Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 AM EDT Thu Oct 20 2022

Valid 12Z Thu Oct 20 2022 - 12Z Sun Oct 23 2022

...Pacific Northwest through the Central Rockies...
Days 2-3...

Early season winter event likely to develop across the Pacific
Northwest and then spread across the Northern and Central Rockies
as a mid-level trough amplifies. The lead shortwave responsible
for this system will drop along the British Columbia coast and
into Washington State Friday morning, and then broaden/deepen into
the Great Basin by Saturday. Lobes of vorticity circulating around
this trough will combine with height falls and upper diffluence
within a complex jet structure to drive impressive synoptic ascent
across the region. At the same time, a lead Pacific jet streak
will surge a modest AR (IVT 300-500 kg/m/s) eastward into the
Rockies to rapidly moisten the mid-levels of the atmospheric
column and drive a ribbon of PW anomalies above +2 sigma according
to NAEFS ensemble tables. The overlap of the deep layer ascent
with the increased moisture will result in widespread
precipitation spreading across the region, initially in the
Pacific Northwest and then expanding into the Rockies on Sunday
/D3/. By later D3, theta-e ridging will begin to intensify across
the Plains as a surface low pressure system strengthens in the lee
of the Rockies. While there remains considerable spread into the
placement and intensity of this feature by D3, it is likely that
some enhanced low-level moisture will begin to wrap back into this
system with a possible TROWAL developing aloft. This suggests an
enhancement in QPF/snowfall along easterly/upsloping terrain
beginning D3, especially from the Uintas northward into the
Absarokas and Little Belts.

The thermal profiles for this event will gradually cool as a cold
front surges eastward, and snow levels are progged to fall to as
low as 3500 ft across WA, and to around 4500 ft in MT/WY. Despite
this, the airmass is generally sourced from marginally cold
Pacific air, and SLRs will likely be only 8-10:1, higher in the
highest terrain, but still only around the 25th percentile
according to the Baxter climatology. This is reflected by pWSSI
probabilities that are driven most strongly by snow load, further
indicating that heavy snowfall will be of the wet variety and
impactful even where accumulations are lower.

As far as accumulations, heavy accumulating snowfall is likely to
begin D2 /Friday/ across the Washington Cascades and Spreading
into the Northern Rockies where WPC probabilities for 6 inches
reach 30-50%, but generally remain above pass levels. However, by
D3 as snow levels continue to fall and forcing/moisture become
more significant, WPC probabilities for 6 inches reach above 60%
in the Oregon Cascades, as well as across the Uintas, Northern
Rockies, and most impressively in the ranges surrounding the
Absarokas and NW WY where more than 12 inches of snow is likely in
the higher terrain. Significant accumulations of heavy wet snow
are likely at many of the passes above 5000 ft, including Marias,
Reynolds, and Willamette, where there is a moderate to high risk
for more than 4 inches according to WPC probabilities.

Additional heavy snow is likely into D4 as well for parts of the
Central and Northern Rockies and expanding into the Northern
Plains.

The probability of significant icing is less than 10 percent.

Weiss