Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 AM EDT Tue Apr 06 2021

Valid 12Z Tue Apr 06 2021 - 12Z Fri Apr 09 2021

 ...Northern Rockies onto the North-Central High Plains...
Day 1...

A closed southern stream low over northern UT will shift ESE to KS
through tonight. The lee-side surface cyclone currently near the
CO/KS border is directing Gulf moisture west over western SD/Neb
and into WY which will continue until the 700mb low shifts to
central Neb this evening, with snow rates dropping this evening as
the system moves into warmer, lower elevations.

00Z CAMs/regional QPF guidance notably higher than 00Z non-NCEP
global guidance in the TROWAL region from the Black Hills to Pine
Ridge west to the Big Horns, particularly this afternoon. This led
to an increase in confidence for higher snow potential
particularly in eastern WY to the Pine Ridge. However, the
majority of the QPF falls from 18Z-00Z which is peak diurnal
heating and it is April, so SLRs should be limited and elevation
should play a role in accums (though snow accums in fgen bands are
often fairly independent of elevation).

Day 1 snow probabilities are 50 to 70 percent for 6 or more inches
for the Black Hills and from Pine Ridge in far northwest Neb west
into WY with 30 to 40 percent in the Powder River/Thunder Basin.


...Pacific Northwest and Northern Rockies...
Days 2-3...

A shortwave diving down the coast of British Columbia Wednesday
will shift into WA Wednesday night and reach the WY Rockies by
late Thursday night. Briefly backed mid-level flow will bring warm
moist air into the Pacific Northwest, aided by a modest Pacific
jet streak arcing southward from the Gulf of Alaska. The trough
axis and associated surface front will drive the ascent to produce
fairly progressive precipitation across the Olympics and Cascades
D2, and the into the Northern Rockies D2.5/3. Day 2 WPC
probabilities for 6 or more inches are moderate to high in the
Olympics and WA Cascades with snow levels around 3000ft. Day 2.5
snow probabilities are moderate for 4 or more inches in the
northern ID/Bitterroots and around Glacier NP with low Day 3
probabilities for 4 or more inches in the northern Absarokas and
Big Horns.


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

Jackson