northeastward as yet another Pacific upper
low approaches the West Coast (continuing into D4), with snow
levels around 5000 ft and low WPC probabilities for at least 4
inches of snow across the southern Cascades and northern Sierra.
This system will have deeper heights and stronger IVT, but will
also move onshore at a decent clip to limit QPF and associated
snowfall amounts.

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

Snell