Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
436 PM EDT Fri May 3 2024

Valid 00Z Sat May 04 2024 - 00Z Tue May 07 2024

...Cascades through Sierra Nevada, Intermountain West, and
Rockies...
Days 1-3...

Sprawling low over the northeast Pacific west of BC this afternoon
will deepen further from reinforcing shortwave energy tonight as it
reaches the OR/CA coast before shifting east over far northern CA
through Saturday night. This is an anomalously deep system for
early May, with 500/700mb heights dropping 3 standard deviations
below normal across southern Oregon and northern California. Warm
conveyor moisture brings high elevation snow tonight to the
Shasta/Siskiyou and southern OR Cascades before notable height
falls and strong moisture influx along with left-exit region upper
jet forcing and strong topographic lift brings a swath of heavy
snow with rates locally exceeding 2"/hr down the Sierra Nevada
Saturday afternoon and evening. Initial snow levels on the Sierra
of over 7000ft Saturday morning will quickly drop below 5000ft
during the heavy precip rates.

This will generally be a heavy/wet snow with lower SLRs adding a
snow load risk to the snow amounts which continue to trend upward.
Day 1.5 WPC PWPF shows high probabilities for 8 inches or more
falling on the CA Cascades and the northern/central Sierra in areas
above 5000ft. Heavier accumulations of 1-2ft can be expected above
about 7000ft. In addition to the falling snow, strong/gusty winds
and blowing snow are forecast to contribute to moderate to locally
major winter storm impacts as indicated by the WSSI.

The system slowly fills/weakens as it moves from the northern CA/NV
border Sunday morning across northern UT Sunday night. Snow levels
still drop to around 5000ft with the low over the Great
Basin/Intermountain west. Day 2.5 PWPF for more than 8 inches are
40-80% over the higher ranges of eastern OR/northeast NV, central
ID, northern UT (including the Wasatch where local maxima are
expected) to northwest WY.

However, redevelopment is forecast by the 12Z model consensus as
it moves over WY on Monday with Gulf-sourced moisture streaming in
from the Plains enhancing snowfall for the eastern slopes of the
northern Rockies. Day 3 PWPF for more than 8 inches are 30-70% for
the northern CO Rockies, Absarokas and Bighorns in WY and into
ranges of southwest MT.

Furthermore, the next trough off the Pacific crosses the PacNW on
Monday with renewed snow on the Cascades with snow levels around
4000ft. Day 3 PWPF for more than 8 inches are 40-80% north from the
central OR Cascades through the western WA Cascades.


The probability of significant icing is less than 10 percent.


Jackson