Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Tue Dec 05 2023

Valid 12Z Tue Dec 05 2023 - 12Z Fri Dec 08 2023

...Central and Southern Appalachians...
Days 1-2...

An upper level disturbance and associated low pressure circulation
over the Middle Mississippi Valley this morning will initially
produce light snow in the central Appalachians Tuesday afternoon
and into Tuesday night. However, as a trailing upper level
disturbance over the TN Valley catches up to the primary shortwave
trough tracking into the central Appalachians this afternoon,
longwave troughing aloft will deepen and northwesterly 850mb flow
will introduce added moisture flux into the higher terrain. With
temperatures aloft well below freezing and a favorable wind regime
for upslope enhancement, look for snowfall rates to increase
Tuesday night and continue into Wednesday morning. Snow should
taper off by Wednesday night as the best forcing and moisture
quickly moves off the Mid-Atlantic coast. Snow accumulations will
generally be observed above 2,000ft, but minor totals below
2,000ft are not out of the realm of possibility. WPC probabilities
for >4" of snowfall show moderate chances (40-60%) along the
windward slopes of east-central West Virginia's Potomac Highlands.
The latest WSSI shows Minor Impacts from east-central WV and
western Maryland to as far north as the Laurel Highlands,
suggesting there could be hazardous travel conditions (snow
covered roads and reduced visibilities) in these areas Tuesday
night and into the day on Wednesday.

...Pacific Northwest into the Northern Rockies...
Days 2-3...

The Pacific Northwest and Northern Rockies can expect a prolonged
period of unsettled weather thanks to an active Pacific jet stream
pattern directing storm after storm at the northwestern U.S.. An
upper level disturbance ejecting out of the base of the longwave
trough in the northeast Pacific will be the next feature that
forces snow levels to fall low enough for snow to become the
dominant precip type in mountainous terrain by Wednesday
afternoon. As the cold front clears the region and 700-500mb
height falls ensue, snow will pick up in intensity first along the
Cascade Range and as far south as the Shasta and Salmon Mountains
of northern California, then over the Blue Mountains of northeast
Oregon and both the Boise and Sawtooth of central Idaho. Snow
levels in the Pacific Northwest will be as low as 3,000ft in parts
of western Washington by Thursday morning with similar snow levels
anticipated in the Northern Rockies by Thursday night. In
addition, to the falling snow levels, as the trough advances
inland, the left-exit region of a 250mb jet streak and a
continuous supply of 700mb moisture flux will be present. This
provides strong vertical ascent beneath the left-exit region aloft
as well as a sufficient moisture source for snow. Add in favorable
upslope flow for many mountain ranges and this setup should lead
to heavy snowfall as far inland as the Tetons of western Wyoming.

WPC probabilities show a high chance (>70%) for snowfall >6" in
the Cascade Range, the Boise and Sawtooth, the Lewis Range of
northern Montana, and the Tetons south of Yellowstone NPS late
Wednesday and through Thursday. The Oregon Cascades and the Boise
Mountains most notably have a moderate-to-high chance (50-70%) of
seeing >8" amounts. The latest WSSI shows mostly Minor impacts for
most of the ranges mentioned above, although some Moderate Impacts
are highlighted in the highest terrain of northern California.
Travel on roads and passes may be treacherous due to a combination
of snow covered roads and reduced visibilities, so motorists
should exercise caution while traveling in these mountain ranges
Wednesday and Thursday.

Mullinax