Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 PM EDT Fri Oct 29 2021

Valid 00Z Sat Oct 30 2021 - 00Z Tue Nov 02 2021


...Northern Rockies to the central High Plains...
Days 1-3...
A deepening closed low near the Hudson Bay will result in an
amplifying trough across the central CONUS through the weekend.
Around the periphery of this trough, periodic shortwave energy
will dive southeast out of Canada, accompanied by intensifying
upper jet streaks. At the surface, this will manifest as a wave of
low pressure moving across southern Canada, draping a cold front
from the Northern Rockies Saturday, as far south and east as Texas
and the Ohio Valley by Monday. The interaction of the surface
front with shortwave energy and upper jet support will produce
periods of moderate to heavy snow into early next week.

For D1 /tonight and Saturday/, the heaviest snowfall is likely in
the Northern Rockies near Glacier NP, with a secondary maxima in
the Big Horn Range of Wyoming. In these areas, upslope enhancement
to the larger scale ascent will produce additional snowfall, and
WPC probabilities for 4 inches are above 50% near Glacier NP, and
20-30% in the Big Horns. Snow levels falling to around 2500 ft
behind the front could also produce some light snowfall into the
High Plains of MT/WY D1.

A waning of synoptic ascent Saturday night into Sunday will
produce generally a quiet day for snowfall on D2. However, by Day
3 /Monday/ an intensification of a secondary upper jet streak
combined with a modest shortwave dropping out of the Pacific NW
will once again lead to heavy snowfall, this time in the Central
Rockies near the WY/CO border. WPC probabilities are as high as
30% in the Park Range and Front Range near Rocky Mountain NP,
generally above 6000 ft.

Further to the east across the High Plains of CO/WY and into the
Panhandle of NE, there is a modest signal for a band of moderate
to heavy snowfall. The intensification of the upper jet streak
will place the favorable RRQ for ascent atop this region, while
the ageostrophic response of this evolution will combine with the
low-level baroclinic zone to produce an enhanced band of fgen.
While area soundings suggest a marginal thermal structure during
the period of most intense ascent, overnight Sunday into Monday
some light accumulations are likely, and if forcing can overcome
the warming into Monday morning, a low-level isothermal layer
could support heavier snowfall. WPC probabilities for 2" are about
10-20% which is slightly lower than NBM probabilities for the same
amount. At this time do not anticipate a significant snow event,
but a few inches is possible across this area on D3.


For Days 1-3, the probability of significant icing is less than 10
percent.

Weiss