Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 PM EST Fri Dec 31 2021

Valid 00Z Sat Jan 01 2022 - 00Z Tue Jan 04 2022

...Western U.S...
Days 1-3...

A shortwave digging out of the Pacific Northwest will sharpen into
a longwave trough Saturday. Downstream of this trough, pronounced
moist advection on increasing confluent mid-level flow and within
a subtropical Pacific jet streak will surge into the Four Corners,
while height falls ahead of the trough axis drive ascent. The
overlap of this will lead to heavy snowfall in the terrain from
the Mogollon Rim eastward to the Sacramento Mountains, and
northward through the San Juans, Sangre De Cristos, and southern
CO Rockies on D1. Heavy snow is likely, generally above 4000 ft,
with snow levels falling rapidly behind a cold front, but in
conjunction with waning precipitation. WPC probabilities are high
for 8 inches in these ranges, with local maxima above 2 feet
likely in the White Mountains of AZ, and parts of the New Mexico
Sangre de Cristos.

A short wavelength but with impressive amplitude will build in
behind this lead trough, bringing a brief respite to snowfall
across the West. However, by late D2 and into D3 (after 18Z
Sunday) precipitation will again overspread the region from the
Pacific Northwest eastward to the Northern Rockies and southward
towards the northern CA ranges. This precipitation will be driven
by a closed low dropping out of the Gulf of Alaska, with pinched
flow to the south providing moist advection onshore aided by a
modest and zonal Pacific jet streak. An upper divergence maxima
pivoting onshore Monday morning will enhance ascent, and although
snow levels will climb above 3000 ft in northern CA and southern
OR, they will otherwise remain 1000-2000 ft during the period of
heaviest precipitation. This will then manifest as heavy snow,
with WPC probabilities for more than 8 inches reaching 80% D2 in
the Olympics and WA Cascades, with high probabilities for 8+
inches on D3 reaching along the OR Cascades, into the northern CA
ranges near Shasta, and over towards the Northern Rockies. 2-day
total snowfall in the higher terrain of the Olympics and Cascades
could exceed 4 feet, and the low snow levels will also allow for
significant accumulations of more than 1 foot at Snoqualmie,
Stevens, and Santiam Passes.


...Central and Southern Appalachians...
Day 3...

A strong cold front will drop southeast Sunday evening, leaving a
sharp baroclinic gradient across the area. From the west, a
sharpening shortwave will eject out of the Southern Plains, while
a secondary jet streak surges northward to place the favorable
diffluent RRQ atop the Mid-Atlantic. The interaction of this
shortwave, diffluence, and low-level baroclinicity will likely
promote surface low development Monday. The guidance has continued
to trend slower and stronger with the shortwave, and this supports
a more intense low pressure system as it moves from the Southeast
to off the Mid-Atlantic coast Monday. Uncertainty abounds, but
there is potential for rain changing to heavy snow across parts of
the area. At this time, the greatest threat for accumulation will
be confined to the terrain of WV and NC where WPC probabilities
are around 10% for more than 4 inches. However, some light
accumulations are also possible outside of the terrain despite
recent warm temperatures. While this evolution has pretty good
agreement among the models now, it is still a new solution and
there are lots of details to still iron out. At this time it is
most likely accumulating snow will remain in the elevated areas,
but future updates will focus these details.


...Central Plains through the Great Lakes and into Northern New
England...
Days 1-3

...Significant winter storm expected for portions of Central
Plains to Great Lakes This Holiday Weekend...

Phasing shortwave energy across the Four Corners region will
interact with the RRQ of a powerful (200kt) jet streak arcing
across the Great Lakes to produce a surface low pressure and
associated wintry precipitation. The surface low is progged to
move from the lee of the CO Rockies towards Arkansas on Saturday,
and then lift northeast to off the New England coast by Sunday
night. Impressive ascent will accompany this low, with synoptic
forcing driven by upper diffluence and mid-level height falls
combining with WAA to expand precipitation northward. At the same
time, a robust moisture pool noted by PWs of +3 standard
deviations across the Southeast will surge northward as the LLJ
rises into a TROWAL and wraps cyclonically around the system. This
enhanced instability will combine with increasing frontogenesis
into a very deep DGZ to produce impressive snowfall from Kansas
northeastward towards Chicago on D1, with less robust but still
significant snowfall spreading northeast into Northern New England
on D2. The guidance has been pretty consistent today with its
footprint, but an increase in snowfall has been noted across most
models, likely due to a better potential for convective snowfall
rates progged by folded theta-e surfaces coincident with -EPV, and
forecast by HREF 1"/hr and 2"/hr probabilities. The heaviest
snowfall is likely late tonight through Saturday evening from
Kansas through northern Illinois where SLRs could reach 20:1, and
WPC probabilities are more than 70% for greater than 6 inches.
Local maxima of 10" are probable as noted by the WSE and NBM 90th
percentile across this area, with a secondary maxima possible near
the Chicago metro where NE winds off Lake Michigan should create
some LES enhancement. Farther northeast, the ascent and available
moisture both wane on D2, but WPC probabilities still indicate a
high chance for more than 4 inches across the central L.P. of MI
into the Adirondacks of NY and parts of northern VT and NH. By D3
some lingering snowfall may produce a few inches of accumulation
across far northern ME, where WPC probabilities are 10-30% for
more than 4 inches.

South of the heavy snow axis, a swath of mixed precipitation is
expected, with a period of freezing rain and sleet producing
significant accumulations. The guidance has trended down on the
freezing rain accretions today, likely due to a more rapid
changeover as strong CAA rapidly deepens the sub-freezing layer
despite a pronounced warm nose persisting. While the probability
for a stripe of freezing rain accreting to more than 0.1" is still
above 50% in central MO, accretions much greater than that are
less likely as the period of freezing rain may correlate with more
intense precip rates leading to less efficient accretion. Sleet
may accumulate to 0.5" in some locations after the changeover from
freezing rain.

Weiss


~~~Key Messages for the Central Plains to Great Lakes Winter
Storm~~~
Heavy snow and moderate freezing rain/sleet will spread from
Kansas northeast into Michigan late tonight through Sunday.

The most significant snow impacts are expected from northern
Kansas to the Chicago metro area. Snowfall rates n excess of 1"/hr
combined with strong winds will lead to dangerous travel and near
zero visibility at times.

Significant mixed precipitation including freezing rain and sleet
is likely to the south of the heavier snow. Ice accretions greater
than 0.1", and sleet accumulations near 0.5", are possible from
eastern Kansas into western Illinois.

Bitterly cold temperatures will follow with dangerously cold wind
chills Saturday night into Sunday morning.


Weiss