Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 AM EDT Sat May 4 2024

Valid 12Z Sat May 04 2024 - 12Z Tue May 07 2024

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

Deep/anomalous upper low currently centered west of the Oregon
coast early this morning will move east/southeast through southern
Oregon and northern California by late tonight/early Sunday. THis
system is anomalous for early May, nearing 3 standard deviations
below the climatological mean at 500 mb. Meanwhile, on the leading
edge as the front passes through, an axis of higher moisture
characterized by PW anomalies between +1 and +2 sigma will bring a
period of widespread moderate/locally heavy precipitation
initially this morning over northern CA and southern OR then
spreading southward into the Sierra as well as eastward into the
Intermountain West by later today/tonight. Snow levels will
initially be at or above 7000 ft along the warm front passage but
are expected to quickly drop in the wake of the cold front and as
the core of the upper level low moves overhead. The latest guidance
shows snow levels falling down to as low as 2500-3000 ft in places
and generally at or below 4500 from southern OR through the Sierra
Nevada. THe strong jet forcing and orographic lift will likely
produce snow rates locally exceeding 2"/hr down through Sierra
Nevada today.

The greater moisture available will make for a heavy/wet snow
across the region and the SLRs are likely to lean toward the lower
end of the guidance/climatology. The latest WPC snow probabilities
for Day 1 (12Z Sat-12Z Sun) are high (>70%) for at least 8 inches
across the Sierra Nevada and reach moderate levels (at least 40
percent) through the Oregon Cascades. Some higher peaks in the
Sierra could top 12-18 inches (20-30 percent chance) before the
event winds down later in the weekend. 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 into early
Monday morning. Snow levels still drop to around 5000ft with the
low over the Great Basin/Intermountain west. The latest WPC snow
probabilities show moderate (30 to 60 percent) chances for at
least 8 inches over the higher terrain areas of eastern
OR/northeast NV, central ID, northern UT (including the Wasatch
where local maxima are expected) to northwest WY.

As the southern vort max swings through the Rockies, it's forecast
to strengthen and take on a negative tilt as it moves into the
Plains. This enhanced lift/forcing along with a source of higher
moisture will lead to to heavy snowfall across the northern CO
Rockies, northern Wasatch in UT and across much of the Absarokas
and Bighorns in WY and into ranges of southwest MT. The latest WPC
snow probabilities for at least 6 inches peaks between 60 and near
80 percent for the Day 2.5 period.

Finally, after a period of upper ridging during Day 2-2.5, another
upper trough is forecast to approach and move through the Pacific
Northwest after 00Z Tuesday. 500 mb height anomalies are about
1-1.5 std below normal and PW anomalies are weak/modest at just
+0.5 to +1. With snow levels down to about 4000 ft, the higher
ranges of the OR/WA Cascades stand to see additional snowfall and
the latest WPC probabilities for at least 6 inches are 40 to
locally 70 percent.


The probability of significant icing is less than 10 percent.


Taylor