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Winter Weather Forecast Discussion
 
(Latest Discussion - Issued 0844Z Dec 04, 2022)
 
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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 AM EST Sun Dec 04 2022

Valid 12Z Sun Dec 04 2022 - 12Z Wed Dec 07 2022

...Western United States...
Days 1-3...

A closed mid-level low will start to unwind today and eventually
open up into a positively-tilted trough on Monday (and perhaps
close off again) as it generally weakens and moves into northern
California. In the process, it will detach several vort maxes to
the east across OR/ID/northern NV eastward into the base of a
sizable upper low rotating over Hudson Bay. Moisture plume
associated with an older system to its southeast will weaken
through the day as well, but will take time to a couple days to
return to more typical values for early December. By Tuesday (Day
3), the now broader trough will push through the southern Great
Basin with less overall QPF.

For Day 1, focus for heavy snow will be over the northern and
central Sierra as the strong moisture plume to start (precipitable
water values around +2 sigma) coincides with a 110kt jet streak
into Nevada, driving snow rates >1-2"/hr. High snow levels above
7000-8000ft will crash through the day as the front moves through
and colder air rushes in from the northwest, helping to lessen QPF
as well. However, moisture associated with the upper low itself
will move in from the northwest into the northern California
ranges through the day with modest accumulations. WPC
probabilities of at least 12 inches of snow are above 50% over the
northern Sierra. Probabilities of at least 6 inches of snow are
high over the Klamath, Shasta-Siskiyous, and Trinities. Farther
north, eastern lobe of extended vorticity will move through
eastern Oregon into Idaho on the LFQ of the jet promoting modest
to locally heavier snowfall from the Bitterroots to the Tetons.
There, probabilities of at least 6 inches of snow are high (>70%).
Additional snow will fall over southwestern Montana where upslope
enhancement will bring local totals over 6 inches.

On Monday, Day 2, snowfall will spread eastward across the
Wasatch/Uintas to the central Rockies as broad southwesterly flow
just ahead of the opening upper low/trough shifts the moisture
plume into the Four Corners region. Mid-level frontogenesis should
help maximize totals over the northern CO Rockies where more than
6 inches is possible. Moisture and lift around the ex-upper low
itself will bring additional modest snow to northern California as
well. Over Montana, successive areas of high pressure will slip
southeastward out of Canada into the Plains, re-emphasizing
easterly upslope flow and continuing modest snowfall around
Glacier National Park and light snowfall over much of the rest of
the state, continuing into Day 3.

Elsewhere on Tuesday, Day 3, trough over the Sierra/Great Basin
will progress into western UT/AZ by early Wednesday, continuing
the light to modest snowfall threat to the Four Corners region and
mostly to the Colorado Rockies, lying at the intersection of the
best dynamics and moisture. Upper jet will strengthen to >130kts
from AZ to KS which may help maintain the snow over CO beyond the
forecast period, though amounts will likely remain on the lighter
side. WPC probabilities of at least 4 inches of snow are more than
50% for the San Juan Mountains, Flat Tops, Elks, and Sawatch
Range. Over the northern Plains, models bring a stripe of snow to
mostly ND and far northern SD into MN as a weak surface feature
scoots by, but the probability of at least 4 inches remains less
than 10%.


For Days 1-3, the probability of significant icing exceeding 0.10
inch is less than 10%.


Fracasso