Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Tue Mar 31 2020

Valid 12Z Tue Mar 31 2020 - 12Z Fri Apr 03 2020


...Northwest...Northern Rockies...and Northern Great Plains...
Days 1-3...

A large upper low centered near Alberta, Canada will spin nearly
in place through late-week while pieces of vorticity
energy/shortwaves shed around it into the Mountain West and
Northern Plains. There will be two primary shortwaves: one moving
onshore this morning that will race east-northeast into Manitoba
by Thursday morning, and a second following quickly on its heels.
These features will be potent despite remaining positively tilted,
and each will drag a cold front across the region beneath it. The
first of these will be accompanied by a modest jet-streak and LFQ
diffluence to enhance ascent, with mid-level confluence spreading
moisture well inland from the PacNW. Day 1 snow is likely to be
heavy from the Olympics through the Cascades, and into the terrain
of ID, NW WY, and western MT where WPC probabilities are high for
6 inches, and locations above 4000 ft may exceed 12 inches,
highest in the Tetons and Wind Rivers. During D2 as the leading
shortwave and associated cold front progress further east, forcing
weakens in WA/OR, and heavy snow becomes spread into the Big Horn
range, and into the High Plains of eastern WY where WPC
probabilities for 6 inches are moderate to high.

This initial shortwave and cold front will produce a tight
baroclinic gradient across the High Plains which combined with
continued upper diffluence and approaching height falls in advance
of the second shortwave will encourage surface low development in
the lee of the Rockies. While this low is not expected to deepen
much as it moves eastward, filling into more broad warm-advection
as it encounters an amplified ridge to its east, moisture will
become significant as low/mid level supports strong moist
advection. The synoptic ascent combined with ample WAA and strong
850-600mb frontogenesis will support heavy precipitation from NE
WY through SD/ND and into MN. While guidance still differs on the
exact placement of the heaviest precipitation, and some
uncertainty exists into the thermal profile of where the
snow/mix/rain line will be, it is becoming more likely that a
period of heavy snow will spread northeast early Thursday through
Friday. Banded precipitation is likely with heavy snowfall rates
as well. WPC probabilities are currently moderate to high for 6
inches from NE WY into the Black Hills of SD, with a secondary
maximum possible in eastern ND. SE of the heaviest snow, a band of
freezing rain potentially accreting to over 0.1" is also possible.
The current forecast is a blend of the ECMWF/NAM/UKMET, with the
GFS a bit too fast and progressive, while the CMC appears a little
slow with its evolution.

Weiss