Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 AM EDT Sat Mar 21 2020

Valid 12Z Sat Mar 21 2020 - 12Z Tue Mar 24 2020



...Northeast...
Day 3...

A mid level shortwave moving through the Ohio Valley/Great Lakes
will produce a swath of light snow on Sun and then lift northeast
towards New England while slowly beginning to tilt negatively. A
developing surface low is likely to deepen as it heads E/NE to
south of New England, potentially reaching near the Benchmark 40n
and 70w. As this low deepens, warm advection will spread
northward, with the warm air rising over the dome of cooler air
entrenched at the surface from northeast PA across eastern NY and
New England.

The column will initially be cold enough for snow from central PA
across eastern new York through New England, the exception
potentially being along the immediate coast. However, as the low
deepens and moves northeast, the warm air advection will
eventually transition the precip type to rain over the valleys
from northeast PA across southeast NY and southern New England.

WPC probabilities are high for 4 inches from the Poconos, through
the Catskills, and Berkshires, southern Green Mountains, and NH
Monadnocks with lower probabilities elsewhere. There remains
uncertainty into the intensity of track of this low, but
significant snowfall is becoming more likely just inland from the
urban I-95 corridor Sunday night through Tuesday. There is
potential fr 4-8 inches of snow across the favored terrain in the
aforementioned mountains.

CO San Juan Mtns Day 1...
The persistent upper low over the Great Basin will gradually wind
down and shear out to the east through Sunday.
Downstream from the low, an upper level jet maximum crosses NM. 
This produces difluent flow with a modest upper divergence maximum
that cross southern CO today. A 700 mb wave crossing southern CO
produces a period of light snow in the San Juan mountains of
southwest CO.  The wave's departure leads to decreasing snow
coverage. Most areas are expected to receive 3-5 inches of snow.

...Mountains of southern CA Day 2 into the southern Great
Basin/UT/western CO Day 3...
On Sunday, an upper low will drift 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 D2/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 deamplfying
mid level wave, potential for heavy snow is limited.

...Mountains of the Pacific Northwest to the Northern Rockies Day
3/Monday...
An amplifying mid-upper level wave digs south from the eastern
Pacific and British Columbia south across WA/OR on Monday.  This
drives a jet maxima south across WA into OR with moisture fluxes
streaming inland across OR into ID/WY. 700 mb convergence maxima
crossing the WA/OR Cascades as the 700 mb front moves through
result in confidence in snow developing and accumulating several
inches.  The 700 mb convergence maxima crossing the ranges of ID
(such as the Clearwater Mountains) also support snow there as well
as the GFS shows a defined 300 mb jet maxima and associated
divergence crossing ID Mon afternoon/evening. There is potential
for several inches of snow, supported by the 00z
GFS/Canadian/ECMWF/NAM QPF and precip types.  

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

Petersen