Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Mon Oct 12 2020

Valid 12Z Mon Oct 12 2020 - 12Z Thu Oct 15 2020

Days 1-3...

...Pacific Northwest to the Northern Rockies...
A 130+ kt upper jet streak and associated flat shortwave will
quickly traverse the northern Rockies and High Plains on Monday.
Flat upper riding will follow in an otherwise continued fast,
quasi-zonal flow aloft, before another fast-moving shortwave
reaches Vancouver Island and WA on Tuesday. This second wave will
allow for more appreciable upper height falls across the northern
Rockies and into the Plains Tuesday night and Wednesday.

Higher elevation snowfall associated with the initial shortwave
and surface front will taper from west to east today across the
northwestern U.S. Highest totals during day 1 are expected above
5500 ft over the WA Cascades and northern Rockies in northwest MT,
though 24hr probabilities of 6+ inches will again be localized to
the highest peaks. Snowfall coverage picks up on day 2 (Tue-Tue
night) associated with the next powerful upper jet streak (150+
kts at 250 mb) and associated vort lobe. Snow levels initially
climb to 8000-9000 ft ahead of the shortwave, then drop to
4500-5500 ft across the WA Cascades and northern Rockies Tuesday
night. Day 2 24hr probabilities of 8+ inches peak between 60-80%
over the northern WA Cascades and northern Rockies in northwest
MT. By day 3 (Wed-Wed night), as the shortwave energy deepens the
longwave trough east of the Divide, snowfall over the northern
Cascades and Rockies will dissipate both in coverage and
intensity. Day 3 24hr probabilities of 8+ inches drop to 30% or
less across northwest MT.

The probability of significant icing is less than 10 percent.

Hurley