Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
442 PM EST Mon Dec 28 2020

Valid 00Z Tue Dec 29 2020 - 00Z Fri Jan 01 2021

Days 1 to 3...

...Lower Great Lakes..

A swath of locally heavy lake-effect snow is expected downwind of
especially the lower Great Lakes this evening and overnight in the
wake of deepening surface low pressure exiting through southern
Quebec and a cold front crossing the Northeast. Strong cold-air
advection should yield locally heavy snow showers and squalls,
with the heaviest snowfall expected to be across western NY off
Lake Ontario and generally just to the southwest of the Tug Hill
plateau. Across this region, a low-level fetch involving both Lake
Huron and Ontario will be noted and should tend to favor somewhat
more organized and heavier lake-effect snow with several inches
likely.


...Four Corners Region to the Central Plains and Upper Midwest...

A deep upper trough and associated closed low will pivot across
the Four Corners region on Tuesday. This will initially drive very
heavy snowfall with the aid of strong mid/upper-level forcing,
moisture transport, and orographic ascent for areas of western CO
and especially the San Juan mountains where as much as 1 to 2 feet
of new snow is expected through midday Tuesday. Lesser
accumulations are expected back to the west across areas of the
Wasatch.

Meanwhile, a leading shortwave associated with the broader upper
trough is expected to eject out across the central Plains and
upper Midwest and this energy coupled with the arrival of northern
stream shortwave energy from southwest Canada should generate a
broad and strong warm advection pattern across the broader Plains
and Midwest as surface low pressure develops over the lower MO
Valley by Tuesday night and aims for the upper Great Lakes region
by Wednesday morning. A broad swath of light to moderate snowfall,
with some occasional heavy snowfall is expected for areas of
southern SD, central/eastern NE and IA going through late Tuesday,
with then areas of far northwest IL, central/southern WI and lower
MI seeing a relatively focused threat of heavy snowfall Tuesday
night and early Wednesday. Generally as much as 4 to 8 inches of
snow can be expected with isolated heavier amounts approaching 10
inches possible going through midday Wednesday. Along the southern
periphery of the accumulating snowfall axis, the expectation is
also for some sleet and freezing rain, as a strong warm nose aloft
moves in. The sub-freezing boundary layer temperatures will likely
result in some areas of a 0.10+" of ice accretion which will be
most likely to occur from northeast KS through far northern MO,
southern IA and into northwest IL. Lesser accretion values are
expected for areas immediately adjacent to this corridor.


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

As the cold front associated with the upper Midwest winter storm
on Tuesday and Wednesday moves steadily southeastward, cold high
pressure will be building in behind it and should be in place
across large areas of the Midwest and Great Lakes region by
Thursday. Meanwhile, the aforementioned upper trough and closed
low over the Four Corners region will be advancing gradually off
to the east and should reach the southern Plains on Thursday. This
will in turn generate strengthening surface low pressure over the
lower MS Valley with increasingly strong warm-air advection and
Gulf of Mexico moisture transport fostering an expanding area of
moderate to heavy precipitation. The northern 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.
As much as a 0.25+" of ice accretion will be possible. The threat
of freezing rain will extend all the way down across areas of
central OK and into north-central TX, and also downstream across
the remainder 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.

Some mix with snow will be possible along the northern periphery
of the precipitation shield involving the middle MS/OH Valleys,
but there will be notably bigger concerns for accumulating
snowfall down across areas of central to 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. Locally several
inches of accumulating snowfall will be possible, although there
is currently considerable uncertainty as to the amounts and
placement of the snow.


...Pacific Northwest and Northern Rockies...

A fairly deep upper trough and associated closed low will advance
inland across the Pacific Northwest by late Wednesday along with a
cold front. Strong onshore flow into the higher terrain of the
Cascades will yield robust Pacific moisture transport which
coupled with lowering snow levels should foster heavy snowfall
accumulations from late Wednesday through Thursday. As much as 1
to 2 feet of snow is expected with the heaviest totals likely in
the WA Cascades. Locally heavy snow though is expected downstream
across portions of the northern Rockies by late Thursday including
the Bitterroots where spillover moisture and energy will be
arriving.


Orrison