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

Valid 00Z Fri Oct 21 2022 - 00Z Mon Oct 24 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 and 850 mb moisture
flux +2 sigma according to the 12z GEFS) eastward into the Rockies
to rapidly moisten the mid-levels of the atmospheric column while
also driving 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 (Day 3). By later
Day 3, 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 this time frame, 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. These expected snow
ratios also continue to be notably lower than the NBM and other
12z guidance.

As far as accumulations, heavy accumulating snowfall is likely to
begin late 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
Saturday 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 Yellowstone region of northwest WY, and
southwest MT, 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 to linger into Day 4 (Monday) for
parts of the Central and Northern Rockies, with the potential for
impactful snow to expand into the Northern Plains as well.

The probability of significant icing is less than 10 percent.

Weiss/Snell