Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 AM EDT Sun Oct 23 2022

Valid 12Z Sun Oct 23 2022 - 12Z Wed Oct 26 2022

...Northern/Central Rockies through the Northern High Plains...
Days 1-2...

Anomalous mid-level closed low approaching -4 sigma according to
NAEFS ensemble tables will continue to advect eastward through
Monday, moving across the Central Rockies and into the Northern
Plains. As this feature shifts, forcing will continue to intensify
through impressive height falls and PVA as lobes of vorticity
swing around the closed feature. Aloft, the subtropical jet streak
will phase with the northern stream across the Southern Rockies by
this evening, providing impressive LFQ diffluence to further
enhance deep layer ascent, and lead to lee cyclogenesis in eastern
WY. This surface low will then lift northeastward while deepening
before ejecting into Manitoba by Monday evening. Impressive
meridional moist advection downstream of this trough will surge
PWs to +2 sigma across the Upper Midwest and Central Plains, with
the associated theta-e ridge progged to lift cyclonically with the
WCB into a TROWAL late Sunday into Monday, focusing across eastern
MT and the far western Dakotas. The position of this TROWAL has
focused among the guidance recently, and confidence is increasing
that a band of heavier snowfall will occur across eastern MT,
especially tonight into early Monday as 700-600mb fgen overlaps
with modest deformation to provide enhanced ascent. While the most
robust theta-e ridging is progged to be elevated at around 500mb,
this seems to position well within the elevated DGZ, and is
directly above the most impressive omega, all suggesting the
potential for heavy snow rates within this band. The DGZ is
somewhat narrow and is still elevated which will likely limit the
true intensity of snowfall due to lower than climo SLR, especially
as dendrites fall through a deep thickness of strong winds which
may lead to fracturing, but the WPC prototype snow band tool
suggests snowfall rates of 1-2"/hr are likely at times.

While the most intense snowfall rates may occur within this band
Sunday night, additional heavy snowfall where impressive synoptic
ascent overlaps with strong upslope flow to produce snowfall rates
of more than 1"/hr in the higher terrain from the Wasatch eastward
through the San Juans and northward into the Big Horns, Absarokas,
and other surrounding ranges. While mesoscale ascent may not be as
impressive in this region as within the aforementioned snow band,
higher SLRs in the terrain, especially as CAA occurs to steepen
lapse rates and deepen the DGZ, will result in impressive snowfall
accumulations here as well.

On D1, WPC probabilities for more than 6 inches are highest in the
CO Rockies, Big Horns, Absarokas and surrounding ranges, where
more than 12 inches of additional snowfall is possible in the
highest terrain. A secondary max for more than 6 inches exists in
the High Plains of eastern MT beneath the anticipated
deformation/fgen band, where WPC probabilities are as high as 70%,
and locally up to 10 inches could occur. Most of the snow moves
out of the region during D2, but an additional several inches of
snow is likely near the Dakotas/Montana border before winding down
Monday night.

Farther to the south, a piece of the full latitude trough may
break off as a separate weaker closed mid-level low over NM and
then race eastward into the Southern Plains Tuesday. This feature
is likely to be more transient than the low to the north, but will
still be accompanied by impressive upper diffluence and some
upslope flow, especially into the Sangre De Cristos, which should
result in periods of light to moderate snowfall as far south as
the White Mountains of AZ. WPC probabilities for more than 4
inches are above 40% in the Sangre De Cristos, and 20-30% in the
White Mountains on D1.


...Pacific Northwest into the Northern Rockies...
Days 2-3...

As the strong trough from Friday moves into the Rockies and
Northern Plains, flow in the wake becomes confluent once again out
of the Pacific which will allow multiple shortwaves to dive
rapidly southeast into Pacific Northwest and then into the
Northern Rockies by the end of the forecast period. Two distinct
shortwaves are progged in the next 3 days, one moving into
Washington Monday evening before shifting eastward with modest
amplitude, followed by another sharper trough advecting into WA/OR
Tuesday night with greater amplitude. The airmass ahead of the
first shortwave is cool from the longwave trough to the east, and
each shortwave will likely result in further column cooling with
lowering snow levels. Brief WAA ahead of the first shortwave on
Monday will raise snow levels to 5000 ft in WA and as high as 9000
ft in OR, but a cold frontal passage will then drop these back to
around 4000 ft by Tuesday, with further lowing to around 3000 ft
all the way into the Northern Rockies by Wednesday. With
impressive upslope ascent likely into this cold airmass, SLRs
could be dramatically higher than with the previous event, with
the colder airmass supporting more efficient accumulations. WPC
probabilities on D2 are as high as 60% for more than 6 inches in
the WA Cascades, and 30-40% across parts of the Northern Rockies.
By D3, the lowering snow levels and trailing shortwave will spread
heavy snow additionally into the OR Cascades and as far southeast
as the Tetons, and WPC probabilities for 6 inches or more on D3
are high in the Cascades and moderate elsewhere. Total snowfall
could exceed 2 ft in the highest WA Cascades, with significant
accumulating snow likely at many of the important mountain passes
including Willamette, Stevens, Washington, Lookout, and Lolo.


The probability of significant icing is less than 10 percent.

Weiss