Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
236 PM EDT Wed May 15 2019

Valid 00Z Thu May 16 2019 - 00Z Sun May 19 2019

...Western U.S....

As an anomalously deep upper low/trough moves across the region,
winter is forecast to return to much of the terrain of the West.

This upper low will close off west of California before beginning
to fill and open as it ejects eastward near the California/Oregon
border late on Thursday. As it moves inland, PWAT anomalies surge
on deep layer moist advection, while 500mb height anomalies crash
leading to lowering snow levels through the period. At onset, snow
levels will be generally 8-10 kft, but fall to as low as 5kft
across the Sierra into day 2. Beyond that timeframe and into the
weekend, snow levels will continue to fall, becoming as low as
5000 ft into the Great Basin and Northern Rockies. This will allow
snow to overspread much of the mountains and even some of the
higher valleys from day 2 into day 3.

The heaviest snow is likely in the Sierra where WPC probabilities
are high for 12 inches on day 1 thanks to orthogonal low-level
winds producing upslope enhancement combined with robust moist
advection and jet level diffluence, and total 3-day snows in the
Sierra could eclipse 2 feet in some places. As the upper trough
weakens and swings northeastward, the best forcing accompanies it
driving the best snow probabilities to the northeast as well.
Despite weakening overall forcing and lower column moisture, WPC
probabilities remain moderate for 8 inches from the highest
mountains of NV, into the Wasatch, and north through the ranges of
ID and into the Northern Rockies. By day 3, the best probabilities
for 8 inches continue further east into Colorado and Wyoming, with
snow levels hovering around 5000-8000 ft, highest in Colorado.


The probability of significant icing is less than 10 percent.


Note: This will be the last scheduled winter weather discussion
until next winter season which begins September 15, 2019. Any
discussions during the summer will be issued on an event-driven
basis when the threat of a significant winter storm develops.


Weiss