Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 AM EST Fri Feb 28 2020

Valid 12Z Fri Feb 28 2020 - 12Z Mon Mar 02 2020


...Great Lakes...
Day 1...

A strong low pressure vertically stacked across SE Canada will
maintain robust CAA across the Great Lakes much of today before a
vorticity maxima surges southward within the mean trough tonight.
Before this occurs, unidirectional and favorable flow combined
with steep lake-850mb lapse rates will support heavy Lake Effect
Snow in the favorable W/NW snow belts downwind of Lake Erie and
Ontario, with moderate LES downwind of Superior and Michigan. The
heaviest snow is likely east of Lake Ontario and into the Tug Hill
Plateau where a high inversion and effective fetch/moisture tap
from Lake Superior and Huron aid in producing heavy snow. WPC
probabilities are moderate to high for 12 inches, with event total
snowfall from Thursday and today potentially exceeding 4 feet in
places. Heavy snow is also likely east of Lake Erie where a dearth
of ice cover will allow LES to persist today, and WPC
probabilities are high for 6 inches or more. Additional moderate
snow accumulations are likely in the eastern U.P. and northwestern
L.P. of MI.


...Central and Southern Appalachians/Cumberland Plateau...
Day 1...

Deep upper low centered over southeast Canada will maintain a
longwave trough and cyclonic flow across much of the eastern
CONUS. Embedded within this trough, two vorticity impulses will
rotate southeast through the trough and across the Southern
Appalachians. The first of this will occur this morning and bring
light snow from the Southern Ohio Valley through the mountains of
northern GA. Much of this will be light, but where upslope
enhancement occurs into the terrain of WV and along the NC/TN
border, WPC probabilities are low to moderate for 4 inches.

A secondary impulse will race southeast across much of the same
area this evening/tonight. While available moisture is less with
this feature, forcing is robust and there is potential for a few
snow squalls across KY/TN and into the terrain of VA/NC late on
D1. Additional accumulations are likely to be light, but briefly
heavy snow rates are possible.


...Western U.S....
Days 1-3...

A shortwave will lift onshore Washington State today and then
split off to the east through Sunday racing across the Canada/US
border. This feature will be accompanied by modest jet energy and
diffluence as well as moderate ascent and moisture ahead of the
700mb associated trough axis. Heavy snowfall on D1 should be
confined to the Cascades and Olympics where WPC probabilities are
high for 6 inches, and snowfall may locally exceed 12 inches in
the highest terrain.

As the leading shortwave splits off to the east, a secondary
shortwave will deepen and drop southeast into central CA before
closing off atop the southern Great Basin on D3. This second
deepening feature will be accompanied by an intensifying
subtropical jet stream arcing west to east across southern CA and
into the Southern Plains, spreading moisture into the West.
Forcing becomes much more significant, and increasing 700mb
divergence and moisture on SW flow will allow for heavy snowfall
to spread across much of the Great Basin on D2, and into the
Central Rockies by day 3 as a surface low develops in the lee of
the terrain. The heaviest snow D2 is likely from the Cascades of
Washington through the Sierra, and into the ranges of NW WY where
WPC probabilities are high for 6 inches. By day 3, forcing and
moisture coalesce more significantly, producing heavy snow in the
ranges of Utah/Wyoming/Colorado with high probabilities for 8
inches. The deep trough dropping so far south on D3, almost into
Mexico, will allow snow levels to drop significantly such that
even the transverse ranges of Southern California will receive
light accumulations Sunday.     

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

Weiss