Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Sun Nov 08 2020

Valid 12Z Sun Nov 08 2020 - 12Z Wed Nov 11 2020


...Northern Rockies and Montana...
Days 1-2...

...Blizzard expands across Montana today...

A mid-level shortwave which has ejected out of the Southwest will
amplify and close off across Montana today and then lift into
Manitoba Monday. Rapid height falls and PVA associated with this
feature combined with mid and upper level divergence will enhance
cyclogenesis across WY, with this low moving slowly to the
northeast through Monday.

Gulf of Mexico and Pacific moisture being drawn northward in the
warm conveyor belt ahead of this system is anomalous with PWs of
+2 standard deviations above the climo mean and mean mixing ratios
along the 300K isentropic surface as high as 6g/kg. This moisture
will drawn into a rapidly developing TROWAL over central MT which
is further focused by a 1030mb surface high shifting south along
the Canadian Rockies. Deep layer ascent through the aforementioned
synoptics will combine with intense mesoscale forcing through
upslope flow, isentropic upglide, and strong deformation to cause
heavy precipitation to spread across much of eastern and central
MT. Initial precipitation is snow in the northern Rockies and will
be rain on the High Plains of central MT. However, rapid dynamic
cooling of the column, especially as the low strengthens and
isallobaric acceleration occurs from the north between this low
and the aforementioned high, will cause an abrupt transition to
snow, with a swath of mixed rain/sleet/freezing rain in a stripe
northeast of the surface low over northeast MT and northwest ND.
Freezing rain should only feature minimal accretion with Day 1 WPC
probabilities for 0.1" less than 10%.

Snowfall rates are likely to be intense as strong omega intersects
a saturated DGZ. The WPC snowband probabilities indicate a high
likelihood for 1-2"/hr snowfall rates through this evening,
leading to rapid accumulation across parts of the state. It is
likely that most of the precip will occur as snow, and WPC
probabilities continue to indicate a high likelihood of 12 inches
for much of north central and northeast MT, including Great Falls
and Glasgow. Within this region there are probabilities for 18
inches as high as 40%. Accumulations in excess of 20" are possible
in the favored upslope terrain, or where any banding lingers for a
long duration, and this event could challenge November record
snowfall across parts of Montana. In addition to this heavy snow,
strong winds are expected to produce blizzard conditions at times.


...Western U.S....
Days 1-3...

A reinforcing closed low over the Pacific NW will drop south from
Washington early Sunday to Arizona early Monday, when by then will
have formed an anomalous and large upper gyre that spans the Great
Basin and northern Rockies. This feature opens and shifts east
onto the Great Plains Monday with a northern Plains/Upper Midwest
low expected to develop Tuesday. A subsequent shortwave in advance
of a weak AR will approach the Pacific NW coast on Tuesday.

Both of these upper lows will be accompanied by Pacific moisture
within associated jet streaks, through which the combination of
moisture and ascent will produce widespread moderate to heavy snow
across much of the western terrain through Sunday night. The
anomalously low heights will produce falling snow levels all the
way to the Mexican border, so heavy snow accumulations are
expected for the San Gabriels and San Bernadinos, as well as the
Mogollon Rim, and most higher ranges of the Great Basin. The
heaviest snow is expected in the San Juans today and tonight where
orthogonal mid-level flow, and instability will enhance
precipitation, and the HREF probabilities show a high chance for
snowfall rates reaching 4"/hr there. Day 1 WPC probabilities are
high for 12" in the San Juans, with additional snow Day 2 bringing
event totals in excess of 3 feet. Other ranges that will likely
receive significant snowfall include the Sierra Nevada and San
Gabriels/San Bernadinos of CA, the Uintas and southern Wasatch in
Utah, and parts of the Mogollon Rim. Here, Day 1/2 WPC
probabilities are moderate high for 8 or more inches, with local
amounts greater than 12 inches likely above 4000-5000 ft.

On Monday the complex upper low begins to fill and shift east with
shortwave ridging shifting into the West. This will shunt the
forcing, at least briefly, leaving lingering light to moderate
snow across the Four Corners. However, quickly behind this exiting
system the next shortwave digs into the Pac NW with an
accompanying Pacific jet streak and weak AR to provide moisture.
Snowfall will spread across the Olympics Monday night and then
spread into the WA and OR Cascades, and eastward into the Northern
Rockies on Tuesday. WPC probabilities are moderate for 8 inches in
the higher terrain.


...Central Plains to Upper Midwest...
Day 3...

A cold front moving across the Plains will leave a sharp
baroclinic gradient into the Midwest Tuesday. Forcing aloft
including a potent shortwave and diffluence within the LFQ of a
potent jet streak will impinge upon this baroclinic zone to drive
surface cyclogenesis Tuesday morning, and this low will likely
deepen quickly as it shifts northeast into the Great Lakes by the
end of D3. There remains considerable spread in the track guidance
of this system, affecting the thermal structure of the column.
However, strong synoptic ascent suggests a quickly deepening low
will be slightly less progressive, and a blend of the ECMWF/GFS
was preferred for this forecast. This suggests some light freezing
rain is possible along the transition zone between rain and snow,
with a band of heavy snow possible to the NW. WPC probabilities
are low for 0.1" of freezing rain Tuesday, highest in northern IA
and into southern MN. NW of there, the potential exists for some
mesoscale banding as robust deformation/fgen overlap within a
developing comma head. WPC probabilities have increased for 4
inches of snowfall, with the great potential along Lake Superior
from NW Wisconsin into the Arrowhead of MN.


Weiss