Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 AM EST Mon Dec 05 2022

Valid 12Z Mon Dec 05 2022 - 12Z Thu Dec 08 2022

...California...
Day 1...

Long-lived upper low off the coast of NorCal/southwest OR this
morning will finally start to progress eastward and weaken today,
but will still direct some moisture into the Sierra and northern
California ranges for one more day before ending early Tuesday.
Snow levels near 5000ft will continue to lower by tonight as
precipitation tapers off. However, rates this morning may still
eclipse 1-2"/hr north of Lake Tahoe where accumulations over a
foot are likely in the higher elevations. WPC probabilities of at
least 8 inches of snow are greater than 50% generally above
5000-6000ft.

...Great Basin/Rockies...
Days 1-3...

Weakening upper low/trough into NorCal and the Great Basin will be
responsible for a widespread light to modest snowfall for Nevada
eastward through the Four Corners states as it slowly but steadily
streams eastward over the next three days. Broad southwesterly
flow beneath a strengthening upper jet (>130kts) will provide
synoptic lift over the region. Lingering modest moisture
(precipitable water values +1 sigma) will be fed by the eastern
Pacific but mostly to the south of the snowy areas. Nonetheless,
smaller surges in moisture may advance through the area early this
week aligning with lower-level FGEN and capitalize on favorable
orographic upslope to yield several inches of snow for many areas
and significant accumulations for the Colorado Rockies. Multi-day
totals may exceed 1-1.5 feet in some areas. Snow levels between
5000-7000ft (from northwest to southeast) today will drop by
around 1000ft by tomorrow and further still on Wednesday as colder
air moves in behind the trough.

...Montana/northern Idaho...
Days 1-2...

Northern Idaho and western Montana will be on the southwest side
of a large upper low anchored over Hudson Bay during the next
couple of days. A surface boundary over the region will provide
some lift as mid-level FGEN slices through the area from the
north-northwest astride another Canadian front. Surface highs in
between systems will reinvigorate easterly upslope flow which will
favor the western MT ranges around Glacier NP but also into the
northern Bitterroots. In addition, northwesterly flow around the
lumbering Canadian upper low will also enhance periods of light
snowfall across central Montana through the next 48 hours,
particularly as a jet streak races through the region Tuesday. WPC
probabilities of at least 6 inches of snow over the two-day period
are high (>70%) in western Montana and also over northern central
Montana around the Little Belts and also toward Havre into the
Bears Paw Mountains.


...Pacific Northwest...
Day 3...

The next system out of the Gulf of Alaska will approach the
Washington coast by early Thursday. Warm advection coinciding with
a narrow plume of moisture will push into the Olympics and
northern Cascades by the end of Day 3, where several inches of
snow are likely per the current timing. Snow levels will be around
2000-3000ft closer to the coast but perhaps down to 1500ft nearer
to the Cascades. More appreciable snow is forecast just beyond
this period.


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

Fracasso