597
FOUS11 KWBC 052009
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 PM EDT Sun May 5 2024

Valid 00Z Mon May 06 2024 - 00Z Thu May 09 2024

...Cascades through Rockies...
Days 1-3...

A mid-level low currently centered over southeast OR per GOES-WEST
water vapor imagery is progged to shear open tonight as a
shortwave rounds it over the southern Great Basin. This makes for a
potent base of the trough pushing northeast over CO late tonight
before reforming a mid-level low again over the northern Plains
Monday. This low then phases with a reinforcing trough that
approaches from the PacNW Tuesday, drawing the low center west from
ND into MT Tuesday night before lingering over SD Wednesday.

Pacific-sourced moisture streaming in ahead of the low over the
Great Basin will continue to bring warm air advection precip over
UT this afternoon and CO/WY this evening. High snow levels of
8000-10,000ft are present in this warm air, but rapid height falls
under the approaching low/trough will quickly bring snow levels
down to around 5000ft yet tonight over the Wasatch/Uinta in UT,
Wind River and Bighorns in WY, and the western slopes of the CO
Rockies. Decent instability under the trough will continue to allow
shower activity to spread east in the colder/lower snow level air.
Day 1 PWPF for these terrain areas are 40-90% for >8" with the
highest values in the Flat Tops (just west of the Sawatch) in CO.

Meanwhile, the next wave arrives into the PacNW late tonight with
snow levels around 4000ft and moderate precip rates. Day 1 PWPF for
>8" are 40-90% along the OR/WA Cascades.

The main low shifts onto the northern Plains Monday and the
phasing keeps its precip focus over the Plains where it`s mainly
rain into Tuesday. The trough crossing the Northwest allows snow
levels to drop to around 3000ft, but it mostly has continental air
to work with, so Day 2 snowfall is somewhat limited - PWPF for >8"
is 30-70% over the WA/OR Cascades, north-central ID and northwest
MT (around Glacier NP) ranges, Wasatch, and the Park Range in CO
with the highest values (over 80%) for the Tetons.

The phasing low that retrogrades toward the northern Rockies
Tuesday then shifts southeast into SD maintains NNE flow into the
eastern slopes of the MT Rockies and the Bighorns with snow levels
around 4000ft. This persistent flow could make for some notable
snow totals. Currently the Little Belt Mtns in west-central MT
are progged to receive over 2" of QPF and 2 feet or more snow. Day
3 PWPF for >12" are 30-60% around Glacier NP, the northern
Absarokas/Bighorns with >80% for the Little Belts.


The probability of significant icing across the CONUS is less than
10 percent.


Jackson


$$