Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
223 PM EST Tue Nov 14 2023

Valid 00Z Wed Nov 15 2023 - 00Z Sat Nov 18 2023

...Northern Rockies...
Days 1-2...

An amplifying upper trough tracking south and east from British
Columbia will coincide with a cold frontal passage to provide a
sufficiently cold enough air-mass for snow across portions of the
Northern Rockies. Moisture will come in the form of a ribbon of
700mb moisture flux originating off the Pacific Northwest coast
and streaming east over an area of 700mb frontogenesis Wednesday
night. During the peak of the event, snowfall rates could range
between 1-2"/hr in the Bitterroots and Lewis Range. This is
highlighted in the 12Z HREF which showed high chances (80-90%) for
parts of these ranges seeing >1"/hr snowfall rates. WPC
probabilities show a moderate-to-high risk (70-90%) for snowfall
totals >6" in the Lewis Range, which includes the Marias Pass.
There are even moderate chances (40-60%) for snowfall totals >12"
in and around Glacier National Park. Lesser amounts are
anticipated in the Bitterroots, but still, probabilities sport a
Moderate risk (40-60%) for >4" of snowfall. The WSSI generally
shows Minor Impacts in the affected ranges, however Moderate
Impacts are possible in the highest peaks of Glacier National Park
and the Swan Range south of Kalispell, MT.

...Sierra Nevada...
Day 2...

An upper low off the California coast will direct a surge of
subtropical Pacific moisture northeast towards the "Golden State"
and lead to strong upslope flow into the southern and central
Sierra Nevada Wednesday evening. NAEFS shows an atmospheric river
(AR) directing an impressive 750 kg/m/s IVT at northern Baja.
Central CA will not see direct impacts from that branch of the AR,
but IVT values will still be above the 90th climatological
percentile 00Z Thurs. The air-mass in the West is quite marginal,
making this particular event highly elevation dependent. The
heaviest snowfall totals will be confined to elevations above
9,000ft with WPC probabilities showing moderate chances (50-70%)
for >4". Above 10,000ft, some snowfall totals could top 8" in
spots. Snow is likely to taper off by Thursday morning.

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

Mullinax