Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Fri Oct 21 2022

Valid 12Z Fri Oct 21 2022 - 12Z Mon Oct 24 2022

...Pacific Northwest through the Central Rockies and Northern High
Plains...
Days 1-3...

An impressive and long lasting early season winter storm is
expected to begin this evening across the Pacific Northwest and
then expand through the weekend into the Rockies and eventually
the Northern High Plains. Periods of moderate to heavy snow are
likely, resulting in significant snowfall accumulations,
especially in the higher terrain above 4000-6000 ft.

The driver of this system will initially be a shortwave embedded
in confluent mid-level flow diving along the British Columbia
coast and moving onshore WA state this evening. This shortwave is
expected to rapidly amplify into a closed low and resulting sharp
longwave trough centered across the Great Basin Saturday night
before shifting into the Four Corners and elongating into the High
Plains late Sunday /D3/. This evolution will result in substantial
height falls and periods of PVA as spokes of vorticity rotate
around the primary closed low. Higher up in the atmosphere, a
complex jet evolution is likely to develop with coupled northern
stream jet maxes phasing with the subtropical jet streak as full
latitude trough amplification occurs, resulting in secondary
coupled jetting arcing into the High Plains during Sunday. This
will help drive more enhanced deep layer ascent, and lee
cyclogenesis is progged to occur Sunday east of the Rockies. This
surface low will then lift northeast while deepening, combining
with at least modest fgen as a result of the ageostrophic response
to the sharpening upper jet to drive locally even more robust
lift. While total ascent will likely be broad, pockets of upslope
enhancement are likely both on the W/SW side of terrain today and
Saturday, and then on the east side Sunday, as flow downstream of
the surface low intensifies out of the Gulf of Mexico and rich
theta-e air returns cyclonically around the low upsloping into the
High Plains into Monday.

Moisture will become significant as the Pacific jet streak advects
an AR characterized by IVT of 400-500 kg/m/s onshore coincident
with PWs of +2 sigma. However, the AR is generally transient so
that the highest moisture anomalies do not linger across any area
for an exceptionally long time. However, where the best ascent
does overlap with these higher anomalies, the result will be
rounds of heavy precipitation, which in some areas could occur
over a 48-hr period. The moisture is generally sourced from the
Pacific so is marginally cold, but snow levels are progged to fall
to as low as 3000 ft in the Pacific Northwest, 4000 ft in the
Intermountain West, and to around 1000 ft in the Northern High
Plains, resulting in heavy snow above these levels. SLRs will
likely be somewhat reduced due to the marginal thermals and
antecedent warm air, falling generally between 7:10-1, which is
below the 25th percentile according to the Baxter climatology.
This suggests snow will be more heavy and wet, which is also
supported by pWSSI that is driven most significantly by snow rate
and snow load, rather than snow amount. However, this suggests
even modest snow amounts could be impactful due to the wetter
nature of the snowfall.

The heaviest snowfall during the period is likely in the terrain
above 5000 ft centered near the Absarokas and surrounding terrain
as far south as the Wasatch and north as the Northern Rockies.
Here, WPC probabilities for more than 8 inches are above 60% both
D2 and D3, resulting in local total snowfall that may exceed 20
inches - most likely in the highest terrain of the Wind Rivers,
Absarokas, Crazies, Big Horns, and Little Belts. Otherwise, more
than 12 inches is likely by Monday across the highest terrain of
the WA and OR Cascades, as well as the Wasatch, Uintas, San Juans,
CO Rockies, and the higher terrain of ID/MT. Lower elevations down
to 3000-4000 ft could also experience several inches of snowfall.
Significant accumulations of heavy wet snow are also likely at
many of the passes above 5000 ft, including Marias, Reynolds,
Lolo, and Willamette, where there is a moderate to high risk for
more than 4 inches according to WPC probabilities.

By D3, the heaviest snow should shift into the Northern High
Plains as far east as Williston, ND, and WPC probabilities for 4
inches or more of snow are as high as 40% from the Black Hills of
SD northward through eastern MT, but an expansion of moderate snow
as far south as the Mogollon Rim and White Mountains of AZ may
lead to a few inches of snow there as well.


The probability of significant icing is less than 10 percent.

Weiss