Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 PM EST Mon Nov 09 2020

Valid 00Z Tue Nov 10 2020 - 00Z Fri Nov 13 2020

...Central Plains to Upper Midwest...
Days 1/2...

A cold front associated with the low that had been over the
northern Plains in recent days and is now over northern Ontario
will continue slowly shift east across the central Plains/upper MS
Valley/Upper Midwest tonight. A potent southern stream trough
currently over the Four Corners will eject northeast tonight and
track along this baroclinic zone into Tuesday before it phases
with a northern stream impulse over the upper MS Valley late
Tuesday and close into mid-level low that tracks across western
Lake Superior Tuesday night. 12Z global guidance consensus has
featured this deeper/slower system with confidence increasing in
the intensifying comma-head-associated precip/snow Tuesday/Tuesday
night.

Ahead of this mid-level low and above the frontal zone, a powerful
southwesterly jet will allow some surface cold air to subduct the
frontal zone and raise some freezing rain potential on the back
side of the cold frontal precipitation this evening from northeast
Neb to Duluth and the Arrowhead of MN where a light glaze is
possible. The right entrance region of this jet streak will be
just ahead of the mid-level wave, aiding upper level divergence
and promote surface cyclogenesis as the low tracks from MO to WI
on Tuesday. The strong PVA above and west of the baroclinic zone
will wrap in the warm conveyor belt and allow comma head precip to
flourish over Neb, southeast SD, northeast IA, and southern MN
where a narrow band of moderate to locally heavy snow is now
expected. Day 1 snow probabilities for 4 or more inches are
moderate to high over central and northeast Neb into southern MN.

Farther northeast, and especially over east-central and northeast
MN and northwestern WI and the far western UP is where the
combination of cyclogenesis and upper level dynamics should
produce a heavy swath of snow. Day 1.5 snow probabilities are low
to moderate for 6 or more inches from south-central MN, across MSP
to the Arrowhead of MN. Lake enhancement from cyclonic flow over
much of Lake Superior should produce the maximum snowfall in the
far northwest corner of WI where Day 1.5 probabilities for 8 or
more inches area above 30 percent.

A narrow corridor of rain to freezing rain to sleet to snow is
expected to track on the northwest side of the surface low from
eastern Neb into WI. Day 1 freezing rain probabilities are 30 to
40 percent along the central IA/MN border.


...Pacific Northwest and Northern Rockies...
Days 1-3...

The next shortwave trough has dual vort impulses, of which the
leading one reaches western WA this evening which tracks across
the northern Rockies into MT through Tuesday. The reinforcing
impulse digs the trough farther south and it reaches the WA/OR
coast Tuesday afternoon and crosses the ID/WY Rockies Wednesday.
Decent Pacific moisture and low snow levels (around 3000ft in the
WA Cascades tonight and Tuesday and around 2000ft in the ID/MT
Rockies Tuesday) allow moderate snow in some areas of concern with
heavy snow for higher terrain. Day 1 snow probabilities for 6 or
more inches are high for the Olympics, WA and northern OR Cascades
and moderate for the higher OR Blue Mtns, and the highest ranges
of northern ID/western MT. Snow probabilities for 6 or more inches
on Day 2 are high for the central OR Cascades, OR Blue Mtns,
ranges in north-central ID and eastern ID/western WY as well as
moderate for far northern UT terrain. 48hr probabilities for 12 or
more inches through Day 2 are high in the WA/OR Cascades, Blue
Mtns, Clearwater Mtns of northern ID and the Grand Tetons of WI.

The next shortwave (in a long line of activity for the Northwest
CONUS - as is common with La Nina) approaches the WA/OR coast
Thursday with snow levels generally around 3000ft for the Olympics
and WA/OR Cascades.


Jackson