Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Mon Oct 31 2022

Valid 12Z Mon Oct 31 2022 - 12Z Thu Nov 03 2022

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

Troughing will sink southeastward out of the Gulf of Alaska today
and dig into the Pacific Northwest through Tuesday into Wednesday.
This will carry a surface cold front through the region over the
next few days with lowering snow levels. A small shortwave in the
mid-latitudes will outpace the main trough from the northwest and
help spur some enhanced UVV across northwestern Montana as low
pressure organizes over eastern Montana into southern Canada by
Thursday morning. Back to the coast, incoming vort max will ride a
~150kt jet through Oregon, acting to pull the trough axis
southward through Nevada. Though the moisture source ahead of the
cold front will be waning with time over the Pac NW, southern
stream surge will provide additional moisture through the Great
Basin/Four Corners region as a strengthening jet takes shape in
response to the sharpening upper pattern.

Snowfall will be light to modest over much of the West, starting
as rain in some places before changing to snow as colder air moves
in behind the cold front. Significant accumulations are possible
in the Oregon Cascades, Blue Mountains, and over northern Idaho
and the western Montana ranges around Glacier National Park today
through Tuesday. Snow levels above 7000-8000ft early today will
drop to below 4000ft over WA into northern OR by Tuesday morning.
For Tuesday into Wednesday, precipitation will sink southeastward
through the Sierra and into the Great Basin and central/northern
Rockies, first as the lead mid-latitude shortwave lifts through,
then as the main trough brings in stronger height falls. Though
lower level frontogenesis should allow for some snow totals of a
few inches over far northern CA into southeastern OR, core of
additional moist flow looks to be aligned farther southeast across
AZ and NM. Nonetheless, upslope will enhance totals across the
eastern NV ranges and across UT northeastward through the Uintas
by Day 3. By late Day 3, San Juans in CO and the Wind River and
Bighorns in WY will see accumulating snow as the system progresses
eastward. WPC probabilities for at least 8 inches of snow are
moderate (>40%) above 5000ft or so for northern areas and above
7000ft for southern areas (Sierra).

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

Fracasso