FOUS11 KWBC 301932
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 PM EDT Wed Oct 30 2024

Valid 00Z Thu Oct 31 2024 - 00Z Sun Nov 03 2024

...Northeastern Minnesota to Upper Michigan...
Days 1-2...

A vigorous upper level trough entering the Central Plains tonight
will lead to a strengthening area of low pressure that tracks into
the Upper Mississippi Valley by Thursday morning. This setup is
driven heavily by strong synoptic-scale ascent over eastern MN,
northern WI, and upper MI Thursday morning thanks to robust
250-500mb upper level divergence. As the storm strengthens Thursday
morning, dynamic cooling within the column coupled with intense
vertical velocities just below of, and including some lower
sections, of a saturated DGZ will support periods of heavy snow
from east-central MN to the MN Arrowhead, northern WI, and the
western most communities of MI's Upper Peninsula. Latest WPC
Snowband Probability Tracker does suggest the potential for 1-2"/hr
snowfall rates from just north of the Twin Cities metro area to
more prominently around Duluth, its surrounding communities, and on
north and east to include the coastal portions of the Arrowhead and
northwest WI. This event is a fairly fast moving setup with snow
starting mid-morning in parts of eastern MN, then concluding close
to sunset. The start and end times across northwest WI and the MI
Upper Peninsula would be closer to early afternoon for a start time
and ending around midnight local time.

Snowfall totals will likely be heavily driven by which areas reside
beneath the deformation axis, which is still not fully agreed upon
by guidance across the board. That said, most guidance does show an
opportunity for heavy snow in areas that would be experiencing
there first measurable snowfall of the season. Given that is would
be the first snow, the potential impacts are more amplified for
residents in these areas. The latest WSSI does show a large Minor
Impact area that stretches from the northern Twin Cities suburbs to
as far north as the MN Arrowhead and as far east as the western
most section of MI's Upper Peninsula. It is northwest WI to the
east of Duluth that show localized areas of Moderate to even Major
Impacts. These impacts are driven largely due to a combination of
the algorithm's Snow Amount and Snow Load components, signifying
that this is likely to be a heavy, wet snow in areas that could
result in detrimental impacts to trees branches and possibly power
lines. The good news is the storm will be racing east Thursday
night and any lingering snowfall is effectively finished around
sunrise Friday morning.

...Northwest to the Northern Rockies...
Days 1-3...

A potent storm system is arriving in the Pacific Northwest today
that is ushering in the next round of unsettled weather as far east
as the Northern Rockies and northern Great Basin. Initially, a cold front
will track through the Pacific Northwest today that begins the process
of lowering snow levels below 4,000ft by this evening throughout much
of the Pacific Northwest. NAEFS shows an upper level trough, sporting
500mb and 700mb heights below the 10th climatological percentile,
barreling into the Pacific Northwest and southern British Columbia
early Thursday morning. This upper level feature is also directing
a 300-400 kg/m/s IVT (above the 90th climatological percentile
over northern CA) into portions of the Northwest. There is not much
in the way of a sub-freezing air-mass either in front of, or in
wake of the upper low. This should keep most heavy snow to
elevations above 4,000ft in the Cascade Range, Siskiyou, and
Shasta. This same stream of moisture will reach the northern
Rockies late tonight and into Thursday where heavy snowfall
>5,000ft is expected in the Blue, Sawtooth, Boise, and Teton
Mountains.

The upper low lingers along the Pacific Northwest coast through
Friday morning, then finally advances inland through the Northwest
Friday night. A cold front will advance south through northern
California and help to cause lowering snow levels throughout the
northern and central Sierra Nevada on Saturday. Periods of snow are
possible in the higher ridge lines of the northern Great Basin on
Saturday and as far east as the Tetons and Wasatch Saturday
afternoon.

WPC 72-hr PWPF shows high chances (>70%) of snowfall totals >18"
in the Oregon Cascades above 5,000ft, with some of the tallest
peaks above 6,000ft having similar high probabilities for snowfall
totals >30". Farther east, the Blue and Sawtooth Mountains at
elevations above 6,000ft have moderate-to-high chances (50-70%) for
snowfall amounts >18". Even portions of the Washington Cascades
are forecast to see some heavy snow with moderate-to-high chances
(>70%) for >12" of snowfall at elevations above 4,000ft. Farther
south, the Sierra Nevada and Siskiyou sport moderate-to-high odds
(50-70%) for snowfall >8" on Saturday with additional snowfall
likely to occur Saturday night. In terms of impacts, the
Probabilistic WSSI shows Moderate chances (40-60%) for Moderate
Impacts along the Oregon Cascades and the southern peaks of the
Washington Cascades. These areas can expect the more treacherous
travel conditions, particularly where heavy snow falls around pass
level. Elsewhere, the other mountain ranges listed sport generally
low-to-moderate chances (30-50%) for Minor Impacts.


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

Mullinax





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