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

Valid 12Z Sun May 05 2024 - 12Z Wed May 08 2024

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

Unsettled and active winter weather will continue through the
forecast period as a deep and anomalous upper low, currently over
southern Oregon and northern California, moves eastward into the
Intermountain West and Rockies over the next few days. This system,
characterized by low heights and a cold air mass for early May,
will lead to falling snow levels across the Intermountain West,
while for the Cascades into the Sierra Nevada, levels have bottomed
out around 3000 ft early this morning.

For today, the bulk of the heaviest precipitation is expected over
Oregon, as moisture pivots north/northwest around the mid/upper
level low. Some enhancement due to terrain effects will be possible
and some snow rates above 1"/hr will be possible from the OR
Cascades as well as eastern OR. The latest WPC snow probabilities
for this region reach moderate to high levels (at least 60 percent)
and are highest for the Cascades where the peaks are likely to see
an additional 12 inches today/tonight.

Further east, the terrain areas of the Wasatch in UT, Big Horns in
WY, and to some degree to the CO Rockies will see increasing
moderate/heavy snowfall through early Monday morning. Here, the WPC
snow probabilities are moderate (40-60 percent) but do peak above
70 percent for the Wasatch and Big Horns. 

By Day 2 (12Z Mon-12Z Tue), the vort max swinging through the
Rockies will quickly take on a negative tilt as it ejects into the
High Plains. Deepening low pressure over western SD will spread
precipitation back into parts of WY, eastern MT where thermal
profiles are marginal for snowfall though intense precip rates and
higher terrain areas may see dynamic cooling enough to changeover
to wet/heavy snow. This will be especially true further west across
the Wasatch, Big Horns, and terrain areas of central ID where an
additional 6-12 inches will be possible.

Meanwhile, another quick moving shortwave trough will approach the
Pacific Northwest Monday into Monday night, spreading precipitation
across the region. Here, snow levels to around 5000 ft will support
additional snowfall accumulations for the OR/WA Cascades.

Finally by Day 3 (12Z Tue-12Z Wed), there are trends in the latest
model guidance for the deep surface low over the Northern High
Plains to occlude and stall over the northern Rockies with some
phasing occurring with the secondary shortwave passing through
Mon/Mon Night. This could bring widespread moderate/locally heavy
precipitation to parts of Montana and for the western areas,
locally heavy snow is possible. There remains model uncertainty in
how this will evolve and the amount of cold air that will be in
place for more snow than rain, but the latest trends and WPC snow
probabilities show 60 to 80 percent for at least 6 inches and some
signal (10-30 percent) for at least 12 inches for the higher
terrain ranges.


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


Taylor