Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
412 PM EDT Fri Oct 23 2020

Valid 00Z Sat Oct 24 2020 - 00Z Tue Oct 27 2020


...Northern Rockies to the Central Plains...
Days 1-2...

As a well-defined system drops south from western Canada into the
western U.S., widespread snows, with moderate to heavy
accumulations, are expected from the Pacific Northwest to the
north-central U.S. over the coming days.

Heavy snow will become widespread across the Northern Rockies to
as far south as Wyoming as a potent shortwave digs through the
mean longwave trough across the CONUS. This shortwave will be
accompanied by a robust Pacific jet streak transporting anomalous
PW (edge of the AR) into the region, and the combination of the
ascent through height falls, PVA, diffluence, and low-level
upslope flow will produce heavy snow on day 1 /Saturday/. WPC
probabilities for 12 inches are high in the highest terrain of the
Northern Rockies, Absarokas, and Tetons.

Late D1 and into D2 /Sunday/ the swath of snow will likely
separate into two main areas. The first will be associated with
the surface low moving southeast through the Rockies beneath the
shortwave and the LFQ of the subtropical jet streak from the
Pacific. This will continue to provide robust ascent for heavy
precipitation, which will be snow in the higher terrain
throughout, and gradually become snow as the column cools at lower
elevations of the High Plains in MT/WY/CO. WPC probabilities are
high for 12 inches in the Tetons and CO Rockies on Sunday, with
lower probabilities for 12 inches along the Cheyenne Ridge and
Front Range. Lower accumulations, but still in excess of 4 inches,
are likely in the High Plains, with a relative minimum possible in
eastern WY where the split jet streaks may produce an overall
weakness in synoptic ascent.

Further to the east the leading jet streak will race off towards
the Great Lakes, leaving the favorable diffluent region for ascent
atop the northern and central Plains. At the same time, isentropic
ascent will increase at 300K to spread precipitation north and
east coincident with a band of fgen, possibly enhanced by the
aforementioned jet. While forcing for CSI does not appear present
for excessive snow rates, an exceptionally deep DGZ with a cold
and nearly isothermal profile suggests high SLRs with the
potential for large aggregate snowflakes despite temps well below
0C. This had led to an increase in snowfall amounts, especially
across SD and NE where the fgen band sinks slowly southward within
this favorable environment. WPC probabilities are high for 6"
across parts of SD and NE, with isolated amounts to 10" possible.
As this band sinks southward by D3, it should weaken, but will
also encounter a more marginal temperature profile so
probabilities for heavy snow accumulations wane rapidly across
Kansas.


...Southern Rockies into the Southern Plains...
Day 3...

The same upper trough will amplify while remaining positively
tilted late Sunday into Monday. Surface low pressure dropping
along the Rockies will continue to produce heavy snow from the San
Juans and Sangre De Cristos into the Wasatch of Utah, with lighter
amounts spreading eastward into the High Plains of NM. Moisture
will be slightly anomalous embedded within the Pacific jet streak,
noted by PWs that are 0.5 standard deviations above the climo
mean. WPC probabilities for 8 inches are high in the Sangre De
Cristos and San Juans Sunday night into Monday, with local amounts
to 20 inches possible. Lighter snows are likely spreading eastward
into the High Plains.

Additionally, as a surface cold front drops into Texas, persistent
SW flow aloft will produce an overrunning precipitation event from
north Texas through southeast Kansas. Guidance differs
considerably in the location and amounts of mixed precip during
this event, but there are increasing chances for some light
accretion of freezing rain. WPC probabilities are currently less
than 10% for 0.1", highest in central Oklahoma.


Weiss