Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
126 PM EST Sun Feb 05 2023

Valid 00Z Mon Feb 06 2023 - 00Z Thu Feb 09 2023


...Western U.S....
Days 1-3...

Two distinct troughs moving into the West through the middle of
the upcoming week will spread dual atmospheric rivers (AR) onshore
to result in waves of heavy snow, especially across the higher
terrain.

The first of these impulses is moving onshore CA this morning as a
sharpening longwave trough, with its axis surging into the Great
Basin tonight. This feature will become increasingly sheared out
to the northeast in response to northern stream energy surging
into the Upper Midwest to split the trough, but maintain a long SW
to NE oriented axis from the Four Corners into the Great Lakes by
D2. The southern end of this trough is then progged to deep into a
closed low over the Four Corners, providing additional ascent into
the Southern Rockies and southern High Plains. Confluent flow
south and downstream of this trough combined with the intensifying
sub-tropical jet streak will drive high PWs and an AR from CA into
the Southwest, characterized by IVT of around 250 kg/m/s. While
the most intense ascent across CA will weaken during D1, broad,
large scale lift will overspread much of the region from the
Cascades through the CO Rockies, resulting in moderate to heavy
snow above generally 2500-4000 ft across the West, with some
enhanced snowfall rates of 1-2"/hr likely at times early on D1 in
the Cascades, Blue Mountains, and Wasatch where upslope flow and
some mid-level fgen will overlap. WPC probabilities for more than
6 inches of snow are moderate to high for a smattering of terrain
from the WA Cascades through the CO Rockies. In general, the
heaviest snow will be above 4000 ft and focused in upslope terrain.

A second impulse will then approach the Pacific Northwest coast
Tuesday aftn /D2/, and while this feature is progged to race
eastward thanks to confluent and progressive 500mb flow surging
into the coast, it will provide robust ascent through height falls
and PVA, in a region of impressive moisture, which will persist
even beyond the most intense forcing as the confluent flow
persists behind the shortwave. IVT is progged by both the ECENS
and GEFS to exceed 500 kg/m/s into WA/OR during this period, and
the guidance has become increasingly robust with its snowfall
potential Tuesday and Wednesday before ridging blossoms across the
Pacific coast late in the forecast period. Strong WAA associated
with the warm front will drive snow levels to around 5000 ft along
and west of the Cascades Tuesday, coincident with the heaviest
precipitation, but the subsequent cold front and CAA will drive
snow levels down below 2000 ft Wednesday with continued at least
moderate snowfall. This could produce significant pass level
impacts which is supported by high pWSSI probabilities for
moderate impacts due to snow rate near Snoqualmie, Stevens, and
Lookout Passes. WPC probabilities for 6+ inches of snow are above
80% in the northern WA Cascades and Olympics on D2, with 50-80%
probabilities extending into the OR Cascades, Northern Rockies,
and Bitterroots by D3.

The probability of significant icing is less than 10 percent.

Weiss