Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Sat Nov 13 2021

Valid 12Z Sat Nov 13 2021 - 12Z Tue Nov 16 2021


...Pacific Northwest...
Days 1-3...

After a brief break in between systems, a warm front will push
into Washington from the southwest this evening. Snow levels will
rise again toward 9000 ft after a brief period of warm advection
snow as temperatures rise. Levels are expected to remain high
until Monday when an amplified upper shortwave and its associated
cold front push into the region. This will take some light to
modest snow down through pass levels to perhaps 3000 ft as
precipitation exits the region. WPC probabilities of at least 4
inches of snow on day 3 are around 50-60% near Snoqualmie and
Stephens Passes.


...Northern Plains, Upper Midwest, and Great Lakes...
Days 1-3...

A fast-moving shortwave trough at the nose of a 150 kt jet will
track east-southeastward through the High Plains today and deepen,
promoting an organized surface low along the baroclinic zone
between milder air in central Montana (50s) versus a stout cold
dome over the Dakotas (20s). Warm advection precipitation will
continue to push eastward this morning resulting in a period of
freezing rain for central and eastern North Dakota. Ice
accumulations should generally be light (glaze to a few hundredths
of an inch) as WPC probabilities of at least 0.10" ice remain are
10-20%.

Sharp, but progressive height falls and brief upper diffluence
will support a concentrated swath of snow just to the north of the
surface low track (from North Dakota southeastward into northern
Minnesota) later today. Guidance continues to come together with
the CAMs generally trending a bit heavier since yesterday closer
to the global guidance across MN day 1. WPC probabilities of at
least 4 inches of snow are 10-35% with a higher-end potential near
6 inches per the HREF and NBM with an axis of 700mb FGEN through
MN this afternoon.

Snow is expected to shift from northern Minnesota into the upper
Great Lakes region tonight and Sunday, with the better forcing
expected to center the heavier amounts from northwestern Wisconsin
to central and southern Lower Michigan. WPC probabilities indicate
than at least an inch or two is likely, with a small threat for 4
inches or more in areas downwind of favorable across-lake
trajectories along with slightly higher elevations. By Monday into
Tuesday, low pressure will weaken through the St. Lawrence Valley
and merge into a developing wave near the Gulf of Maine as the
upper trough sharpens, supporting only light snow across northern
Maine and waning snow east of Lakes Erie and Ontario, where WPC
probabilities of at least 4 inches are around 10 percent.


Fracasso/Pereira