Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 PM EDT Mon Apr 05 2021

Valid 00Z Tue Apr 06 2021 - 00Z Fri Apr 09 2021

 ...Northern Rockies onto the North-Central High Plains...
Days 1-2...

A strong closed low dropping out of the Pacific Northwest this
afternoon will maintain intensity as it moves southeast towards
Colorado Tuesday morning and then into the Central Plains
Wednesday morning. This low will be accompanied by a modestly
coupled jet structure to drive intense deep layer ascent, with
height falls and mid/upper divergence helping to drive pressure
falls at the surface. This will result in lee cyclogenesis east of
the Rockies, with the slowly deepening low then lifting towards
the Midwest by the end of D2.

As the entire system shifts eastward, Gulf of Mexico moisture will
advect northward as rich theta-e advection develops, eventually
lifting into a TROWAL structure around the north of the 700mb low.
Guidance has trended subtly southeast today with the mid-level
feature, and there remains some uncertainty into where the
heaviest snow will occur. However, robust warm and moist advection
aided by elevated instability within the TROWAL should support
ample moisture spreading into the region. Initially, this should
be all rain east of the terrain, but as the wave moves eastward
CAA will commence, which when combined with dynamic cooling within
strong ascent maxima will lead rain changing to snow, with periods
of heavy snow likely. Antecedent conditions are hostile for snow
accumulations due to warmth and rain, but within the terrain, and
eventually in the lower elevations, snow should accumulate,
although SLRs will likely be below climo. However, the significant
snow accumulations should be confined to the higher elevations
above 4000 ft in the mountains of WY, the Black Hills, and the
Pine Ridge of NE, with highest snowfall on the E/NE slopes where
upslope enhancement will occur. WPC probabilities on D1 are high
for 6 inches in the WY ranges including the Wind Rivers and Big
Horns, with moderate probabilities in the Black Hills. By Day
1.5-2, additional light accumulation is expected to be confined to
the Black Hills and Pine Ridge.


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

A shortwave diving down the coast of British Columbia will move
onshore Washington State Thursday morning and then eject quickly
southeastward into MT/WY by the end of the forecast period
(Thursday evening). Brief locally backed mid-level flow will bring
warm moist air into the region, aided by a modest Pacific jet
streak arcing southward from the Gulf of Alaska. This moisture
will stream in ahead of the trough axis and associated surface
front, which will then drive the ascent to produce precipitation
across the Olympics and Cascades D3, lifting into the Northern
Rockies D3. WPC probabilities for 4 inches are high in the
Olympics and WA Cascades D3, increasing on D3 while also extending
into the Northern Rockies. Highest snowfall during the period is
likely in the WA Cascades where more than 12 inches is likely in
the higher terrain, and more than 6 inches possible at Snoqualmie
and Stevens Pass level.


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

Weiss