171
FOUS11 KWBC 222030
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 PM EST Fri Nov 22 2024

Valid 00Z Sat Nov 23 2024 - 00Z Tue Nov 26 2024

...Pacific Coast through the Interior Northwest and Great Basin...
Days 1-3...

Persistent closed low centered off the Pacific Northwest coast
will remain energized by vorticity maxima swinging through the
associated trough this period. This will result in amplification of
the gyre, noted by an expansive region of 500-700mb height
anomalies reaching below -1 sigma, and approaching -2.5 to -3 sigma
in the core of the trough. Beneath this trough, surface low
pressure off the WA coast tonight will continue northward then
northwestward, resulting in persistent onshore and divergent mid-
level flow. The accompanying upper level jet energy will help
spread moisture onshore as a continuation of the ongoing
atmospheric river (AR) across northern CA D1. While IVT values
exceeding 500 kg/m/s wane and pivot southward tonight, this will
still result in plentiful moisture and widespread moderate to at
times heavy precipitation pushing onshore the Pacific Coast into
the interior Northwest.

Snow levels within this AR will be quite high, generally 6000-8000
ft, except east of the WA Cascades tonight where they will remain
entrenched around 4000 ft before rising. This indicates that most
of the heavy snow will be above pass levels, except in the higher
Cascade Passes D1, and the Sierra Passes D1-D2. WPC probabilities
D1 are >50% for more than 6 inches of snow in the northern WA
Cascades, the Salmon River Range, and portions of the Northern
Rockies near Glacier NP. Into D2, snowfall will be a bit more
expansive but overall lower in magnitude. Farther south, the Sierra
will be favorably located to be impinged upon by moisture by
starting tonight, leading to pronounced upslope snow. WPC
probabilities are high (>70%) for 12+ inches above 7000ft or so
into Saturday, with 2 feet possible above 8000 ft.

For D2-3, snow levels crash again as a cold front pushes onshore,
but this is accompanied by reduced moisture advection as the column
begins to dry out until the end of D3. Some moderate snow will
persist through onshore flow, especially in the Sierra where an
additional 6-12 inches is possible, but otherwise WPC probabilities
for more than 6 inches are between 30-50% and focused across the
Wasatch, Uintas, and ranges of NW WY. By Monday afternoon, the next
surge of moisture will push into central CA and straight into the
Sierra where moderate snow amounts will likely continue into D4.


...Appalachians and Northeast...
Days 1-2...

Stacked/wrapped upper and sfc low over the Northeast will continue
its counterclockwise loop through Saturday before lifting out
through Atlantic Canada. Upslope snow into the central Appalachians
will persist through Saturday evening via NW flow as moisture has
circulated from the Atlantic up and around southeastern Canada back
across the lower Great Lakes. WPC probabilities for at least
another 4 inches of snow D1-2 are >50% above 3000ft or so across
eastern WV.

Into New England, trailing vorticity lobe will spur a new surface
low southeast of Cape Cod tonight that will track northward,
favoring increased northerly/cooler flow over interior portions of
Maine southwestward into VT. The column is fairly mild to start
over New England via SE flow ahead of the initial front, but that
will change overnight with rain changing to snow as the low
approaches and deepens. QPF will be focused closer to the coast,
but light amounts of snow are likely for the interior and
especially the higher terrain above 2500ft or so (White Mountains
and northern ME) where WPC probabilities of at least 4 inches are
>40%.


...Northern Rockies through the western Northern High Plains...
Days 1-2...

A shortwave ejecting through the base of an anomalous trough off
the Pacific Coast will swing eastward into the Northwest Saturday
and then begin to amplify into a potent but fast-moving impulse
across the Northern Rockies and into the Northern Plains Sunday.
This deepening system will drive downstream ascent through height
falls, PVA, and divergence. At the same time, a zonally-oriented
jet streak will pivot south of the region, placing its favorable
LFQ atop the far northern High Plains, interacting with increasing
baroclinicity as a front wavers near the international border. The
overlap of synoptic lift with the enhanced mesoscale ascent through
fgen will result in weak cyclogenesis, leading to an expanding
area of precipitation falling as snow. The column will be plenty
cold for wintry precipitation thanks to high pressure from Canada
extending southward. As the WAA along the warm front lifts north,
an exceptionally deep DGZ will support fluffy aggregate dendrites
which should accumulate efficiently, but the most favorable
environment and most impactful snowfall will likely be just north
of the U.S.-Canada border.

The heaviest snowfall is likely in the terrain near the Northern
Rockies where upslope flow will enhance ascent, and WPC
probabilities support a high risk (>70%) for at least 8 inches near
Glacier NP. With a drier trend farther east, WPC probabilities for
at least 4 inches of snow are <40% along the Canadian border.


The probability of significant ice across the CONUS is less than
10 percent.


Fracasso/Snell







$$