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

Valid 00Z Sun Nov 27 2022 - 00Z Wed Nov 30 2022

...Pacific Northwest through the Rockies into the Plains...
Days 1-3...
An increasingly unsettled period is set to return in earnest for
much of the West to close out November. A robust closed low over
the Alaska Panhandle/northwestern British Columbia this evening
will slide southeastward through western Canada and across the
prairies early next week, bringing in lower heights to the Pacific
Northwest and Rockies along with a surge in moisture and much
colder air. This will bring significant snow to not only the
mountains but down through many mountain passes across the
Cascades. Some light snow is even possible down to many valley
floor locations by Tuesday.

Tonight into Sunday, height falls coinciding with increasing upper
divergence on the poleward exit region of a ~100kt jet and
interacting with a surge of moisture from the Pacific (IVT values
300-400 kg/m-s) will provide for modest to heavy snow on Day 1.
Westerly flow will maximize upslope enhancement into the
Washington Cascades where PWPF indicates moderate or high chances
(>40%) of at least 12 inches of snow, including both Snoqualmie
and Stevens passes. Snow rates could exceed 1-2"/hr in some
locations across the Cascades overnight into early Sunday per the
WPC Snowband Tool.

The upper trough and surface cold front are 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. Trailing vort max on the southwest
side of the upper trough will quickly move into Washington by Day
2, with another shot of snow for many areas above about 1000ft as
snow levels continue to fall. Snow will spread farther south
through the Oregon Cascades and into the northern Great Basin as
the upper trough broadens over the region. Two-day totals may
exceed 12 inches over much of the Cascades above about 3000-4000ft
per the latest PWPF.

By Day 3, 500mb heights may max out at their deepest levels over
Utah where the GFS shows about -2 to -2.5 sigma departure from
average. Modest divergence aloft combined with lower-level
convergence and orographic ascent will help promote modest snow
totals to areas of the Wasatch and Uintas eastward through much of
the Colorado Rockies into southeastern Wyoming by Tuesday. By
then, as height falls push east of the Rockies, lee-side
cyclogenesis will spur low pressure over southeastern Colorado
that is forecast to lift northeastward along the surface boundary
astride the Corn Belt. An area of snow is likely on the cold side
of the boundary though without a tap to Gulf moisture. However,
lower to mid-level frontogenesis should encourage more modest
totals in a narrower axis from southwest to northeast by the end
of the period (and continuing into Day 4). The models show various
degrees of available QPF dependent upon the northern stream
evolution and thus a large spread in snowfall amounts. For right
now, PWPF indicates an area of low probabilities (10-40%) of at
least 4 inches of snow from the SD/NE border into southwestern MN,
and moderate probabilities (40-70%) over southeastern WY into
northwestern NE and southwestern SD (where there are also low
probabilities of at least 8 inches of snow).

Also late on Day 3, lowering snow levels in the Pacific Northwest
all the way to sea level could bring some accumulating snow to
elevations around 500ft. PWPF shows low probabilities of at least
1" of snow down to 500ft around Puget Sound, depending on how
quickly precipitation moves into the region.


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

Fracasso/Pereira