Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
514 AM EDT Wed Mar 18 2020

Valid 12Z Wed Mar 18 2020 - 12Z Sat Mar 21 2020


...Great Basin and California...
Days 1-3...

An orphaned closed low (a remnant of the low that brought the
recent heavy snow to the Sierra Nevada) over northwest NV will be
subducted by stronger shortwaves circling to the south allowing
this low to slowly die in place over the Great Basin (or perhaps
over CA after some westward drifting) through Friday. Precip along
the northern CA/NV border and west to the Sacramento Valley will
diminish as the low begins a southward drift down the east side of
the Sierra Nevada through Thursday as the snow level slowly rises
from around 3000ft to around 4000ft. Day 1 probabilities for 4 or
more inches are moderate to high for the Warner Mtns of far
northeast CA and low to moderate over the northern Sierra Nevada.
The moderate risk for 4 or more inches drifts down to the central
Sierra Nevada as well as central and southern NV ranges on Day 2,
expanding to the mountains of southeast UT on Day 2.5.


...Four Corners States through the North-Central Plains and
Western Great Lakes...
Days 1-2...

The closed low off the northern Baja Peninsula will open and eject
northeast across the Four Corners tonight and over the central
Plains Thursday before joining a northern stream shortwave
approaching the Great Lakes from the eastern Canadian Prairies
Thursday night. The resultant low tracks over interior Quebec
Friday.

The falling heights and increasing south to southwest flow bring
to Pacific moisture with widespread precip across AZ/UT today
(with snow levels around 6000ft) before shifting to NM/CO tonight
(snow levels 6000 to 7000ft). Day 1 snow probabilities for 8 or
more inches are moderate to high for the Mogollon Rim and White
Mtns of AZ and Kaibab Plateau (north of Grand Canyon North Rim) up
the UT Wasatch (and low probabilities for the Uintas) before
expanding to the San Juan/Sangre De Christos and west-central CO
Ranges to the Front Range. 

As this system slides northeast, the combination of height falls,
PVA, and increasing jet level diffluence through an increasingly
coupled jet structure will create lee cyclogenesis across eastern
CO. This surface low will then shift east across KS Thursday to
interact with a cold front, and produce an intense band of
frontogenesis between 750-600mb. While the low-level thermals from
eastern WY through the Central and Northern Plains are initially
too warm for snow, this enhanced frontogenesis collocated with the
synoptic ascent will drive the column to cool and change over to
snow. Once this occurs, a swath of heavy snow with the potential
for inch or more snow rates is likely from far NE CO/SE WY through
Nebraska and northeast to southern MN. The heaviest snow from this
system outside the high terrain (Day 2 probabilities are high for
12 or more inches for the CO Front Range) is likely in the High
Plains/eastern slopes of southeast WY and across much northern NE
to southeast SD where Day 2 WPC probabilities are moderate for 6
or more inches.

Farther northeast, there is somewhat lowered confidence due to the
strength of a developing TROWAL and its placement based on the
interaction of the northern stream trough, but WPC probabilities
are moderate for 4 inches from the Buffalo Ridge in MN across the
UP of MI, with snowfall in the western UP likely enhanced by Lake
Superior.

A stripe of modest freezing rain is likely to shift south from
southeast ND and northeast SD today increasing in the north side
of the developing low as it passes to the icing risk area of
eastern SD into southwest MN. Day 2 ice probabilities are 20 to 30
percent along the southern SD/MN border along with a small area
with a 10 percent risk of a quarter inch of ice.


...Northeast...
Days 1/2...

A quick moving wave of low pressure beneath a modest shortwave
shifts from the Ohio Valley today and crosses the northern
Mid-Atlantic tonight spreading precipitation across Upstate New
York, Southern and Central New England. The precipitation will be
driven primarily by WAA and frontogenesis as the surface wave
moves east from Pennsylvania to east of Cape Cod. The thermal
profiles are marginal and strong WAA will limit accumulating snow
to terrain with Day 1.5 snow probabilities around 10 percent for 2
or more inches in the Adirondacks, Greens, and Whites.


Jackson