Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Thu May 04 2023

Valid 12Z Thu May 04 2023 - 12Z Sun May 07 2023

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

The weather over the western third of the Lower 48 the second half
of the week, and even into the upcoming weekend, can be summarized
with a longwave upper trough anchored over the West Coast
containing a series of disturbances revolving around the upper
trough. Thursday features an anomalous upper low in southern
California (below the 2.5 climatological percentile 12-18Z
Thursday) opening up and tracking into the central Great Basin by
Thursday afternoon, with another disturbance hot on its heels
approaching the southern California coast. Out ahead of these
vorticity maxima, a steady diet of 850-700mb moisture will be
directed into parts of California, the central Great Basin, and
central Rockies. Between 12-18Z Thursday, NAEFS showed
850-700-500mb temperatures that are also below the 10th
climatological percentile of southern California, which is
prompting unusually low snow levels in the Transverse Ranges.
Snowfall totals >4" are most likely to occur at elevations >6,000
ft, and localized amounts of 8-12" at elevations above 7,000 ft.
The areas >7,000 feet have been identified by the Day 1 WSSI as
likely witnessing Moderate, to even locally Major impacts,
primarily due to not just the snow amounts but also the heavy/wet
consistency potentially weighing down tree limbs and power lines.

The southern and central Sierra Nevada, particularly above 7,000
feet, will be the most active mountain region the next few days
with steady periods of moderate, to occasionally heavy, periods of
snow. By Friday, there will be two shortwave troughs: one in the
northern Rockies and the other in the Lower Colorado River Basin,
responsible for snow from the Sierra Nevada on east to the Wasatch
and as far north as the mountains of eastern Oregon, central
Idaho, and southwest Montana. The Lower Colorado River Basin lobe
of vorticity will head east for the central Rockies, where it will
generate periods of snow in the Colorado Rockies Friday evening
and into Saturday morning. Meanwhile, the next upper low off the
Pacific Northwest coast is set to deliver the next round of
Pacific moisture into the Cascade Range and northern Sierra
Nevada. This upper low makes its way east to being just off the
Oregon Coats by Saturday afternoon, where it will then direct the
same slug of Pacific moisture into the northern Rockies through
Saturday night.

Despite the prolonged period of snow, the 3-day snowfall
probabilities depict the typical mountains ranges of the West
Coast and Intermountain West receiving modest snowfall totals
that, over the course of three days, keep most impacts from
topping out higher than Minor on the WSSI. This is due to the lack
of an Arctic air-mass in place, which is harder to come by given
the time of year. Snow levels will generally be at or above 6,000
ft in the northern Rockies, and above 8,000 ft in the Wasatch and
Colorado Rockies.

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

Mullinax