Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Sat Mar 20 2021

Valid 12Z Sat Mar 20 2021 - 12Z Tue Mar 23 2021


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

An upper jet extending southwest to northeast across NV to western
WY and central MT leads to a combination of a slowly moving
frontal boundary plus moisture advection from NV across southeast
ID , northwest WY and southern MT. Well defined and persistent 700
mb convergence maxima helps lift the available moisture, in
conjunction with upper divergence maxima and orographics in
windward areas to produce moderate to heavy snow. The heaviest
snow are forecast in the Beartooth Mountains of south central MT,
where up to a foot of snow is expected. WPC probabilities are high
for 6 inches across these mountains. Secondary, lighter maxima are
forecast in the Cascades and Wasatch, where 6 to 10 inches is
possible.

From Day 1 through early Day 2, heavy snow is likely in the Big
Horns where persistent mid-level WAA will produce snowfall that
accumulates 6 to 12 inches. 

On Sunday, a 700 mb low forms in CO.  700 mb ascent and moisture
convergence leads to precipitation developing in the CO high
Plains to the foothills and front range. E/NE low-level flow
around the surface low drives upslope enhancement into the Front
Range and Palmer Divide. WPC probabilities on D2 are 40-50% for 8
inches in the Front Range, and as high as 50% in the Palmer
Divide. The column is marginally cold enough for snow into the
High Plains, so the primary uncertainty is how far east on to the
Plains light snow accumulations are likely Sunday night along the
I-25 urban corridor.

On Monday, the 700 mb low moves northeast across the central
Plains. Initial temperatures are too warm for snow, and cooling as
the low passes may lead to a brief period of snow before precip
tapers on the western side of the circulation in eastern CO to
western KS and Nebraska. Probabilities are low, up to 20 percent,
for 4 inches due to the short duration of snow.

On Sunday, the next shortwave and associated Pacific jet streak
will spread moisture and 700 mb convergence maxima across the
Olympics, Cascades, and later into the Northern Rockies.  The
strong moisture advection and lift results in locally heavy snow
that accumulates 12-18 inches in the Olympics and northern WA
Cascades.

Several inches are expected Sunday into Monday into the Blue
Mountains of OR, Clearwater Mountains of ID, and Tetons of
northwest WY as the upper trough moves inland, with associated 700
mb convergence maxima crossing these mountains, aiding in
producing lift.
Because the convergence maxima are progressive, the probabilities
for each 24 hour period providing 8 inches of new snow are low.

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

Petersen