Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 AM EST Tue Dec 29 2020

Valid 12Z Tue Dec 29 2020 - 12Z Fri Jan 01 2021

Days 1 to 3...


...Central Plains and Upper Midwest and Great Lakes...

A shortwave associated with the broader upper trough is expected
to eject out across the northern to central Plains and upper
Midwest.
Snow has developed in western SD and central Nebraska, and this
area should continue to grow as a broad and strong warm advection
pattern across the broader Plains and Midwest.  The stronger
ascent occurs on the nose of the 850-700 mb jet crossing eastern
KS across northern MO and eastern Iowa.  A band of heavy snow is
expected across eastern Iowa to southern and eastern WI where
temperature remain cold enough for snow. 6 to 10 inches of snow is
likely in this area.

South of this area, a mixed precip event occurs from northern KS
across northern MO and adjacent southeast NE, far southern Iowa,
and northern IL/southern MI.
The expectation is for sleet and freezing rain, as a strong warm
nose aloft moves in. The sub-freezing boundary layer temperatures
will likely result in areas of a 0.10 to 0.25+" of ice accretion
from northeast KS through northwest MO, southern IA and into
northwest IL. Lighter freezing rain amounts are expected into
southern lower MI and northwest IN/central IL. The primary
uncertainty is tight gradients exist in northeast KS and northern
MO, with the high res windows showing potential for locally higher
sleet/icing accumulations in northeast KS vs. the lighter QPF and
resultant ice accumulations in the global models.


...Southern Plains to the Lower/Middle MS Valley along with the OH
Valley and Northeast...

Cold high pressure will be building across large areas of the
Midwest and Great Lakes region by Thursday. The upper trough and
closed low over the
southwest will be drifting east and should reach the southern
Plains on Thursday. This will in turn generate strengthening
surface low pressure over southeast Texas
with increasingly strong warm-air advection and Gulf of Mexico
moisture transport moving inland.  Coupled upper divergence/lower
convergence maxima in conjunction with the moisture fluxes lead to
an expanding area of moderate to heavy precipitation.

The northwest periphery of the precipitation shield is likely to
encounter sufficient cold air from the high center to the north
for an axis of sleet and freezing rain, with concerns for locally
significant ice accretion possible for areas of northwest AR,
southern MO and southern IL on Day 2. As much as a 0.25+" of ice
accretion will be possible. On Day 3, the axis of icing moves
north into northern MO, eastern KS and much of central OK and into
north-central TX.
Light icing is possible in northern portions of the OH Valley.
Farther to the northeast across the interior of PA/NY and New
England, some light icing from freezing rain will be possible on
the warm-air advection side of the aforementioned cold front as it
arrives across the Northeast on Thursday.

West of the 700 mb closed low, accumulating snowfall is forecast
down across areas of southwest TX including parts of the Edwards
Plateau and adjacent areas of the TX Big Bend. This will be tied
into the track of the upper-level low center which is expected to
foster locally strong dynamic cooling and enough of it in
conjunction with low-level cold-air advection for rain to mix with
or change over to snow. Higher accumulations on Day 2 would be in
the Davis Mountains of southwest TX, then extending into west
central Texas on Day 3.   As the closed mid level circulation
moves north out of TX, precip in central to western OK and
southwest MO eventually changes over to snow, so light
accumulations are expected near the 700 mb low as it arrives.

...Pacific Northwest and Northern Rockies...

A strong upper trough and associated closed low will advance
inland across the Pacific Northwest by late Wednesday. Strong
onshore flow into the higher terrain of the Cascades will yield
robust Pacific moisture transport, which should combine with mid
level ascent to bring snow to the WA Olympics and Cascades on Day
1.  As the upper trough moves onshore Wed, cooling aloft leads to
lowering snow level and greater coverage of snow in the ranges of
WA/OR into the northern Rockies.
Heavy snowfall accumulations of 1 to 2 feet of snow are expected
with the heaviest totals likely in the WA Cascades. Locally heavy
snow though is expected downstream across portions of the northern
Rockies Wed in the ranges of northeast WA across northern ID as
the forward branch of the 300 mb crosses the region, accompanied
by upper divergence maxima.  On Thu., the upper trough moves east
of the Rockies, so snow coverage/intensity should taper in the
Pacific northwest to northern Rockies.


Petersen