Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
417 AM EST Thu Dec 31 2020

Valid 12Z Thu Dec 31 2020 - 12Z Sun Jan 03 2021

...Southern Plains/Upper Midwest/Northeast...
Days 1-3...

...A significant and large scale winter storm will is expected
across the eastern half of the CONUS into early 2021...

A deep closed low positioned over the Sierra Madre of Mexico this
morning will deepen as it moves into Texas tonight. This feature
will then gradually begin to fill as it lifts northeast across the
Central Plains and towards the Great Lakes Friday night, and then
into New England Saturday. Beneath this mid-level feature, a
surface low pressure is likely to lift northeast from TX to MO on
D1, and then more rapidly to the northeast to be off the coast of
Maine by Saturday aftn. This low will be accompanied by a leading
warm front, and robust antecedent WAA spreading significant and
widespread precipitation across the eastern half of the CONUS.

Guidance this morning has trended a bit south and slower with the
primary system, and the mass fields of the ECMWF/GFS were used in
the blend, with the NAM thermals preferred for its low-level
thickness evolution. Cool surface high pressure ahead of this low
will be reinforced by mid-level confluence ahead of the upper
feature, and as WAA spreads precipitation northward, an expansive
area of freezing rain is likely. There are still considerable
uncertainties into how much freezing rain will accumulate due to
model uncertainty in the strength of the low-level warm nose and
northward extent of precipitation. The WAA is likely to be quite
strong so many areas from eastern OK through the Ohio Valley and
into parts of the Northeast will start as freezing rain but then
change to rain. Further north, a band of significant freezing rain
is likely, and WPC probabilities for 0.25" are greater than 50% on
D1 in eastern KS and much of central MO, lifting into northern MO,
IL, and IN/MI D2, as well as spreading into the higher terrain of
PA ahead of the expansive warm front where a CAD setup is likely.
Isolated amounts to 0.5" are not out of the question, but heavy
rates should somewhat limit accretion efficiency and the
antecedent surface airmass is not extremely cold. Although these
regions demarcate the area of most likely heavy freezing rain,
there is likely to be rather expansive coverage of at least a few
hundredths of freezing rain accretion extending from West Texas
northeast all the way into Maine, with pockets of 0.1-0.2" likely
in central TX, the Midwest, and the terrain of New York and
western New England.

In addition to the freezing rain, areas of heavy snow are also
likely on the northern and western edge of this system. On D1,
heavy snow is likely in West Texas beneath the upper low where
lapse rates and cold air aloft combine to drive instability and
heavy snow rates. The strong height falls, intense upper
diffluence, and steep lapse rates will likely combine with
mid-level fgen to drive a corridor of heavy snowfall, and WPC
probabilities are high for 4 inches, with locally as much as 8"
possible. During D2 as the low shifts northeast, a TROWAL is
likely to develop throwing rich theta-e air NW within the WCB and
combining with an axis of deformation as northern stream energy
interacts with the primary trough. This should lead to a band of
heavy snow NW of the center from north-central OK into eastern KS
and far NW MO. WPC probabilities for this area have increased as
the HREF snowband probabilities indicate a high chance for 1"/hr
snow rates as the low pulls off to the northeast. There is a
moderate risk for 4 inches across this area on D2. As the low
moves further northeast and begins to spread precipitation across
New England D2-3, heavy snow is likely to overspread much of
upstate NY and northern New England which will be cold enough for
primarily snow due to a southern shift in the guidance overnight.
Eventually the WAA may still cause a changeover to freezing rain
in some places, but WPC probabilities for 4" of snow are high in
the Adirondacks, Greens, Northeast Kingdom of Vermont, and much of
northern NH and ME.

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

A series of shortwave troughs, separated by brief shortwave
ridging will move onshore the PacNW through the weekend, each one
accompanied by a surge of moisture on increasing Pacific jet
energy. The first is a potent wave moving onshore this morning
which will drive ascent through jet level diffluence combined with
mid-level PVA and height falls. Periods of heavy snow are likely
in the terrain of the WA Cascades, Blue Mountains of OR, and
eastward into the Northern Rockies. This shortwave will advect
quickly eastward shutting off the heavy precipitation by this
evening. Before snowfall ends, heavy accumulations are likely and
WPC probabilities are high for 6" in these ranges, with more than
12" possible in isolated parts of the WA Cascades.

Late D2 into D3, another shortwave moves onshore bringing rapid
height falls accompanied by a more robust just streak to provide
deep layer ascent. The overall column is a bit colder with this
second, and mid-level WAA/moist advection ahead of the shortwave
will provide plenty of moisture for heavy precipitation noted by
IVT above the 90th percentile according to the NAEFS tables moving
onshore Washington Saturday evening. Heavy snow is likely in the
Olympics and Cascades of WA, and spreading into the OR Cascades
southward, and towards the Northern Rockies eastward. WPC
probabilities are high for 8 inches in these ranges D3, with
locally more than 2 ft possible in the highest terrain of the WA
Cascades and Olympics where upslope flow will contribute to
enhanced snowfall.



Weiss