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

Valid 12Z Wed Oct 21 2020 - 12Z Sat Oct 24 2020

A significant early season snow event continues for the Northern
Rockies while a swath of heavy snow is expected tonight through
Thursday for the Dakotas and northern Minnesota and far northern
Wisconsin.


...Northern Rockies...
Day 1...

The next in a long line of disturbances rounding a sprawling low
centered over northern Hudson Bay is a shortwave trough currently
over Vancouver Island that will dig over the Northwest CONUS
through Thursday before shifting east to Lake Superior through
Thursday night. This trough is accompanied by a 150kt jet max that
veers along the BC coast through tonight as the trough digs and
the downstream jet backs to southwesterly tonight/Thursday over
the northern Plains. Lee side surface cyclogenesis is beginning in
WY and will strengthen in place through today before shifting
southeast to eastern CO tonight before ejecting east across the
Plains Thursday. Precip ahead of the trough axis is currently
across southern over BC and will shift southeast along the
northern Rockies of MT to the ID border today with upslope
enhancement from a surface ridge nosing down the immediate lee of
the Rockies from a 1040mb high centered over northern BC. With
cold air in place from the northern Rockies and east, nearly all
of the Day 1 precip will be snow with probabilities for 6 or more
inches moderate to high for most MT ranges to the ID border and
the Absarokas and Big Horns extending into WY. A secondary area
along the northern MT border, north of an inverted trough also has
low values for 6 or more inches in Day 1.


...Northern Plains to Lake Superior...
Days 1/2...

Lee side cyclogenesis expanding into an inverted trough on the
Plains through tonight that shifts east to WI on Thursday sets up
a swath of significant low level frontogenesis and snow across the
northern Plains. 00Z Guidance continued a southerly trend with QPF
preferences focused on the 00Z GFS. The band of heaviest snow
looks to center on the SD/ND border tonight before shifting east
across northern MN/far northern WI and into the western UP of MI
on Thursday. There is still uncertainty about the p-type
transition zone with the 850mb low track over Neb and southern MN
Thursday sending a warm nose well into SD where sleet and freezing
rain mix in. Strong low level dynamics should cause dynamic
cooling that overcomes this warm nose, especially close to the
ND/SD border, with more robust WAA driving a better chance for
mixed precip for eastern SD into MN. Day 1.5 ice probabilities are
20 to 30 percent for a tenth inch or more of ice over over eastern
SD to the Buffalo Ridge of southwest MN, then again over
north-central WI. Day 1.5 snow probabilities are moderate for 8 or
more inches along the SD/ND border where the axis pivots a bit
before ejecting northeast Thursday. Day 2 probabilities for 6 or
more inches of snow are 30 to 40 percent from southeast ND across
north-central MN to the Arrowhead where an easterly flow over Lake
Superior (a longer fetch than the southeasterly flow from the past
two waves) enhances snow potential for the North Shore/Arrowhead
of MN.


...Washington Cascades and Northern Rockies...
Day 3...

Yet another shortwave trough ejects from Alaska/Yukon shortwave in
the Thursday timeframe, closing into a low over southern interior
BC Friday night before tracking to the ID/MT border. Pacific
moisture is able to spread across WA ahead of the trough on
Friday. Height falls, positive vorticity advection, and left exit
ageostrophic flow from a northerly jet will produce ascent as the
low to mid-levels saturate ahead of the a 700mb wave Friday night.
Furthermore the surface ridge from a 1045mb high noses down the
immediate lee of the northern Rockies aiding upslope flow to the
eastern slopes of the MT Rockies. This results in QPF of 0.5 to
1.5" across the WA Cascades and northern Rockies of eastern
WA/ID/MT. Snow levels remain low over the MT Rockies with sub
1000ft MSL spread into eastern WA (with levels 1500 to 2000ft over
western WA/Puget Sound Friday night making for heavy snow at
rather low elevations for late October. Day 3 snow probabilities
for 8 or more inches are moderate to high eastern slopes of the MT
Rockies west across northern ID, some of the higher ridges of
eastern WA, and the WA Cascades and Olympics.


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

Jackson