Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 AM EST Sat Nov 26 2022

Valid 12Z Sat Nov 26 2022 - 12Z Tue Nov 29 2022

...Pacific Northwest to the central High Plains...
Days 1-3...
Heavy mountain snow is expected to return to the Olympics and
northern Cascades by the second half of the weekend. Deep onshore
flow and large-scale ascent ahead of a well-defined shortwave
diving southeast from the Gulf of Alaska is expected to bring
widespread precipitation into the region by Sunday.  Lower snow
levels will support accumulating snows at the Cascade passes, with
WPC PWPF indicating moderate to high probabilities for
accumulations of 6 inches or more at both Snoqualmie and Stevens
passes on Sunday. 

This initial wave is forecast to move east across the northern
Rockies by late Sunday and into Plains by early Monday.  This will
carry the potential for locally heavy mountain snows farther east,
with 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. 
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 Sunday night 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 northern to central Oregon Cascades. 

Models show this second wave continuing to amplify Monday 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 on
Monday.  By early Tuesday, WPC PWPF indicates that three day snow
totals are likely to exceed a foot over most of the major
Washington and Oregon Cascade 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,

Beginning Monday night and continuing into early Tuesday, 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 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.

The probability of significant icing exceeding 0.1 inch is less
than 10%.

Pereira