Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Wed Apr 01 2020

Valid 12Z Wed Apr 01 2020 - 12Z Sat Apr 04 2020


...Pacific Northwest to the Upper Midwest...
Deep upper trough centered near Alberta, Canada will drift
eastward while spinning off lobes of energy across the Northwest
and into the Northern Plains.

The first of these will push across WA/ID/MT today before lifting
into Manitoba early Thursday. This feature will drag a cold front
southward across the High Plains and into the Central Plains
beneath it. Favorable upper jet diffluence will help spread
precipitation across this region, with enhancement likely along
the strongest baroclinic gradient. A band of heavy snow has become
more likely across far eastern MT into ND, where WPC probabilities
for 4 inches are high, and isolated amounts to 6 inches are
possible. Also on D1, the second shortwave will combine with LFQ
jet level ascent to drive modest cyclogenesis in the lee of the CO
Rockies. WAA ahead of this feature combined with the synoptic
ascent will spread moisture into the High Plains of WY and western
SD, including the Black Hills, with enhancement likely along the
southward sinking cold front. Additionally, upslope flow behind
this boundary will likely cause snow amounts to increase in the
Black Hills, Big Horns and other ranges of WY. WPC probabilities
on D1 are high for 6 inches in this terrain. Lighter snows
amounting to several inches are likely as well across the Cascades
and Northern Rockies thanks to lingering synoptic ascent.

The surface low on D1 will weaken into a WAA/broad synoptic ascent
pattern as a ridge amplifies across the middle of the CONUS.
Strong WAA atop the low-level cold front will produce intense low
and mid level frontogenesis in an axis from western SD northeast
into far NW MN. While guidance still features some discrepancy
into where the best banding will occur, it appears a narrow band
of heavy snow, with rates exceeding 1"/hr are likely. Antecedent
conditions are less than ideal with warm air ahead of the system,
and rain changing to snow, but WPC probabilities for 6 inches have
increased and feature a moderate risk on D1.5-D2 across this area.
Isolated amounts over 8 inches are possible.

Southeast of the heaviest snow axis, a narrow band of freezing
rain is likely as cold air sinks southward to cool the surface
layer beneath the WAA. The HREF mean is quite aggressive with
accretions, and an extended period of freezing rain is possible.
However, warm antecedent conditions and potentially high rain
rates should limit ice accretion somewhat, and WPC probabilities
for 0.25" are less than 30%, highest in eastern SD into western MN.

By D3, the primary system will shift off to the northeast, while
another mid-level shortwave digs into the PacNW from the Gulf of
Alaska and around the parent vortex in Western Canada. Height
falls, jet diffluence, and moisture confluence will drive snowfall
in the Cascades and Northern Rockies, and WPC probabilities for 6
inches are moderate in the highest terrain.

Weiss