Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Mon Nov 29 2021

Valid 12Z Mon Nov 29 2021 - 12Z Thu Dec 02 2021


...Great Lakes through the Northeast...
Days 1-3...

Broad cyclonic flow will persist across the east through the
middle of the week, with embedded shortwaves producing periods of
precipitation within the region.

For today, the mid/upper level trough axis will continue to push
east across the Northeast and the favored lake effect snows will
begin to taper off as the better forcing and flow subsides.
Downstream surface low development off the coast of MA is expected
to remain far enough offshore to limit any potential for
significant/heavy snowfall. Lake effect snows should be fairly
localized and confined to mainly Lake Erie and Ontario with
additional accumulations under 2" through 00Z this evening.

A fast moving shortwave trough upstream over southern Canada will
race through fast quasi-zonal flow toward the Upper Midwest later
today. With it, a stripe of precipitation is expected as the
surface low tracks from northern North Dakota to western Wisconsin
then central Michigan. The better frontogenetical banding setup
favors accumulating snow from northeast WI, U.P. of Michigan, and
much of central/northern L.P. Michigan where 2" probabilities max
out at 40 percent off Lake Superior and are in the 20-30 percent
for northern L.P. Michigan. Once the wave passes the eastern Great
Lakes, the flow off Lake Erie and Ontario becomes more favorable
and should lead to localized lake effect heavier bands tonight
into Tuesday. Here the 2" probabilities are moderate (Lake Erie)
to high (Lake Ontario) for the favored snow belt areas.  

...Pacific Northwest...
Days 1-2...

Shortwave energy pushing toward the Pacific Northwest and its
associated atmospheric river will bring a round of widespread
moderate to heavy precipitation to the region. However, the
warm/tropical Pacific origin will lead to very high snow levels,
generally above 6000 ft today/tonight, rising well above 9000 ft
Tuesday into Wednesday. This should keep any threat of heavy
accumulating snow confined to the very high peaks of the WA
Cascades in the day 1 period where WPC probabilities for 4" are
slight/moderate for the high peaks. By Day 2 and Day 3 however,
the snow levels at or above 9000 ft will really limit accumulation
potential and the WPC probabilities for 4" fall to less than 10
percent.


For Days 1-3, the probability of significant icing is less than 10
percent.

Taylor