Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 AM EST Sun Nov 27 2022

Valid 12Z Sun Nov 27 2022 - 12Z Wed Nov 30 2022

...Pacific Northwest through the Rockies into the Plains...
Days 1-3...
Unsettled weather will prevail across much of the region through
the first part of the week as a series of well-defined upper-level
shortwaves and their associated frontal boundaries impact the
region.

The leading wave diving into the Pacific Northwest this morning is
forecast to continue to move steadily to the east across the
northern Rockies today, bringing a sharp cold front south across
the northwestern corner of the CONUS. This will carry the
potential for locally heavy mountain snows east, with the WPC PWPF
indicating moderate to high probabilities for accumulations of 6
inches or more across some of the higher elevations of the Blues,
as well as the northern and central Idaho, northwestern Montana,
and the northwestern Wyoming ranges.  Favorable upper jet forcing
in addition to low-to-mid level frontogenseis is likely to enhance
snowfall rates across portions of the region.
  
This leading wave will be quickly followed by a second wave diving
southeast from the Gulf of Alaska, which will bring additional
precipitation and a reinforcing shot of colder air into the
Northwest.  This second wave is expected to gradually spread
organized, heavier precipitation south along the Washington into
the Oregon Cascades tonight into Monday.  Across Washington and
northern to central Oregon, accumulating snow is likely fall well
below all pass levels, with snow levels dipping below 1000 ft
across most of Washington by early Monday.  By early Monday, WPC
PWPF shows moderate to high probabilities for accumulations of 8
inches or more across most of the major passes of the Washington
and the northern to central Oregon Cascades.
 
Models show this second wave continuing to amplify tomorrow into
early Tuesday, carving out a deep trough centered over the
northern Rockies and Great Basin.  By early Tuesday, the GFS
indicates 500 mb heights of 1.5 to 2.5 standard deviations below
normal across the region.  This amplifying system is forecast to
bring additional heavy snows to parts of the Oregon Cascades, with
additional totals of 8 inches or more expected across a good
portion of the central to southern Oregon Cascades, including
along the major passes. 

Meanwhile, favorable jet forcing along with low-to-mid level
frontogenesis will support light to moderate snows spreading east
across the northern Great Basin and into the central Rockies. 
With snow levels continuing to fall, accumulating snow will be
possible across much of the region, including the lower valleys. 
While widespread heavy totals are not expected, some locally heavy
totals are possible, especially for the northern Utah,
south-central Wyoming, and north-central Colorado ranges.

This second system will continue to amplify into the Four Corners
region, bringing at least some snows south through the southern
Colorado and into the northern New Mexico ranges.

Meanwhile back the west, a shortwave ridge is forecast to offer a
brief period of dry weather across the Northwest early Tueday
before the next system begins to impact the region late in the
day.  Models show a deep, compact low developing and dropping
south along the coast of British Columbia, with strong onshore
flow and favorable forcing starting to bring organized, moderate
to heavy precipitation back into the Northwest starting Tuesday
night.  Snow levels are expected to rebound some, especially
across western Washington and Oregon.  However, heavy snow
accumulations are still likely for the Olympics and portions of
the Washington Casades, with additional impacts expected at and
below the major passes.

...Central High Plains to the western Great Lakes...
Days 2-3...
Beginning tomorrow night and continuing into early Tuesday, as the
trough in the West continues to amplify and shift east, moist,
easterly, low level winds along with increasing upper divergence
and low level frontogenesis will support snow developing over
portions of the central High Plains.  At least some light
accumulations can be expected from southeastern Wyoming into
southwestern South Dakota, and the Nebraska Panhandle, with WPC
PWPF showing slight probabilities for accumulations of 4 inches or
more for parts of 
the region by early Tuesday.

By early Tuesday, surface low pressure developing over eastern
Colorado is forecast to track east across the central Plains
before turning northeast into the upper Mississippi Valley late in
the day.  Models are presenting a classic setup for the
development of banded precipitation, supported by a coupled upper
jet overlapped by low-to-mid level frontogenesis.  A widespread
swath of light-to-moderate snow is expected from northeastern
Nebraska and southeastern South Dakota to the western U.P. of
Michigan and the Minnesota Arrowhead.  Overnight guidance showed
the greatest potential for heavy amounts centered across portions
of northern Wisconsin and the western U.P., with the WPC PWPF
showing some moderate probabilities for accumulations of 6 inches
or more.  Given the setup, it is expected that those probabilities
will increase for parts of the region in subsequent runs.

For Days 1-3, the probability of significant icing exceeding 0.1
inch is less than 10%.

Pereira