Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Sat Nov 27 2021

Valid 12Z Sat Nov 27 2021 - 12Z Tue Nov 30 2021


...Northeast...
Days 1-3...

The ongoing storm system will begin to pull away from the region
with the best forcing and potential for accumulating snow winding
down after 18Z today. The greatest probabilities for widespread
accumulating snow will be confined to northern/down east Maine
this morning as well as portions of the higher peaks of northern
VT/NH where WPC probabilities for 2" remain as high as 40 percent.

Attention then turns to another potential storm system during the
late day 2 into day 3 time frame /late Sunday into Monday/. A fast
moving shortwave trough swinging through the large scale cyclonic
flow present is forecast to close off at 500 mb over Ontario. The
increasingly negative tilted trough will spawn a surface low off
the Northeast/New England coast that is then forecast to lift
quickly east/northeast. There is considerable model spread in the
timing and location of the potential offshore surface low and its
track. The 00Z deterministic models continue to show some
variability, with the CMC/GFS/ECMWF being more offshore, keeping
the bulk of any accumulating snow well offshore. However, the NAM
advertises a much closer storm track to the coast and would wrap
around accumulating snow along coastal New England.

The WSE plumes and viewer highlights well the forecast uncertainty
with its large spread of potential solutions. Despite the lower
confidence, the WPC probabilities for 4" this cycle did trend
higher and now are in the 20-30 percent range (with some isolated
maximum of 40 percent). This system bears a close watch given the
current uncertainty and potential for heavy snow along coastal New
England.


...Pacific Northwest...
Days 2-3...

Modest shortwaves embedded within fast mid-level flow will move
onshore British Columbia, Canada Sunday, and Washington State on
Monday. Each of these shortwaves will produce enhanced ascent in a
column characterized by PWs of around +2 standard deviations above
the climo mean. Each shortwave will be accompanied by a frontal
system and WAA, which will produce additional ascent and provide
enhanced moisture, but also raise snow levels as high as 9000 ft
during the period of heaviest precipitation. This will keep heavy
snow well above pass levels, and WPC probabilities for 4 inches
are high only on D2 into the first part of D3 in the northern
Cascades of WA.


...Great Lakes...
Days 1-3...

Within the larger cyclonic flow across the Great Lakes, several
fast-moving embedded shortwave troughs will swing through over the
next 48-72 hours. The first tracks through the Upper Midwest and
lower Great Lakes later today through Sunday and overall is the
expected weaker system of the three. Quick on its heels is a much
stronger shortwave trough that is forecast to pass a bit further
north Sunday, eventually closing off a low at 500mb over Ontario.
Finally, on Monday /D3/, another shortwave trough will race
through the Great Lakes, bringing renewed chances of snowfall to
the region.

The first system is expected to produce a narrow swath of
generally light snowfall across northern/northeast WI, both
peninsulas of Michigan, and downwind of Lake Erie, where the
combination of synoptic forcing and lake enhanced flow could lead
to slightly higher totals. Day 1 WPC probabilities for 2 inches
are moderate (50-60 percent) with the greatest signal (still
slight) of 4 inches being immediately downwind of the lakes,
particularly Lake Erie in the favored snow belt region of western
NY and northwest PA. The second, stronger system will favor more
lake effect snow, particularly downwind Lake Erie and Ontario
where WPC probabilities for 4" reach moderate to locally high
levels, especially across the NY Southern Tier region.

Finally, the third shortwave passing through for Monday /D3/ still
has some uncertainty in its eventual track and also strength. A
multi-model consensus approach yields a storm track across the
northern Great Lakes which would favor lake effect snows off
Superior, northern Michigan and Lake Erie. Still time to iron out
more details and specifics but potential exists for another round
of 2-4 inches with highest totals confined to downwind of the
lakes in the favored snow belt regions. The highest WPC
probabilities for 4" are downwind Lake Erie at this point.

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

Taylor