Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Sat Oct 21 2023

Valid 00Z Sun Oct 22 2023 - 00Z Wed Oct 25 2023

...Northern Rockies...
Day 3...

Forecast remains on track for potentially heavy snow to enter
parts of the Northern Rockies and especially NW Montana during the
D3 forecast period. A subtle shortwave ejecting from southern
British Columbia to Alberta on Tuesday ahead of a strong upper low
approaching the Pacific Northwest may keep better QPF and winter
dynamics north across Canada initially, but as the cold front
eventually dives southward along the Lewis Range and NW Montana
upslope flow will aid in moderate to heavy snow late Monday night
into Tuesday. WPC snowfall probabilities for more than 4 inches
are 40-70% along the Lewis Range on Montana above 6000 ft.

Snell

Previous Discussion:
Split flow emerging across the western CONUS will result in a fast
northern jet streak arcing into the Pacific Northwest D3. On the
leading edge of this feature, the favorable left-exit region will
drive modest upper diffluence for ascent, aided by weak PVA as
vorticity streamers race through the pinched mid-level flow. At
the same time, a surface cold front will drop southward out of
Alberta/Saskatchewan, causing post-frontal upslope flow into the
terrain of the Northern Rockies late in the forecast period. As
snow levels collapse to around 2000 ft, this should result in
periods of accumulating snow driving moderate accumulations. WPC
probabilities for more than 4 inches of snow are confined to the
area around Glacier N.P., where they reach 20-30%, highest above
6000 ft.


Weiss