Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
314 PM EDT Thu Nov 03 2022

Valid 00Z Fri Nov 04 2022 - 00Z Mon Nov 07 2022


...Four Corners and Central Rockies...
Day 1...

Shortwave trough will continue to dig through the base of an
anomalous longwave trough centered over the Great Basin through
tonight and is expected to close off east of the Four Corners
region by Friday morning before taking on a negative tilt and
quickly ejecting into the Plains through the end of the Day 1
period. Height anomalies are between -2 and -3 sigma. Strong
height falls, downstream divergence with accompanying PVA, and
increasing upper diffluence within the left exit region of a 130kt
poleward streaking jet will produce strong UVVs, and lead to lee
cyclogenesis later today across Colorado. As the lee low develops,
moist flow originating from the Gulf will lift isentropically
along the 295-300K surface with additional lift due to upslope
enhancement. There is the potential for heavier snow rates, on the
order of 1-2"/hr rates, across much of Colorado west of the Front
Range and into southern Wyoming thanks to the overlap of
saturation in the DGZ and stronger lift. Crashing snow levels as
colder air seeps south will bring the threat of accumulating snow
eastward into the High Plains including the I-25 urban corridor
where a few inches may accumulate.

Based on the latest WPC probabilities, the heaviest additional
snowfall will be found for the higher peaks with additional
accumulations of 2-4 inches likely (isolated higher amounts up to
6-8" possible). Across the Front Range, High Plains, and into
Nebraska panhandle, WPC probabilities for 2" remain slight in the
20-30 percent range.


...Pacific Northwest through the Northern Rockies...
Days 1-3...

Impressively confluent mid-level flow angling onshore from the
Pacific will combine with a zonally oriented upper jet streak
reaching 170 kts to drive a robust atmospheric river (AR) onshore
the Pacific Northwest and into the Northern Rockies late this
week. IVT from CW3E is progged by both the GFS and ECMWF to exceed
750 kg/m/s, possibly approaching 1000 kg/m/s, which is nearly 4
sigma above the climo mean according to the NAEFS ensemble tables.
This will support widespread heavy precipitation in the Pacific
Northwest, with moisture spilling over into the Northern Rockies
as well. Dual shortwaves embedded within this AR and confluent
flow will help to drive periods of more intense ascent, the first
moving onshore Friday night. With the AR being of Pacific origin,
and strong WAA preceding the lead shortwave, snow levels will
climb to 7000-9000 ft Friday, before falling to 2000-3000 ft by
the end of the period as a cold front gets dragged southward
behind a low pressure moving along the Canadian border. This will
leave the heaviest snow above pass levels through the event, but
moderate accumulations down to the passes are likely.

By Day 3, the anomalously deep 500 mb low, closing off just
offshore British Columbia, will sag southward, bringing another
reinforcing shot of moderate to locally heavy precipitation to the
Pacific Northwest through Sunday night and then southward into
northern California Sunday night into Monday. Snow levels will
begin to crash with the approach of this system (down to 2000-3000
ft), bringing more widespread snowfall accumulations to much of
the WA and OR Cascades and northern CA ranges.

WPC accumulation probabilities are high for 6+ inches in the WA
Cascades with more than 12 inches possible in the highest terrain,
and are also high for 6"+ eastward across portions of the Northern
Rockies, Bitterroots, Tetons, Wind Rivers, and Blue Mountains
where isolated amounts up to 2 feet are possible. With the second
wave of precipitation arriving Sunday into Sunday night, WPC
probabilities for 8" or more are 70+ percent for the WA and OR
Cascades.



The probability of significant icing greater than 0.25" is less
than 10 percent.

Weiss/Taylor