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

Valid 12Z Sun Mar 22 2020 - 12Z Wed Mar 25 2020



...Northeast...
Days 1-2...

Low pressure developing near the VA/NC coast early Monday will
lift northeast south of Long Island and off the New England coast
through Tuesday. This low will develop and deepen in response to a
shortwave moving out of the Midwest and increasing diffluence from
a subtropical jet streak pivoting northeast. As the low lifts
towards the benchmark 40N 70W, strong warm air advection will
spread northward through the northern Mid-Atlantic and northeast.
Initially, the column will be cold enough for all snow from the
Laurel Highlands of PA, northeast into New York and New England.
The strong WAA will likely produce a burst of snow across these
areas, but persistent warming of the column will gradually change
the p-type from snow to rain across PA, southeast NY, and 
southern New England.

The heaviest snowfall is likely in the elevated terrain of eastern
New York and central New England, where WPC probabilities are high
for 4 inches. The heaviest snow amounts, possibly exceeding 6
inches, are expected in the Catskills, Berkshires, Southern
Greens, southern Adirondacks, Wapacks/Monadnocks of southern NH,
Northern Worcester Hills, and potentially into the White Mountains
of NH as well.

The NAM model had the strongest cyclone and greatest QPF and
resultant snow, showing potential up to a foot of snow in the
aforementioned areas.  Both the ECMWF and Canadian global had
slightly higher QPF amounts on the 00z run with more snow.  The
00z GFS was the southern outlier with the storm track and given
the least weighting, given the 00z GEFS Mean low track north of
the GFS better matched the Canadian/Ukmet/ECMWF/Nam Conus Nest.

The precip type uncertainty persists across the valley areas of
northeast PA to southeast NY and interior New England, with
multiple precip types possible.


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

Later today, a closed upper low will move slowly east towards the
CA coast.  South of the low is expected to be a 130kt subtropical
jet that streams across southern CA. This jet produces strong
ascent and moisture fluxes into California. As 700mb flow becomes
increasingly orthogonal to the Sierra, heavy snow will develop and
spread northeastward. WPC probabilities on D1/Sunday are high for
8 inches, with localized amounts over 12 inches likely.

As the 700 mb wave progresses inland into the southwest, both the
moisture plume and ascent cross NV into UT and western CO, so
light snows should pick up Mon in the UT Wasatch and then
mountains of western CO.  Given modest ascent and a deamplifying
mid level wave, potential for heavy snow is limited, and WPC
probabilities for 6 inches are low and limited to the highest
peaks of the Wasatch, Uintas, and Colorado Rockies.

On Tue, the models show an amplifying trough off the west coast. 
700 mb flow is confluent across CA/NV/UT/western CO, with several
areas peaking near 90 percent.  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.  There is potential for up to a foot of snow in the central
to northern CA Sierra Nevada mountains due to closer proximity to
the Pacific moisture source.

...Mountains of the Pacific Northwest to the Northern Rockies...
Day 2/3...

A closed mid-level low will drop southward from the northeast
Pacific Monday towards Washington State, accompanied by a weak
surface low and modest upper level diffluence. Moist advection
will stream eastward well inland from Washington through Montana
and Wyoming south of the mid-level feature.  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 for 6 inches
are moderate for the Olympics and Cascades of WA and OR, and
eastward into the Sawtooth range of ID.
The probabilities on Day 3/Tue expand east into the Glacier
National Park and mountains of south central MT and northwest WY
as overlapping high 700 mb relative humidity and periods of ascent
occur.

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

Petersen