Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 PM EST Tue Nov 24 2020

Valid 00Z Wed Nov 25 2020 - 00Z Sat Nov 28 2020

Days 1 to 3...

...Pacific Northwest/Northern Rockies/Intermountain West...
An upper-level trough will be advancing inland across the Pacific
Northwest and the northern Rockies going through tonight and
Wednesday. The surge of Pacific moisture aided by the arrival of a
cold front, and the larger scale forcing with the upper trough
should yield moderate to heavy precipitation over the Washington
and Oregon Cascades. Snow levels will be lowering as the upper
trough and associated height falls cross the region, and this
should result in heavy accumulating snowfall for the higher
terrain. As much as 6 to 12 inches of snow is expected going
through Wednesday with locally heavier amounts over the favored
west-facing slopes. By Wednesday night and Thursday, the bulk of
the energy aloft and the influx of mid-level moisture should
fostering accumulating snow over the northern Rockies and adjacent
areas of the Intermountain West. This will include as much as 6
inches of snow for the Sawtooth, Tetons, and Wind River range with
even a few inches of snow possible farther south down into the
Wasatch. By Friday, the latest model guidance supports the idea of
an upper low digging down into the Four Corners region and this
should bring a threat for locally a few inches of snow to areas of
southern UT, southwest CO and northern NM.

...Central Plains/Midwest/Great Lakes/Northeast...
Initially there will be a rather weak, transient piece of energy
that will cross the Great Lakes region and the Northeast tonight
through Wednesday as a broad warm air advection pattern sets up
over the region ahead of a stronger upper-level trough and
embedded upper low ejecting east tonight across the central
Plains. The first surge of moisture and stronger isentropic
lift/ascent will favor an axis of light to moderate precipitation
with thermal rather marginal, but still sufficiently cold enough
for a few inches of wet snow to occur over northern Lower MI
tonight before warmer air arrives and allows for a changeover to
rain. On Wednesday, somewhat colder air will initially be in place
over northern New England, and locally a few inches of snow will
be possible here too with a focus on northern ME before this
impulse shears out.

With respect to the second stronger system back over the central
Plains, there will be the arrival and passage overnight of a
fairly robust mid-level deformation zone with impacts on areas of
western/central KS and up across southeast NE going through early
Wednesday morning. Thermal profiles get to be very marginal and
somewhat conducive for a rain/snow mix or a changeover to wet snow
across these areas for a few hours as stronger dynamic cooling in
the deformation zone/comma-head region of the mid/upper-level low
arrives overhead. Meanwhile, cold air advection will be increasing
in the low-levels around the back side of departing surface low
pressure. It appears that there could at least locally be a few
hours of moderate snow that may foster a small snowfall
accumulation (1 to 2 inches) before the system exits Wednesday
morning across the Midwest. By Wednesday night and Thursday, the
energy will quickly move across the lower Great Lakes region and
into the Northeast. Strong warm-air advection ahead of the
advancing surface low will bring mainly rain along its path, but
as the system encounters shallow cold air across northern New
England Wednesday night and early Thursday, some light freezing
rain will be possible before changing over to all rain.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Orrison