Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Mon Nov 06 2023

Valid 12Z Mon Nov 06 2023 - 12Z Thu Nov 09 2023

...Northwest, California through the Northern Rockies and then
Colorado...
Days 1-3...

The tail end of an unsettled weather pattern for the
Northwest/northern Rockies is a one-two punch of troughs on the
back side of the strong (130kt+) zonal jet that extends from the
northern Pacific into far northern CA. A low pressure system is
off the WA/OR coastal border which will swing into western WA
today. A reinforcing trough is digging around this low and will
bring a positively-tilted trough down to central CA on Tuesday
before shifting east across the Great Basin Wednesday, lingering
around the Four Corners Thursday.

Broad onshore flow with snow levels generally 5000-5500ft over
WA/OR and 6000-7000ft in northern CA will bring mountain snows to
the Cascades and down to the central Sierra Nevada as well as the
Sawtooth Mtns and western WY ranges (Absarokas/Tetons/Wind River)
where Day 1 snow probs for >6" are moderately high (40 to 70%).
The weakening of the low as it moves inland and the positive-tilt
to the next trough results in a diminished onshore flow/shunt
south for tonight/Tuesday. Day 2 snow probs are more limited
geographically with moderate probs for >6" limited to the higher
Cascades, Bitterroots, and ranges around Glacier NP.

Then on Wednesday, the positively-tilted trough stalls over the
Desert SW (northern AZ/southern UT) which does allow high pressure
to dig southeast down the northern Rockies and bring an upslope
easterly flow over CO Wednesday into Thursday. Much of CO is in
the favored right entrance region to the WSWly jet ahead of the
trough axis Wednesday into Wednesday night with potential for
decent precip and snow levels dropping from around 7000ft to
4000ft/below high plains elevation by Wednesday night. The 00Z GFS
is the most bullish with QPF in CO on Day 3 with the 00Z ECMWF
more timid. The 00Z NAM had no Day 3 precip in CO, but the 06Z
just came in with some. Given the broad lift and upslope
component, this case is something to pay attention to for eastern
slopes of CO where Day 3 snow probs for >6" are currently moderate
(40 to 60%).


Northern Plains, Northern Great Lakes, Northeast...
Days 2/3...

The next wave of warm advection precipitation will spread across
North Dakota ahead of a weak shortwave on Tuesday with light icing
possible again Tuesday morning in northern ND, then Tuesday night
in portions of the Upper Midwest such as northern WI. Reinforcing
troughing over the northern Plains Wednesday night does send a
decent round of warm air advection precip into the Northeast late
that night. Enough overrunning looks to be present to bring a
light wintry mix/freezing rain to interior portions of the
Northeast where Day 3 ice probs are low for >0.1" around the
Catskills and southern Adirondacks at this time.


Jackson