Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 AM EST Sun Mar 01 2020

Valid 12Z Sun Mar 01 2020 - 12Z Wed Mar 04 2020


...Great Basin...Central Rockies...Four Corners...
Days 1-2...

Powerful subtropical jet evident on WV imagery streaking northeast
into Mexico and Southern California will persist and advect
eastward through early next week. This jet is progged to reach
above 170kts as it races west to east across the Mexico/US border,
before lifting poleward in response to a diving upper trough down
the coast of California. A secondary weak jet streak will be
rapidly lifting off to the northeast into the Northern Plains,
providing subtly enhanced ascent across the region in response to
jet coupling. In the mid levels, a closed low will dig towards
Baja and then eject slowly eastward towards the Big Bend of Texas
Tuesday, with downstream divergence driving ascent in conjunction
with the aforementioned upper forcing. While all this occurs, a
low-level cold front will drop southward, producing modest fgen
and enhanced easterly flow in its wake, which will lead to upslope
flow into the terrain of WY/CO. While the combined forcing will be
significant, it is also likely to be transient as the upper low
and jet advect eastward quickly, and there has been a drying trend
in the guidance both in QPF output and a faster loss of RH in
forecast soundings across WY late on D1.

Despite the relative short duration of strong ascent, PWAT
anomalies of +1 to +1.5 standard deviations above the climo mean
will support heavy snow across the terrain of the Great Basin
eastward into Utah, Wyoming, and Colorado. In this area WPC
probabilities are high for 8 inches, with local totals to 15"
possible. Also on D1, moist advection on orthogonal 700mb flow
into the Sierra will drive upslope snowfall in this range as well,
with WPC probabilities for 8 inches high. Light accumulations are
possible into the transverse ranges of southern CA as well through
tonight.

By Day 2 /Monday/ the best forcing shifts southward and a weak
wave of low pressure dives into Arizona. This will produce light
to moderate snow in the Mogollon Rim, where WPC probabilities are
moderate for 4 inches.


...Pacific Northwest...
Days 2-3...

Multiple shortwaves embedded within confluent mid-level flow and
an intensifying Pacific jet streak angling into British Columbia
will drive a weak wave of low pressure and associated cold front
into the PacNW. This will provide ascent for wave of precipitation
moving from Washington State eastward into the Northern Rockies,
with snow confined to the high terrain above 4000 ft. WPC
probabilities on D2 are mdt/high for 8 inches in the Cascades and
Northern Rockies, with similar snowfall expected on D3. Total
snowfall during the 2-day period may exceed 2 feet in the Cascades.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss