Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 AM EST Tue Nov 21 2023

Valid 12Z Tue Nov 21 2023 - 12Z Fri Nov 24 2023

...Northeast...
Days 1-2...

Multi-stream positively-tilted trough over the MS Valley this
morning will lift into Ontario this evening and then turn eastward
across northern New England on Wednesday. High pressure over
Quebec has planted cold air over much o the Northeast today with
the precipitation shield expanding across the Ohio Valley ahead of
a frontal boundary and parent low moving through the Mid-South and
Midwest. As the system tracks northeastward, the evolution is a
fairly typical Miller-B setup, with mild air surging northward
west of the Appalachians via the initial surface low while colder
air holds on at the surface along and east of the mountains from
the Mid-Atlantic northward. By Wednesday morning, the parent low
will weaken and dissipate over the St. Lawrence Valley as a new
low deepens across the DelMarVa and becomes the new main low as it
continues northeastward across southeastern MA and toward Nova
Scotia by early Thursday.

A mix of snow, sleet, and freezing rain day 1 over the interior
Mid-Atlantic will gradually transition to plain rain as southerly
flow eventually scours out all the cold air at the surface just
ahead of the front. Icing may accumulate over a tenth of an inch
from eastern WV northward across far western MD and into central
PA. Probabilities for more than a tenth of an inch are around
20-50%. The wintry mix will continue through much of interior NY
including the Adirondacks where the deeper cold air will support
snow at onset but transition to freezing rain as warmer air moves
in aloft. As the new coastal low deepens into day 2, the flow will
turn to easterly and northeasterly rather than southerly, allowing
snow to remain dominant closer to the Canadian border and the
mixture of sleet and freezing rain to diminish. Snow rates could
exceed 1-1.5"/hr over northern NY into the Green and White
Mountains per the 00Z HREF probs and WPC snowband tool. Total
two-day accumulations may be heaviest over northern NH
northeastward across interior Maine. In addition, low SLRs would
increase the chance of impacts due to heavier snow load, per the
WSSI. There, WPC probabilities for at least 4 inches of snow are
high (>70%) and for at least 8 inches are 20-50%. Over the North
Country in NY, WPC probabilities for at least 0.25" icing are low
(generally near or below 10%), but are moderate (>40%) for at
least 0.10" of ice.


...Northern Rockies to Central Rockies...
Days 2-3...

Mid-level flow out of the northeastern Pacific will dig into the
Pacific Northwest and northern Rockies late Thursday and close off
into an upper low over the northern Great Basin by early Friday
(end of D3). At the surface, a weaker Pacific system will push
inland day 2 as a stronger cold front dives southward and eastward
through Montana. This front will slow around the Divide which will
enhance low-level convergence on increasing easterly flow out of
the northern Plains as the RRQ of the upper jet skirts eastward,
providing broader lift to the region. Snow will expand and change
over from rain as colder air moves southward on Thursday.
Continued upslope flow and 700 FGEN forcing will maintain and
increase snow over much of southern and southeastern MT into WY
Thursday into early Friday. Overlap of favored DGZ with vertical
ascent will likely support rates nearing 1"/hr and totals well
over 6 inches in at least the mountains. WPC probabilities for at
least 8 inches of snow days 2-3 are maximized over the Bighorns
and northern Absarokas into south central MT as well as the Wind
River Range. Even at lower elevations, accumulating snow is likely
for much of Wyoming westward across the southern Idaho ranges into
the northern Wasatch and Uintas as the upper low tracks near that
region by the end of the period. WPC probabilities for at least 4
inches are moderate (40%-70%) over southeastern WY into the Black
Hills and extending northwestward along and east of the Divide up
to Glacier NP.


Fracasso