Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
239 PM EDT Thu Oct 31 2024

Valid 00Z Fri Nov 01 2024 - 00Z Mon Nov 04 2024

...U.P. of Michigan...
Day 1...

A compact and strengthening surface low will move rapidly
northeast from northern WI through the U.P. and into southeast
Canada early on D1. Although this low will be pulling away the
first 12 hours of the forecast period, a strong comma head driven
by an elongated axis of mid-level deformation will help dynamically
cool precipitation on the NW side of the departing to low to result
in a band of heavy snow pivoting across the U.P. Although the
strongest ascent is progged to occur before the 00Z Friday (start
of the forecast period) some residual lift into the DGZ combined
with some enhanced moisture on northerly flow off of Lake Superior
will result in moderate snow accumulations, especially in the
higher terrain of the Huron Mountains where WPC probabilities for
4+ inches are as high as 10-30%.


...Much of the terrain of the West...
Days 1-3...

Slow amplification of a mid-level trough will occur through the
weekend across much of the western CONUS, with the trough both
deepening and broadening through the forecast period. This
evolution begins with split flow encroaching onto the Pacific coast
characterized by dual jet streaks pushing onshore the Pacific
Northwest and southern California, respectively. As the trough
begins to amplify along the Pacific coast, especially by Saturday
/D2/, the northern stream jet will dig into the Great Basin while
mid-level energy begins to phase into one larger trough. This
evolution will then continue into D3, with the upper trough
beginning to take on a more neutral tilt towards the Four Corners
by the end of the forecast period.

The amplification and evolution of the synoptic pattern will help
to push a surface low southward along the coast, with this feature
digging into the southern Great Basin by Sunday. A cold front
accompanying the low will act as a funnel for increasing PW
reaching as high as +1 sigma, but moisture will otherwise remain
modest across the West as reflected by near normal IVT/PW according
to NAEFS ensemble tables. Still, sufficient moisture within the
region of impressive synoptic lift will result in expanding
precipitation, with two waves likely through the period.

The first wave will be D1 across primarily the Pacific NW as a
weakening surface low pivots towards NW Washington, spreading
precipitation as far south as northern CA and as far east as the
Northern Rockies. This will continue in a weakening form into D2 as
the forcing weakens (dive further south) and moisture remains
modest. Still, the amplifying upper trough will help cool snow
levels to 3500-4500 ft on Friday before slowly warming late D1 in
advance of the next round of WAA/precip. WPC probabilities D1 for
6+ inches of snow are generally 50-90% in the Cascades of OR and WA
above 4500 ft, with additional high probabilities continuing into
the Blue Mountains of OR.

Then during Saturday and Sunday, snow levels vary within periods
of WAA and then cooling as the trough amplifies, but will generally
waver within the 4000-6000 ft range in areas where precipitation
will occur. Rounds of precipitation will expand eastward through
the weekend as well, resulting in axes of moderate to heavy snow,
with the heaviest snow accumulations expected in the higher terrain
or where favorable upslope occurs. WPC probabilities for more than
6 inches are high both D2 and D3, focused along the OR portion of
the Cascades, as well as across the northern WA Cascades, where
3-day snowfall in excess of 2-3 feet is likely in the higher
terrain. Additional heavy snow of 1 foot or more is also possible
across the terrain around Yellowstone NP including the Tetons and
Wind Rivers, with several inches of snow also likely across the
Sierra, Uintas, and San Juans.

With snow levels generally remaining moderately high,
impacts to the passes appear to be modest, which is reflected by
low WSSI-P for moderate impacts except at Santiam Pass.

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

Weiss