Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Mon Mar 08 2021

Valid 12Z Mon Mar 08 2021 - 12Z Thu Mar 11 2021


...The West and Northern Plains...
Day 1-3...
As a closed low currently centered west of Vancouver Island shifts
south off the Pacific Northwest Coast through midweek, a pair of
shortwave troughs moving through the base of the associated trough
will eject east-northeastward. These will cross the Pacific
Northwest and northern Rockies and then California, the Great
Basin, the central Rockies and the northern Great Plains. 

The first shortwave trough axis lifts from Northern California
though the northern Great Basin today and across the ID/MT Rockies
tonight. Favorable upper jet forcing and low-to-mid level
frontogenesis, interacting with increasing moisture supports a
southwest to northeast oriented swath of light to moderate
precipitation shifting northeast across these regions. A brief
period of low-to-mid level northeasterly northeasterly
flow/frontogenesis is expected to support a period of light to
moderate snows from the northwestern Montana ranges into the High
Plains tonight before lifting into central Canada Tuesday.
Meanwhile, the closed low will continue to track southeast toward
the far northern CA coast, with upstream energy ejecting inland
farther south across California Tuesday. During this period, the
potential for locally heavy snow is expected to increase from the
Klamath mountains in southwestern Oregon and northwestern
California to the northern Sierra Nevada. Day 1 snow probabilities
for 6 or more inches are moderately high over the Klamath/CA
Cascades, far northern Sierra Nevada, Wallowa Mtns of northeast
OR, the Clearwater/Salmon River Mtns of ID, and west-central MT
Rockies.

The second shortwave trough ejects across south-central CA into
the southern Great Basin on Tuesday before shifting across
southern UT and central CO Tuesday night. Favorable upper jet
forcing along with low-to-mid level frontogenesis/upslope flow is
expected to support at least moderate accumulations across the
region. Day 2.5 WPC snow probabilities are moderately high for 6
or more inches for the Uinta Mtns of UT and over central and
northeastern WY and the Black Hills.

The threat for heavy snow will continue to spread south across
California Tuesday night/Wednesday as the closed low tracks south
along the coast, drawing cold air and the axis of stronger inflow
farther south across the state on Wednesday.  Periods of heavy
snow are likely to extend from the northern into the southern
Sierra Nevada by early Wednesday, with the likelihood of heavy
snow increasing along the Southern California coastal ranges
during the day.

There is decent agreement above non-CMC 00Z guidance for the Day 3
track of the second shortwave trough from the central Plains to
the Upper Midwest with ample moisture and cold enough air likely
allowing a swath of snow to develop along the north side of the
low level pressure center. As of now Day 3 snow probabilities are
low for 4 or more inches in a broken swath across northeast SD and
northern MN with low probabilities for a tenth inch of ice for far
northern WI into the western U.P. of MI.


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

Jackson