FOUS11 KWBC 300830
QPFHSD

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

Valid 12Z Wed Oct 30 2024 - 12Z Sat Nov 02 2024

...Central Rockies...
Day 1...

Snow showers will continue across portions of the central Rockies,
as an upper trough moves across the region today. While widespread
heavy amounts are not expected, WPC probabilities indicate that
additional accumulations of 4 inches or more are possible across
the higher elevations of the west-central to north-central Colorado
mountains. Snows are expected to diminish as the upper trough moves
east into the central Plains this evening.

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

Precipitation will return to the Northwest and northern California
as a well-defined frontal band associated with a deep low dropping
out of the Gulf of Alaska moves onshore later today. Precipitation
rates are expected to increase later this morning and continue into
the afternoon as the front moves across the region. Rates will
diminish by the evening and overnight, but onshore flow will
continue to support unsettled conditions. Snow levels are expected
to quickly drop behind the front, supporting accumulating snow
across many of the Cascade passes.

The upper low is expected to support another period of organized
heavier precipitation as it continues to drop south on Thursday.
The heaviest precipitation is expected to center from southwestern
Washington through western Oregon into northwestern California.
Accumulations of a foot or more are likely for portions of the
Oregon Cascades, where snow levels are forecast to increase but
remain near or below 5000 ft through much of the period.

Further to the west, this series of systems will bring snow into
the eastern Oregon and northern Nevada mountains beginning this
evening, and the central Idaho ranges overnight, with periods of
snow continuing through Friday. Storm total accumulations of a foot
or more are likely across portions of the Blue Mountains in
northeastern Oregon and the central Idaho ranges, including parts
of the Salmon River Mountains and the Sawtooth Range.

As the upper low remains centered over Washington through much of
Friday, a well-defined shortwave digging to its west will move into
northern California late Friday into early Saturday, bringing
heavier precipitation across northwestern California into the
Shasta-Cascade region and the northern Sierra, where high
elevation heavy snow accumulations can be expected.

...Northeastern Minnesota to Upper Michigan...
Day 2...

The previously noted upper trough crossing the central Rockies
today is forecast to lift northeast through the central Plains and
into the upper Midwest by early Thursday. Guidance shows the upper
trough briefly assuming a negative tilt, with a closed 500 mb
center developing as moves into the upper Midwest Thursday morning.
As the surface, low pressure is expected to quickly deepen and
track northeast across the upper Mississippi Valley into the Great
Lakes. Beginning Thursday morning, models are showing rain
changing to snow within the strong low-to-mid level frontogenesis
band on the northwest side of the low, with some hi-res guidance
indicating snowfall rates increasing upward of 2 in/hr from east-
central Minnesota to the North Shore and northwestern Wisconsin.
While warm ground temperatures and solar insolation may help to
keep amounts in check, these rates, should they develop, will
likely to be sufficient to produce at least a few inches in some
locations. The overall trend in the overnight models was toward a
colder and wetter solution. So while probabilities for anything
above light accumulations have been low, it is expected these will
increase should these trends persist.

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

Pereira


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