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

Valid 00Z Thu Mar 19 2020 - 00Z Sun Mar 22 2020


...California, Great Basin, Four Corners...
Days 1-3...

A potent closed upper low positioned over the northern Great Basin
this evening will begin to fill and rotate slowly southward before
opening into a sheared trough aligned west to east from CA through
the Four Corners early Sunday. Despite the weakening low,
persistent forcing through rounds of PVA, periodic height falls,
and modest jet level diffluence will produce ascent in an
environment characterized by high PW and 700-500mb RH. Downwind of
this feature, there will remain considerable moist advection as
well as 700mb flow persists out of the SW, transporting Pacific
moisture northeastward. The heaviest and most widespread snowfall
is likely on D1 due to stronger forcing, and WPC probabilities for
6 inches are moderate to high in the Sierra of CA as well as most
of the terrain of the Four Corners from the Mogollon Rim to the
Wasatch and into the San Juans, Sangre De Cristos, Uintas, and CO
Rockies. Snow levels will be as low as 3000-4000 ft in CA, but
will be much higher in the Four Corners on D1, 6000-8000 ft. As
the upper low shears to the east, snow levels will fall quickly,
becoming 4000-5000 ft area-wide, but with less forcing to drive
ascent. WPC probabilities on D2 are moderate for 6 inches, and
confined to the highest terrain of the CO Rockies and Utah ranges.
By day 3, a respite will occur with only light snow expected.
2-day totals may exceed 18 inches in some of the higher terrain of
the Uintas, San Juans, and other ranges of Colorado.



...North-Central Plains through the Western Great Lakes...
Days 1-2...

The large closed low pinwheeling across the Great Basin will eject
a strong piece of energy across Baja northeast into the Four
Corners tonight. This feature is progged to deepen briefly to a
closed circulation near the High Plains of Wyoming before opening
once again and advecting northeast Thursday night. By Friday,
interaction of this shortwave with northern stream energy will
cause it to race off to the northeast as it becomes embedded in
the westerlies towards Ontario.

As the upper feature moves into the Four Corners, 250mb jet
streaks will intensify to produce coupling near the CO/WY High
Plains. This is likely to induce lee surface cyclogenesis, and the
surface low is expected to lift to the northeast from CO early
Thursday, to the Great Lakes on Friday. As it moves northeast, the
low will interact with a low-level cold front to produce a swath
of enhanced low/mid level frontogenesis to enhance ascent. While
the column will initially be too warm to support snowfall, as the
low moves northeast and the front drops southward, cooling will
cause p-type transition from rain to snow. There remains some
uncertainty into how quickly this will occur, and with the system
expected to be progressive confidence in snowfall amounts is lower
than normal. However, the enhanced frontogenesis aided by a
developing TROWAL wrapping cyclonically around the low should
produce a band of moderate to at times heavy snow. The highest WPC
probabilities for 4 inches are from southeast WY into central NE
where snowfall rates may reach 1"/hr at times, and isolated
amounts to 8 inches are possible. Further northeast the overall
forcing looks to weaken, but LES on enhanced northerly flow across
Lake Superior creates a secondary area of enhanced WPC
probabilities for 4 inches in the U.P. of MI.


A stripe of modest freezing rain is likely tonight into Thursday
as well across extreme southern ND into northern SD. This is in
response to the drying of the DGZ while low-levels remain
saturated, producing an environment favorable for light freezing
rain or freezing drizzle. Accretions are expected to be light, but
WPC probabilities indicate a low risk for 0.1 inches, focused near
the Buffalo Ridge of SD/MN.

Weiss