Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 PM EDT Wed Oct 16 2024

Valid 00Z Thu Oct 17 2024 - 00Z Sun Oct 20 2024

...Pacific Northwest...
Day 1...

An amplifying shortwave digging south from the British Columbia
coast will advect across OR Thursday morning before continuing
farther south into the Great Basin by the end of D1. This will help
push a cold front farther to the southeast and into the
Intermountain West, providing cooling temperatures and falling snow
levels to as low as 3000-3500 ft. PWs will gradually fall behind
the front as the greatest IVT and accompanying 700-500mb RH surges
eastward, but there will still be sufficient overlap of residual
moisture and ascent (through mid-level height falls, LFQ upper jet
diffluence, and upslope flow) to wring out periods of light to
moderate snow across the Olympics and Cascades. WPC probabilities
for more than 4 inches are confined to the higher terrain of the WA
and OR Cascades, but reach 70-90%, and locally as much as 10" of
snow is possible along the highest peaks. With the snow levels
falling, some light accumulations are also possible at the Cascade
Passes, including Stevens and Washington Passes.


...Central Rockies...
Days 2-3...

**First Significant Winter Storm of the Season to Impact the
 Central Rockies**

A shortwave dropping out of Oregon on Thursday will dig into the
Great Basin Friday and then deepen even more impressively across
the Four Corners by Saturday morning as reflected by NAEFS 500mb
height standardized anomalies approaching -3 sigma. This evolution
will force impressive downstream ascent through mid-level
divergence and height falls, overlapping with coupled jet streaks
to produce intense upper diffluence. At the same time, a cold front
will be pushing eastward beneath the evolving mid-level trough,
with a concurrent baroclinic zone intensifying in response to this
frontal evolution and the position of the upper jet streak. The
resultant frontogenesis and synoptic lift will combine with
periodic upslope flow into terrain feature to drive widespread and
impressive ascent across the Central Rockies from late D1 into D3.

This deep layer lift will occur into an environment that will
gradually moisten as PWs surge to as high as +2 to +3 sigma
according to NAEFS ensemble tables. This will manifest as an
expanding area of precipitation, beginning in the Great Basin
Thursday afternoon before overspreading the Central Rockies by
Friday morning. The ample moisture and pronounced ascent,
especially in areas of upslope or strong fgen will likely result in
areas of heavy precipitation rates as well, leading to ample QPF
through the period.

Snow levels ahead of the upper low will be quite warm, generally
9000-10000 ft using the NBM 50th percentile as a proxy. However,
rapid cooling beneath the approach of the upper low and behind the
cold front should drive snow levels to as low as 4000 ft across the
Great Basin and WY, 5000-7000 ft in UT, and 8000 ft in CO. With
precipitation ongoing and the column cooling, this will result in a
changeover from rain to snow, with heavy snow likely above these
levels, and the WPC prototype snowband tool suggests snow rates
could reach 1-2"/hr, which is supported by the presence of
instability during periods of greatest ascent. These convective
rates could also drag down some colder air, and light
accumulations, at least periodically, are possible well below the
NBM 50th percentile snow levels.

On D1, heavy snow will begin across the Absarokas, and WPC
probabilities for more than 6 inches reach 30-50%. During D2, the
heavy snow becomes much more widespread, reaching as far east as
the Big Horns, and as far south as the San Juans. It is generally
D2 into early D3 when the heaviest snow is likely, but significant
accumulations are likely much of D3 across the San Juans. 2-day
snowfall has a higher than 50% (80%) chance of exceeding 12 inches
in the Uintas (San Juans), with local maxima of 2-3 feet possible
in the highest terrain. These areas are also where the greatest
impacts are expected as the combination of modest SLR but heavy
precipitation causes WSSI-P to feature even a low potential for
major impacts during this early season storm.


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

Weiss