Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 PM EST Fri Nov 26 2021

Valid 00Z Sat Nov 27 2021 - 00Z Tue Nov 30 2021


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

Two separate storm systems may bring periods of heavy snow to the
Northeast into early next week.

The first of these is beginning this evening as a wave of low
pressure weakens over southern Canada while secondary
redevelopment occurs within the Gulf of Maine. This secondary low
will form in response to a shortwave that will close off across
Southern New England and interact with increasing diffluence
within the LFQ of an upper jet streak to lower surface pressures
east of Maine. This low is then likely to deepen quickly, but move
only slowly northward as it gets blocked by mid-level ridging to
the east. As this low pulls slowly to the northeast, precipitation
associated with it will spread into Maine, while NW flow in its
wake will drive upslope precipitation across the Adirondacks and
much of NH/VT.

For D1, the heaviest snow is likely in the favored upslope regions
of the NW Adirondacks, and the western slopes/ridges of the Greens
in VT and Whites of NH/ME. In these areas, despite dry air
advecting in quickly from the west during the aftn, high SLRs and
enhanced ascent via the upslope flow will produce several hours of
heavy snow which may reach 1"/hr at times. WPC probabilities are
high for more than 4 inches, with locally more than 10 inches
possible in the highest terrain. Additional heavy snow
accumulations are likely in northern Maine which will be driven
more robustly by the deep layer ascent, and WPC probabilities for
6 inches are as high as 50%.

As the surface low deepens and moves near Downeast Maine, rapid
cooling of the column will occur in its wake and the guidance
continues to suggest a narrow but potent deformation band will
pivot up the Midcoast and points northeast. This should rapidly
change precip from rain to snow, and folded theta-e/-EPV within
the DGZ and within the intensifying fgen could support briefly
heavy snowfall rates in excess of 1"/hr. There is lower confidence
here due to a more marginal column leading to lower SLRs, and
uncertainty in timing and placement, but WPC probabilities have
increased for 4" in this area and are as high as 50%. While
confidence is lower here, this could produce some high impact
snowfall over a brief period of time.

Snow will wind down from SW to NE Sunday /D2/ bringing a brief
respite to the area before another system approaches D3 /Monday/.
An amplifying shortwave digging rapidly out of Saskatchewan will
close off across Upstate New York while a jet streak intensifies
downstream of the primary trough axis. The interaction of this
trough and the diffluent portion of the upper jet will likely
produce another rapidly deepening low pressure off the New England
Coast Monday aftn. There remains considerable spread in both
timing and placement of this feature, but the trend this aftn has
been for a subtly further west track which could bring heavy snow
back into eastern New England Monday. WPC probabilities are
currently less than 5% for 4 inches, but this needs to be
monitored for potential significant snowfall once again D3.


...Pacific Northwest and the Northern Rockies...
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 D3 in the northern Cascades of WA.


...Great Lakes...
Days 2-3...

Dual shortwaves will drop out of Alberta will race quickly
southeast through the Upper Midwest and Great Lakes. The first,
weaker, impulse will move across Saturday, with the second, more
intense feature dropping across the region on Sunday. The first
impulse may produce a swath of light snow across from Wisconsin to
the L.P. of MI on D1, but WPC probabilities are only moderate for
2 inches. The more significant shortwave will follow quickly
behind the first one, and will amplify into a closed low over
Ontario Sunday evening. This will be accompanied by a
strengthening jet streak arcing eastward from the Pacific
Northwest, which will also transport enhanced moisture into the
region. The overlap of the diffluent portion of the jet streak
with the height falls/PVA associated with the shortwave will lead
to a region of robust deep layer ascent, with some additional lift
provided by modest frontogenesis in response to this evolution.
The guidance has trended northward with these features today, but
a band of briefly moderate to heavy snow could produce several
inches of accumulation in the U.P. and northern L.P. of MI D2,
spreading into OH/PA/NY on D3. WPC probabilities for 4 inches are
as high as 30% within this heaviest axis of snow D2, increasing to
80% on D3 downwind of Lake Erie where LES will contribute to local
amounts exceeding 6 inches.


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

Weiss