Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
150 PM EST Mon Dec 04 2023

Valid 00Z Tue Dec 05 2023 - 00Z Fri Dec 08 2023

...Central and Southern Appalachians...
Day 2...

A reinforcing shortwave to the eastern CONUS longwave trough will
dive from the Upper Midwest Tuesday to the Mid-Atlantic Wednesday,
eventually driving the trough axis off into the Atlantic by
Wednesday night. This evolution will push a clipper-type surface
low southeast beneath it, with a secondary low likely developing
off the NC/VA coast by early Wednesday in response to the height
falls and a strengthening jet streak offshore. To the E/NE of this
clipper, a stripe of WAA collocated with some modest mid-level
deformation will result in an axis of light to moderate snow from
the Upper Midwest through the southern Great Lakes and into the
Central Appalachians, but amounts are expected to be light.
However, as the clipper weakens Wednesday morning and the
secondary low develops offshore at the same time the trough axis
moves east, intensifying NW flow in its wake will drive increasing
upslope flow into the Central and Southern Appalachians. Analysis
of forecast soundings during this time indicate that as the
temperatures cool in the CAA, the DGZ will fall while low-level
lapse rates steepen, especially Wednesday morning. Continuing
saturation within this DGZ and increasing upslope ascent is likely
to result in a period of moderate, to at times heavy, snowfall,
before the column dries out considerably late D2. Snow levels
early D2 will be around 2000 ft, but should fall quickly to
500-750ft before the moisture erodes. Still the heaviest snow
should be above 2000 ft where WPC probabilities for more than 4
inches reach above 50% in WV, and above 50% as well in the highest
terrain of NC, but light snow accumulations are possible down
below 1000 ft before snowfall winds down into D3.


...Pacific Northwest into the Northern Rockies...
Days 1-3...

An extended period of unsettled weather is likely as an
atmospheric river (AR) persists into D2 with high probabilities
for IVT exceeding 750 kg/ms shifting into WA/OR before waning
quickly after 00Z Wednesday. Even after the peak IVT falls into
D2, CW3E suggests high probabilities for at least a weak AR (IVT
of 250 kg/ms) through D2, but focused farther south into OR, with
waves of moisture continuing even through D3.

The most intense AR today will be oriented SW to NE, and
accompanied by very warm snow levels of above 9000 ft before a
shortwave digs southward and drives a cold front across WA
Wednesday aftn. This will cause snow levels to drop to around
6000-8000 ft by D2, but then fall more rapidly to 3000-5000 ft by
D3. Although the IVT will also weaken, several impulses embedded
within the increasingly zonal flow will drive periods of
impressive ascent within a still moist column characterized by PWs
according to the NAEFS ensemble tables remaining above +2 sigma
into Thursday, and still elevated into Friday. This ascent will
also be enhanced at times by modest upper diffluence within the
departing RRQ of a jet streak Tuesday/Wednesday, with the LFQ of
an approaching secondary jet streak helping with some lift
Wednesday night into Thursday. This overlap of ascent and moisture
will result in waves of precipitation, some of which could be
heavy, especially in the terrain where upslope flow will maximize
on the increasingly zonal flow. As snow levels collapse, this will
be heavy snow in the higher terrain, with pass-level impacts
returning by D3. WPC probabilities for more than 4 inches of snow
both D1 and D2 are quite limited in areal coverage, confined to
just the higher peaks of the Cascades and Shasta/Trinity/Siskiyou
regions. However, by D3, WPC probabilities for more than 4 inches
expand considerably and increase to 80% or more for much of the
WA/OR Cascades, the northern CA ranges, and extend east into the
Northern Rockies including the Salmon River/Sawtooth ranges and
areas around Yellowstone N.P. Locally, more than 12 inches of snow
is possible in the higher terrain on D3, with notable snowfall
accumulations likely returning to the passes as well.

Additionally, as the WAA overruns a cold but retreating high
pressure into Canada and the Northern Plains, light easterly flow
should maintain some cold air wedged at the surface to produce a
favorable setup for freezing rain. Total ice accumulations are
expected to be modest as the WAA eventually erodes the surface
cold air, but WPC probabilities for more than 0.1 inches of ice on
D1 are 10-20%.


Weiss