Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Wed Oct 21 2020

Valid 00Z Thu Oct 22 2020 - 00Z Sun Oct 25 2020




...Northern Rockies through the Northern Plains...
Days 1-2...

Shortwave dropping out of the Pacific Northwest will round the
base of a broad upper trough and drop across the Northern Rockies
tonight before shifting northeast across the Northern Plains and
out of the Great lakes by Friday morning. This feature will be
accompanied by a robust upper jet placing the diffluent RRQ atop
the region to drive lee cyclogenesis east of the Rockies. As this
surface low strengthens through today, the combination of
increasing moist advection and upslope flow with the upper
synoptic ascent to drive heavy snowfall in the mountains from
western Montana through the Absarokas and Big Horns of Wyoming.
WPC probabilities for 6" are high in the highest terrain, with
locally in excess of 10" possible in the Big Horns.

As the low consolidates and begins to lift northeast Thursday, a
swath of heavy snow is likely across northern SD, southern ND, and
into Minnesota. During the period of moderate intensification,
robust WAA will lift across SD and isentropically ascend to drive
an increase in both coverage and intensity of precipitation. While
there is likely to be a transition zone of p-type from rain to
freezing rain to sleet to snow, they heaviest wintry accumulations
are expected to be due to snow. As this band of WAA lifts
northward, it will encounter favorable dynamics for CSI, including
-EPV and some theta-e lapse rates near 0 coincident with a
saturated DGZ. This suggests the potential for snowfall rates in
excess of 1"/hr, echoed by WPC snowband probabilities as well
after 08Z Thursday. For this reason increased the SLR above the
NBM reduction, and a narrow stripe of convective snow is likely
somewhere near the SD/ND border. The heaviest snow is likely
within this fgen band, and WPC probabilities for 6" are high for
far northeast SD and southeast ND, with isolated amounts over 10"
possible. As the low pulls further to the east, the fgen band will
pivot laterally while remaining north of the low, but with less
convective potential the probabilities for 6" wane, but 4"
probabilities remain high across north-central MN and into the
Arrowhead. Some modest enhancement is possible along the Iron
Ranges of the Arrowhead due to flow parallel to Lake Superior on
D2.

Additionally, the guidance has trended upward a bit for some
freezing rain and/or sleet accumulation across eastern SD and into
the Coteau des Prairies in SD/MN. There remains uncertainty into
which p-type will be dominant here, as well as how much saturation
will occur within the column thanks to a dry slot modeled by most
guidance. However, the WAA should produce a warm nose to cause a
p-type transition to IP/ZR, and WPC probabilities have increased
to 40% for 0.1" of ice accretion, focused along the Buffalo Ridge.



...Pacific Northwest through the Northern High Plains...
Days 2-3...

Yet another shortwave trough ejects from Alaska/Yukon shortwave
late Thursday, closing into a low over southern interior BC Friday
night before tracking into MT as a weakening wave. Pacific
moisture is able to spread across WA ahead of the trough on
Friday. This feature will be accompanied by a robust 700mb wave
digging towards WY by the end of D3, and a potent 130+kt Pacific
Jet streak. Additionally, upslope flow into the terrain should
become enhanced Friday as high pressure noses down the immediate
lee of the northern Rockies and into eastern MT. Anomalous
moisture and robust ascent should drive heavy precipitation, with
snowfall likely in the terrain from the Olympics of Washington, as
far east as the Grand Tetons and Big Horns in Wyoming. On Friday,
snow levels will still be modest across the Cascades and into
Montana, generally 2000-4000 ft. However, as the 700mb low digs
southeast, snow levels will crash quickly, coincident with heavy
precipitation, and accumulating snowfall is likely by the end of
the forecast period even into the lowlands of eastern MT and other
valleys within the terrain. The heaviest snowfall is likely in the
highest terrain, with high WPC probabilities for 12 inches in the
WA Cascades, and stretching east into the many ranges of northern
ID, MT, and northern WY. As the jet streak shifts eastward late D3
/Saturday/, heavy snowfall will likely spread in a stripe across
the High Plains and into South Dakota/North Dakota. Much of this
snow may occur beyond D3, but WPC probabilities already indicate a
better than 50% for 4".

For Day 3, the probability of significant icing is less than 10
percent.


Weiss