FOUS11 KWBC 160820
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Sat Nov 16 2024

Valid 12Z Sat Nov 16 2024 - 12Z Tue Nov 19 2024

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

As one storm system originally over the Intermountain West last
night departs (leaving some light-moderate mountain snow over parts
of the central Rockies in its wake this morning), attention shifts
to the next Pacific storm system that is set to bring an onslaught
of heavy snow to the Pacific Northwest and Northern Rockies this
weekend and into early next week. Precipitation will arrive
Saturday morning as a warm front approaches from the West. Snow
levels start out around 3,000ft Saturday morning, but strong warm
air advection associated with the warm front will cause snow levels
to rise above 7,000ft in the OR Cascades, while they are more
focused around 5,000ft in the northern WA Cascades Saturday night.
This same moisture plume, which is associated with a >500 kg/m/s
IVT that also happens to be above the 90th climatological
percentile according to NAEFS, will advect rich low-mid level
moisture well inland into the northern Rockies Saturday night. By
Sunday, while moisture advection into the Cascades will not be as
strong, a cold front passing through on Sunday will cause snow
levels to plummet to as low as 2,000ft in the WA Cascades and
2,500ft in the Oregon Cascades by Sunday evening.

These falling snow levels will also occur inland across the
Northern Rockies Sunday night and into Monday. NAEFS by 00Z Monday
shows 500mb heights that are below the 10th climatological
percentile throughout the northwestern U.S.. By Monday afternoon,
700mb temperatures are unusually cold across the Pacific NW with
temps as cold as the 2.5 climatological percentile. This unusually
cold air-mass is in place at the same time as steady onshore flow
perpendicular to the Cascades and northern Rockies continues to
support moderate-heavy snow from the Olympics, Cascades, and
northern Rockies to as far south as the Salmon Mountains of
northern California. Heavy snow not only looks to continue through
Monday night in the Cascades and Olympics, but a much stronger
Pacific storm system arrives on Tuesday with more heavy snow and
much stronger winds.

Through 12Z Tuesday, WPC PWPF shows which chances (>70%) for
snowfall >18" in the Oregon Cascades, the northern Bitterroots of
the Idaho Panhandle, and the tallest peaks of the Lewis Range. The
northern Washington Cascades show high chances (>70%) for over 2
feet of snowfall with some of the tallest peaks above 7,000ft
topping 3 feet. Impacts will be the worst in these aforementioned
areas with Moderate to Major Impacts shown on the WSSI, which
includes some of the notable passes in the OR/WA Cascades.
Elsewhere, WPC PWPF shows high chances (>70%) for >12" of snowfall
in the Blue, Boise, and Sawtooth Mountains. Similar high chance
probabilities exist in the Tetons for >8" of snowfall. The WSSI
shows Minor to localized Moderate Impacts in these mountain ranges.

...Eastern MT & Western ND...
Day 1...

Over the past 24 hours, guidance shows the upper level trough
tracking through southern Montana tonight to be more potent and
compact than originally shown. The region remains beneath the
divergent left-exit region of a 250mb jet streak located over the
Four Corners region. As the 700mb low tracks through eastern
Montana this morning, a ribbon of anomalous 700mb moisture aloft
will wrap around the northern and western flank of the low while
strong vertical velocities aloft and dynamic cooling allow for
heavy snow to ensue over northeast Montana. Latest CAMs show
excellent upward vertical velocities within a highly saturated DGZ
around 600-550mb. The 00Z HREF Snowband Probability Tracker (SPT)
does show a swath of snowfall rates that could hover around 2"/hr,
if not higher in some cases. Residents in these areas can expect
rapidly accumulating snowfall this morning with US Routes 2 and 85
to the north and west of Williston likely to feature treacherous
travel conditions. Slick roads and significantly reduced
visibilities are expected where the heaviest snowfall occurs. By
this afternoon, the storm system will be over eastern North Dakota
and tracking to the northeast. Periods of snow will continue across
western North Dakota before finally concluding Saturday evening.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax

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