Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Thu Mar 26 2020

Valid 12Z Thu Mar 26 2020 - 12Z Sun Mar 29 2020


...California to the Central Plains...
Days 1-2...

A large positively tilted trough will slowly consolidate to a
neutral tilt as it shifts eastward into the Plains during Friday.
As this occurs, prolonged SW flow around the south/east side of
this trough will produce moist advection into California, the
Great Basin, and the Four Corners. Enhanced 700-500mb RH on this
SW flow will become orthogonal into the Sierra producing heavy
snow Thursday, with heavy snow also spreading eastward into Nevada
and the Four Corners states. This synoptic ascent will interact
with a mid-level front extending from the central Rockies into
California as well, focusing heavy snow along this boundary. WPC
probabilities on day 1 for 6 inches are moderate in the Sierra and
other high terrain of eastern Nevada and into the Wasatch. By D2
the heaviest snow spread into the Front Range of CO before forcing
shifts eastward beyond D2.


...Pacific Northwest...
Day 2-3...

A potent shortwave digging out of the Gulf of Alaska will shift
southeast towards the coast of Washington, followed immediately by
a secondary impulse driving a longwave trough into the West Coast
Day 3. Height falls, PVA, and increasing diffluence within the LFQ
of an approaching upper jet streak will drive deep layer ascent
into the Northwest Friday and Saturday, with confluent mid-level
flow providing moisture. Snow levels are forecast to rise from
Saturday into Sunday as a trough/warm front lifts on shore, rising
from 2000-3000 ft Saturday, to 4000-5000 ft Sunday. WPC
probabilities are moderate to high for 6 inches on D2 and D3 in
the Olympics and Washington Cascades, and low on D3 into the
Oregon Cascades and east into the Northern Rockies. Total snowfall
could exceed 2 feet locally in the Cascades.


...Western Great Lakes...
Day 3...

A mid-level trough will amplify east of the Rockies and close off
over the central Plains before lifting northeast on Saturday.
Beneath this feature, a surface low will deepen and lift northeast
as well, becoming a potent cyclone over southern Minnesota late in
the forecast period. Strong WAA ahead of this system will provide
ascent and spread precipitation northward into the Western Great
Lakes. The thermal profile will likely be too warm for snow across
most of the area, but intense ascent within a strengthening TROWAL
could lead to dynamic cooling enough for heavy snow in northern WI
and into the Arrowhead of MN, with some enhancement on upslope
easterly flow into the Iron Ranges. WPC probabilities for 4 inches
are 20-30% invof Lake Superior.

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

Weiss