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

Valid 00Z Mon Nov 28 2022 - 00Z Thu Dec 01 2022

...Pacific Northwest through the Rockies into the High Plains...

Days 1-3...
Unsettled weather will prevail across much of the region through
midweek as one transient shortwave trough progresses into the
central CONUS Tuesday night, while the next positively-tilted
trough moves across Vancouver Island and southwest BC on Wednesday.

The meridional 130+ kt upper level jet streak just offshore the
PAC NW coast Sunday night will continue to drop southeast across
western WA-OR on Monday. This will lead to amplification of the
longwave trough over the Intermountain West and Rockies Monday
night and Tuesday. This 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 500 ft
across most of Washington by early Monday. By that time however,
the bulk of the QPF will have shifted out of the lower terrain
areas. 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 around 2 standard deviations below
normal across the region. Additional snowfall on day 2 (00Z
Tue-00Z Wed) will be light over the NW, however given the cold air
in place some light accums (around an inch or less) are
anticipated over the lower elevations over western WA and
northwest OR. Meanwhile on day 2, favorable deep-layer QG forcing
ahead of the amplifying trough will support light to moderate
snows spreading east across the 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. As
the trough continues to amplify into the Four Corners region,
expect moderate snowfall through the San Juans in southern
Colorado and northern New Mexico.

By day 3, after a period of height rises (shortwave ridging) ahead
of the next system, snow levels are expected to rebound some,
especially across western Washington and Oregon. However, heavy
snow accumulations are still likely for the Olympics into the WA
and northern OR Cascades, along with the intermountain region of
north-central and northeast WA, where probs of >8" on day 3 are
60-80% in the higher terrain north of Spokane. 

...Central Plains and Upper Midwest into the western Great Lakes...

Days 2-3...
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 at lower elevations from
southern SD, into northern NE, northwest IA, southwest and central
MN, and northwest WI, with WPC PWPF showing high probs of at least
1-2" over these areas on day 2.

By later day 2 into day 3 (Tue-Wed), as surface low pressure
tracks from the mid MS Valley northeast to eastern Upper Michigan,
a swath of moderate-heavy snowfall is expect along the northwest
flank (associated with the mid level deformation axis and CCB)
over parts of northern WI and western Upper MI. Latest WPC PWPF
shows highest probs of 4-6" across these areas, highest over parts
of western Upper MI given the likely enhancement off Lake Superior
with the CAA and northerly low-level flow late Tue-Tue night.

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

Hurley