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

Valid 12Z Thu Oct 31 2024 - 12Z Sun Nov 03 2024

...Northeastern Minnesota to Upper Michigan...
Day 1...

Mid-level vort max deepening into a closed low beneath a 130-140kt
upper jet will lift across the Upper Midwest this morning into the
afternoon. Surface low pressure over southeastern MN at 12Z will
move across northern WI into the eastern U.P. of Michigan as
marginally colder air wraps around the system. Rain will change to
snow as dynamic cooling strengthens from ~southern/southeastern MN
northeastward, helping to drive snow accumulation. However,
temperatures and dew points are all above freezing as of the early
overnight hours, so things will have to work in tandem during the
morning and also fight off daylight solar insolation via heavier
rates. The WPC Snowband Tool does show potential for >1"/hr rates
from near MSP northeastward (a slight shift southward/southeastward
from 12 hours ago) and these vigorous systems can often cool the
column quicker than what the models forecast. Still expect a stripe
of 2-5" amounts over eastern MN into northern WI and the hillier
areas of the western U.P., though the WPC probabilities are
generally 20-45%. High- end potential is >6" and probs at that
threshold are ~10-20% over northwestern WI.


...Northwest, Great Basin, Northern Rockies...
Days 1-3...

With a strong upper ridge located well west of California,
troughing will persist then dig into the Western US over the next
few days. A series of systems will move out of the North Pacific
and into/through the region, with cooler than normal temperatures
and at least some snow into many of the passes. Moisture levels
will be about average to start (today) rising toward the 90th
percentile tomorrow afternoon/evening over NorCal as the moisture
source taps more of the subtropics. This will be due to a digging
upper ~150kt jet promoting broad lift over the Northwest that will
progress southward and eastward Fri/Sat. For today/D1, the southern
WA Cascades into the OR Cascades will be in the direct line of the
incoming moisture surge behind the surface cold front. Snow levels
have already fallen post-FROPA to around 3000ft and will continue
to fall behind the front to the south and east today but rebound a
bit over western WA/OR. For D1, WPC probabilities for at least 8
inches of snow are >50% above 4000-5000ft from north to south along
the WA-OR Cascades and above 5000ft over central Idaho.

Into days 2-3, the next wave will promote a deeper trough into the
West with heights crashing into the Great Basin by Saturday
afternoon. Though the initial moisture source will be from the
northern Pacific, as the system drops southward it will tap some
subtropical moisture as it brings in IVT values ~200-350 kg-m/s2
(~90th percentile) to NorCal. Snow levels will rise ahead of the
cold front in the WAA to 6000-7000ft but then fall back to
5000-6000ft Saturday. Farther inland, snow levels will be in the
5000-7000ft range across the Great Basin into the northern Rockies.
This will affect some higher passes where significant snowfall may
be possible. The majority of the snow will be over the Cascades on
Friday and into the Blue Mountains in eastern OR. Into Saturday,
snow will spread into the Sierra and into the Great Basin. WPC
probabilities of at least 8 inches of snow Fri/Sat are >50% above
~5000ft in the north and ~6500ft over NorCal. Snowfall into the
Sierra has low probabilities (10-40%) of at least 8 inches
accumulation.


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

Fracasso