Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 PM EST Sat Feb 01 2020

Valid 00Z Sun Feb 02 2020 - 00Z Wed Feb 05 2020

...Western U.S...
Days 1-3...

Ridging across the west will give way to an amplifying trough
digging into the PacNW Sunday. This trough will deepen quickly as
it drops southeast and closes off in the vicinity of the Great
Basin and then moves very little through Monday. This will drive
an amplified longwave pattern across the CONUS, with Pacific Jet
energy advecting copious moisture through the region reflected by
PWAT anomalies of +1 to +2 standard deviations above the climo
mean. At the same time, a jet streak developing downstream of the
upper trough will intensify and become increasingly
anti-cyclonically curved into the Great Lakes, leaving parts of
the Mountain West in the favorable RRQ for ascent, aided by PVA
and mid-level divergence. Beneath this synoptic lift, a surface
low is likely to develop in the lee of the Rockies, and the
pinched flow between this developing system and high pressure to
the north will drive moist easterly upslope flow into the
mountains of CO/WY. Significant snowfall is likely.

On D1, despite ample moisture flooding into the West, snow levels
are quite high, 4000-7000 ft, but crashing quickly as a cold front
drops into the Northwest. Heavy snow on day 1 will be confined to
the highest terrain of the WA/OR Cascades and ranges of ID/MT
where WPC probabilities are moderate for 8 inches.

As the system becomes better organized D2-D3, snow levels will
continue to drop, reaching the valley floors as far south as
Arizona. This suggests snow will become widespread despite many
places beginning as rainfall Sunday. The closing of the upper low
will allow the 700mb low to drop very slowly southward from WY
into UT/CO, and an extended period of mid-level deformation will
combine with mid-level fgen and the upslope low-level flow to
produce an extended period of heavy snowfall, heaviest across
WY/UT/CO, although some lingering heavy snowfall is likely in the
Cascades on Day 2 /Sunday/ as well. There remains some spread in
the thermal structure despite good mass field agreement in the
models, and this forecast is based on a blend of the ECENS mean
with some contribution of the GEFS/NAM which are slightly colder.
WPC probabilities on D2 are high for 12 inches in parts of WY/UT,
and on D3 in WY/CO. 2-day snowfall in the Absarokas, Wind Rivers,
and Big Horns may exceed 2 feet, with widespread 6+" likely across
most of WY outside of the shadowed regions. During D3 heavy snow
will also likely extend eastward into the High Plains, and WPC
probabilities for 4 inches are moderate into the Panhandle of NE
and western SD.


...Great Lakes...
Day 2...

A potent but fast moving shortwave trough and vorticity lobe will
race across the Great Lakes and New England Sunday. Forcing for
ascent is strong, but short-lived, and a swath of light snow is
likely from Michigan into New England. While most of this snow is
likely to accumulate to less than 2 inches, some enhancement due
to LES of Lake Ontario and into the Tug Hill Plateau allows WPC
probabilities for 4 inches to reach 20-40%.


The probability of significant icing (0.25-inch or greater) is
less than 10 percent.

Weiss