Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EST Sun Mar 07 2021

Valid 00Z Mon Mar 08 2021 - 00Z Thu Mar 11 2021


...Pacific Northwest/California to the Northern and Central
Rockies/Northern Plains...
Day 1-3...
As a closed low centered west of British Columbia continues to
drop south, a series of shortwaves moving through the base of the
associated trough will eject eastward -- impacting the western and
central U.S. through the period. 

Overall, models are in good agreement -- showing a well-defined
shortwave lifting from Northern California though the northern
Intermountain Region and into the northern Rockies on Monday. 
Models continue to show favorable upper jet forcing, along with
low-to-mid level frontogenesis, interacting with increasing
moisture to support a southwest to northeast oriented swath of
light to moderate precipitation developing and shifting northeast
across the region.  Guidance has been been moving into better
agreement over the past 24 hours with respect to the timing and
magnitude of precipitation -- with the GFS backing off some on its
more expansive heavier amounts.  That said, probabilities for
widespread significant accumulations have decreased during the Day
1 period (ending 00Z Tuesday).  However, WPC PWPF still indicates
some potential for locally heavier amounts across the Blue
Mountains into the northern Idaho ranges.  As this energy
continues to lift northeast, snows are expected to continue into
the evening hours across northeastern Oregon and northern Idaho,
while developing farther northeast across western Montana.   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.

This leading energy is forecast to lift northeast into central
Canada, with precipitation diminishing across the northern Rockies
and High Plains on Tuesday.  Meanwhile, the upper low will
continue to drop south off of the Washington and Oregon coasts,
with upstream energy ejecting farther south across California. 
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.

On Wednesday, the threat for heavy snow will continue to spread
south across California. As energy continues to eject inland, the
upper low is forecast to continue its southerly track -- 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 by early Wednesday,
with the likelihood of heavy snow increasing along the Southern
California coastal ranges during the day.

Meanwhile, energy ejecting across California into the Great Basin
on Tuesday is forecast to lift across Colorado and Wyoming early
Wednesday.  As it does, models are showing a pretty good signal
for significant snows developing from central Wyoming into western
South Dakota.  Favorable upper jet forcing along with low-to-mid
level frontogenesis/upslope flow is expected to support at least
light to moderate accumulations across the region -- with WPC PWPF
showing high probabilities for 4-inches of snow across the region,
with locally heavier totals likely across the Bighorn Mountains
and Black Hills.

Models have shown a fair amount of run-to-run variability and
spread with respect to QPF as this system moves downstream. 
Therefore, while at least some measurable snow and/or ice does
appear likely across the Dakotas into Minnesota, details of the
forecast late in the period are far from certain.  

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

Pereira