Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 PM EST Tue Feb 01 2022

Valid 00Z Wed Feb 02 2022 - 00Z Sat Feb 05 2022

...Pacific Northwest...
Days 2-3...

Two shortwaves, one on Thursday and another on Friday will drop
southeast into the Pacific Northwest, separated by brief shortwave
ridging Thursday night. The trailing shortwave is the stronger of
the two, but both will be accompanied by LFQ diffluence at the
leading edge of a Pacific jet streak to enhance deep layer ascent
and spread moisture inland. The shortwaves will be embedded in
otherwise mostly confluent flow, so the duration of forcing to
produce snowfall should be limited as each shortwave is expected
to be transient. However, briefly heavy snow is likely in the WA
Cascades both D2 and D3 where WPC probabilities for 6+ inches peak
above 40%. Snow levels during this time will be rather low, about
2000 ft on D2 and climbing to around 3000 ft on D3. This suggests
that many of the Cascade Passes will also receive accumulating
snow,with more than 6 inches likely at Snoqualmie Pass.

...Southern Rockies through the Central Plains and into the
Northeast...
Days 1-3...

...Several rounds of heavy wintry precipitation likely as a large
scale system expands across the eastern CONUS...

Northern stream and southern stream shortwaves will eject out of
the Western United States and Western Canada and interact to
produce a large scale wintry precipitation event across much of
the eastern CONUS. There are likely to be three distinct areas of
precipitation associated with this system.

1) Southern Rockies D1-2: The shortwave digging out of British
Columbia will surge nearly southward as a ridge amplifies across
the Pacific Ocean. This shortwave digging into the Four Corners
Wednesday will lead to downstream SW mid-level flow which will
help drive moist advection into the Rockies. Height falls and PVA
will accompany this feature, while the associated deepening trough
leads to downstream jet streak amplification leaving the favorable
RRQ for ascent atop NM/CO. The overlap of these features will
occur atop a residual baroclinic zone as a cold front drops
southward and banks into the terrain producing additional ascent
through upslope flow. With PW anomalies of +0.5 to +1 standard
deviations in place, this will lead to heavy snow from the the
Front Range of CO southward through the Sangre De Cristos and into
the Sacramento Range by D2, with heavy snow also extending into
the High Plains. WPC probabilities for 8+ inches are above 80% in
the higher terrain, with local maxima over 2 feet possible,
highest in the Sangre De Cristos. Into the High Plains, more than
4 inches is likely, with some local maxima to 6+ inches occurring
across SW KS where some enhanced fgen behind the front may lead to
more robust ascent.

2) Upper Midwest D1-2: As the southern stream impulse consolidates
over the Four Corners, the northern stream energy will shift
towards the Great Lakes, with confluent flow between the two
occurring into the Ohio Valley. With the front shifting to the
south, this will leave a band of robust mid-level ascent atop a
strengthening baroclinic boundary which will direct intense ascent
into a deepening DGZ. As this occurs, the upper jet streaks will
begin to couple leaving even more deep layer ascent, into which
impressive 290-295K isentropic upglide will occur with mixing
ratios of 3-4 g/kg. This energy will shear off to the northeast,
and it is likely a band of heavy snowfall with rates of 1-2"/hr at
times will shift from Missouri northeast towards Ohio/Michigan
along the main fgen axis, and this could include the Chicago metro
area. While guidance still features spread in the placement of
this axis, and how sharp the NW cutoff will be due to drier air,
it is likely a swath of very heavy snowfall from the intense snow
rates will produce more than 8 inches of snow from central MO
through NW OH D1 into D2, with local amounts over 12" likely where
the bands persist and translate the longest. Near record snowfall
from this first event is possible in some Midwest cities. South of
this band of heavy snowfall, a mix of freezing rain and sleet is
likely in a swath from southern MO into southern OH. There remains
considerable uncertainty into how much of this mix will be
freezing rain and how much will be sleet, but recent model trends
suggest significant sleet could be the primary mixed p-type.
Still, current WPC probabilities are above 80% for 0.1" of
accretion D1 into D2.

3) Texas northeast through Maine D2-3: The primary southern stream
shortwave will begin to eject from the Four Corners D2 while
interacting with northern stream energy to intensify into a closed
low near OK. As this occurs, a pronounced subtropical jet streak
will arc poleward, reaching as high as 190kts near northern
Michigan, near a record for the date according to SPC climatology.
Rapid height falls and pronounced mid-level divergence will
combine with this intense diffluence in the RRQ of the powerful
jet streak to drive deep layer ascent. Within this region of
intense lift, moisture will surge northward with PWs reaching to
+2 standard deviations and IVT exceeding the 90th percentile
according to NAEFS ensemble tables. Together with mid-level
confluence downstream to reinforce the cold surface high pressure,
this is setting up to be a significant snow and mixed-precip event
as far south as Texas, and extending across the Central Plains, OH
Valley, and into the Northeast. The heaviest precipitation from
this second wave is likely to occur just south of the leading
precipitation on D1-2, but there is likely to be some overlap. The
rapid motion of this system should limit truly excessive snowfall,
but heavy rates in the moist column should produce a swath of more
than 6 inches of snow from NE OK through northwest OH, including
St. Louis, MO and Indianapolis, IN on D2, with that heavy snow
expanding into the northeast and northern New England D3 where WPC
probabilities are above 70% for 6+ inches.

Potentially more impactful, south of the heavy snow is likely to
be a zone of heavy mixed precip including sleet and freezing rain.
Guidance again is struggling with how much of this will be IP vs
ZR, but conceptual models indicate a high likelihood for more than
0.25" of freezing rain in some places, with more than 0.5"
possible. WPC probabilities suggest the greatest risk for more
than 0.25" of freezing rain is from northeast TX through northern
AR and then northeast along the Ohio River and into parts of
western PA. WPC probabilities for more than 0.5" are as high as
30% in western TN and along the KY/IL/IN borders. While models
still disagree on the exact placement of this stripe of freezing
rain, there is likely to be a swath of heavy sleet, potentially
accumulating to more than 0.5", just NW of where the heaviest
freezing rain occurs.

Weiss


~~~ Key Messages for Feb 1-3 Winter Storm ~~

-- A large and significant winter storm will impact much of the
central U.S. beginning tonight, bringing a variety of winter
weather hazards including heavy snow, sleet, and freezing rain.

-- The storm will be prolonged with several rounds of winter
weather lasting through Thursday for portions of the central U.S.
before shifting to the interior Northeast.

-- Heavy snow is expected over the southern Rockies and from the
south-central Great Plains through the eastern Great Lakes and
interior Northeast. A corridor of heavy ice accumulation is likely
from Texas through the Ohio River Valley.

-- Locations impacted by snow and/or ice are expected to have
temperatures remain below freezing, and well below average, for at
least a couple days after the wintry precip ends.

-- Plan now for prolonged hazardous winter weather conditions and
disrupted travel.