Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Fri Mar 19 2021

Valid 12Z Fri Mar 19 2021 - 12Z Mon Mar 22 2021


...Pacific Northwest/California/Great Basin/Rockies/central High
Plains...
Days 1-3...

A slow moving closed mid-level low will pivot slowly southeast
towards the WA/OR coast Friday into Saturday, shearing into an
upper trough as it moves inland during the weekend.  For D1 Fri,
the heaviest snow is expected in the WA Cascades and Olympics,
with lighter amounts downstream in the ranges of OR to southern ID
and northwest WY/adjacent southwest MT.  WPC probabilities for 8
inches are high on D1 in
the WA Olympics, and moderate in the Cascades .  A secondary
maxima is for an additional several inches of snow in the Central
CA Sierra Neva range as a slow moving cold front crosses the area,
with moist pre-frontal southwest flow advecting moisture inland
from the Pacific into the terrain. Upslope conditions help augment
the frontal induced lift. The snows wind down as the 700 mb trough
moves further inland on Sat. The probability of 8 inches of snow
on Day 1 in the central CA Sierra Nevada is high.

The longer duration ascent on Day 2/Sat looks to be across the
Beartooth Mountains of far southwest MT to the Yellowstone
National Park and Tetons of WY. The long duration ascent
underneath 300 mb divergence maxima leads to amounts of 6-12
inches across the region, extending east into the Bighorn
Mountains of WY. The trailing cold front crosses the Wasatch, so
several inches of snow are excepted in the terrain of southwest WY
to northern UT.  WPC probabilities for 8 inches are moderate ,
highest in the ranges of northwest WY to adjacent southwest MT.

On Day 3/Sun, the amplifying upper trough builds south from the
central towards the southern Rockies and Great Basin. An upper jet
progresses out of the southwest across the ranges of NM, providing
favorable difluent flow in the left exit jet region of CO.
As a 700 mb low forms in southeast CO, moisture pools near the low
and return flow advects moisture in to the front range of CO down
to the Sangre DeCristo Mountains and Palmer Divide. Snow develops
in the mountains, with uncertainty whether it will be cold enough
for accumulating snow on the CO high Plains. Up to a foot is
possible in Rocky Mountain National Park/CO front range, where the
probability of 8 inches is moderate.

In WA State on Sun, the upper trough approaching British Columbia
drives a zonal jet with moisture advecting onshore from the
Pacific across WA State, in conjunction with 700 mb ascent.
Another round of snow is likely across the higher elevations of
the WA Olympics and Cascades, with clusters of 12-24 inches across
the terrain with the best orographics.  The 00z ECMWF has
increased QPF/snow amounts in the Olympics/northern WA Cascades,
so confidence is increasing in heavy snow potential on Sun. The
probability of 8 inches of snow Sun is moderate to high in the
northern WA Cascades. 

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

Petersen