Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
415 PM EDT Sun Mar 22 2020

Valid 00Z Mon Mar 23 2020 - 00Z Thu Mar 26 2020



...Northeast...
Days 1-3...

Two low pressures will bring snow to the northeast during the next
3 days.

The first is a low pressure which will develop off the NC coast
Monday morning and then lift rapidly to the northeast, passing
near the Benchmark (40N/70W) overnight into Tuesday. This low will
deepen as it lifts northeast, with deepening due in part to height
falls/PVA associated with a shortwave moving from the Upper
Midwest/Ohio Valley, and enhanced 300mb diffluence as coupling of
jet streaks occurs. As the low lifts north, it will be accompanied
by modest to at times robust WAA, causing precipitation to become
widespread and spread from the Mid-Atlantic into New England.
Initially, the column will be sufficiently cold enough for snow
thanks to Canadian high pressure wedging down from the northeast.
This leaves a situation that will likely lead to a burst of
moderate snow as the leading edge of WAA spreads northward.
Thereafter, the continued warm advection will overwhelm the
airmass and turn p-type from snow to rain, first in the southwest
and last in the northeast. The terrain areas from the Poconos
northeast into the Whites of NH may stay snow throughout the
event, and the highest snow totals are likely at elevation. As the
system moves northeast off Cape Cod, a band of
frontogenesis/coastal boundary will likely setup somewhere NW of
Boston, MA, which could enhance snowfall totals into southern NH
as well. WPC probabilities for 6 inches are moderate to high in
the Catskills, southern Adirondacks, Berkshires, Greens, Northern
Worcester Hills, and much of the terrain of NH. The urban corridor
may start as snow with a light slushy accumulation from NYC north,
but no significant snow is expected.

Closely on its heels, yet another shortwave will race across the
Plains and into the Ohio Valley Wednesday, with an accompanying
surface low moving beneath it. This will move eastward and
transfer its energy to a secondary low developing near Delaware
which will become dominant and lift northeast late on D3. This
will again spread warm advection precipitation into the Northeast,
but the antecedent airmass is a bit warmer than with the first
system. There remains considerable uncertainty into the placement
and strength of this second system, some of which is due to the
evolution of the first. At this time WPC probabilities are low for
4 inches, and again confined to the higher terrain of upstate NY
and central New England.



...Mountains of California, the Great Basin and CO Rockies...
Days 1-3...

Potent closed upper low moving onshore California will slide east
Monday while slowly deamplifying. South of this low, a 130kt
subtropical jet with tropical connection will shift into the Great
Basin, producing strong ascent and moisture advection into the
region. This ascent will be enhanced locally by steepening lapse
rates beneath the mid-level low, as well as 700mb flow becoming
orthogonal to some of the ranges, especially the Sierra. Snow
levels on D1 will fall to near 5000 ft, and snow is likely across
most of the terrain from the Sierra through the Great Basin and
into Colorado, including the transverse ranges of Southern
California. WPC probabilities for 6 inches are highest in the
Sierra and on the Kaibab Plateau, with lesser amounts in the other
terrain.

Tuesday into Wednesday, a trough will dig southward from the Gulf
of Alaska and deepen off the West Coast. 700mb flow is confluent
across CA/NV/UT/western CO, driving 700mb RH above 90 percent
across a large portion of the West. Vertical velocity maxima
occurring in the terrain from the CA Sierra Nevada across the
ranges of eastern NV, the UT Wasatch/Uintas, and ranges of
northwestern CO will result in periods of snow, which are expected
to be locally heavy.  After a brief respite Monday night into
Tuesday, WPC probabilities become high once again for 6 inches in
the Sierra and the ranges of northern Utah, with more than 12
inches possible in the Sierra.

...Mountains of the Pacific Northwest to the Northern Rockies...
Days 1-3...


A closed mid-level will drop slowly southward from the coast of
British Columbia towards Washington state into Tuesday, before
slowly filling and advecting southeast onshore Oregon on
Wednesday. A weak surface low and accompanying cold front will
move E/SE ahead of the mid-level forcing, with periods of enhanced
upper diffluence aiding in regional ascent. Increasing 700-500mb
RH and ascent along a mid-level front will drive increasing
precipitation across this area, with snowfall developing and
accumulating Monday into Tuesday.  WPC probabilities through
Tuesday are high for 6 inches in the Olympics, Cascades, Northern
Rockies, Sawtooth, and other terrain of NW WY including the
Tetons, and 1-2 ft of accumulation is possible in many locations.
By day 3 the strongest ascent shifts southeast, leaving the best
probability for heavy snow across Southern MT through northern UT.

The probability of greater than 0.25 inches of freezing rain is
less than 10 percent.

Weiss