Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 AM EST Wed Feb 22 2023

Valid 12Z Wed Feb 22 2023 - 12Z Sat Feb 25 2023

...Long duration, major coast-to-coast winter storm continues...

...Four Corners through the Northern Plains, Upper Midwest, into
the Northeast...
Days 1-3...

The major winter storm will intensify and expand across the Great
Basin, Four Corners, and into the Northern Plains today with
widespread heavy snow producing major to extreme impacts.

The primary trough producing this event is a closed 700-500mb low
which will be centered over the northern Great Basin by Wednesday
morning, with associated height departures approaching -4 standard
deviations below the climo mean according to the NAEFS ensemble
tables. This closed low will slowly retreat to the north into
Canada Thursday morning. However, the retreat of the core of this
gyre will not at all indicate a weakening of this impressive
event. Instead, impressive downstream divergence and and robust
PVA ahead of a sheared by intense shortwave lifting from the Four
Corners into the Northern Plains and Upper Midwest will drive even
more intense ascent and continue this major winter storm and its
wide ranging impacts. Beneath the primary upper trough, an arctic
cold front will dig southward such that while snow levels in the
antecedent WAA will be as high as 4000 ft, they will drop
precipitously to less than 500 ft as far south as the central CA
Valley and southern Great Basin by the end of D1, and then even
lower before leveling off at generally 0-1000 ft across the entire
West. With steep lapse rates beneath the upper low combined with
impressive and increasing upper diffluence as the jet streak
digging along the Pacific coast couples with the downstream
subtropical jet streak, synoptic ascent will be quite robust
through the period, with just a gradual shift southward as the
front and associated fgen maximize omega. During D1 /Wednesday/the
most anomalous moisture noted by PW will be across the eastern
Four Corners and even more intensely into the Upper Midwest as
Pacific moisture within the upper level jet and embedded within
confluent mid-level flow downstream of the primary trough axis
advects eastward, and combines with increasing meridional moisture
advection out of the Gulf of Mexico on intensifying isentropic
upglide. In the central and southern Rockies, this overlap of
Pacific moisture with the deep layer synoptic ascent, and aided by
the combination of strong mid-level fgen along the arctic front
with upslope enhancement will result in heavy snowfall in the
terrain, with lesser but notable accumulations even into the
valleys and foothills. WPC probabilities on D1 for more than 6
inches are 60-90% across much of the Four Corners terrain, with
1-2 ft likely in the higher terrain of the San Juans and White
Mountains of AZ, with modest probabilities for 1 inch down into
the valley floors of UT/CO and AZ. On D2 the coverage of 6+" WPC
probabilities shrinks but still reaches above 50% in the southern
Wasatch of UT and into the Mogollon Rim, as well as the San Juans
once again.

More impressively today, the core of this major winter storm will
take shape and spread tremendous snowfall, both in coverage,
rates, and amounts, across the High Plains and Upper Midwest. The
primary driver of this winter storm, which will likely reach
blizzard criteria in some areas, is intensely coupled upper jet
streaks overlapped with robust height falls along the negatively
tilted shortwave to drive surface low development. The
amplification of these features will support intense downstream
meridional moisture transport from the Gulf of Mexico with PWs
surging to +2-3 standard deviations above the climo mean. This
moisture will wrap within the WCB into a robust TROWAL, supporting
strong elevated instability, while the associated WAA helps to
deepen the DGZ which will almost certainly exceed 100 mb as
reflected by SREF probabilities reaching 90%, with some regional
soundings indicating up to 300mb of DGZ depth, exceptionally deep
for heavy banded snowfall. The intense ascent into this DGZ fueled
by the isentropic ascent, fgen, and increasing deformation
especially late D1 and into D2 could yield snowfall rates in
excess of 2"/hr with high SLR in the cold column limited only by
the likelihood of dendritic fracturing. Additionally, the setup
seems to support that this most intense band will shift in
paradigm from a translating band to a more pivoting band into D2,
which would temporally extend the duration of this heavy snowfall.
Confidence continues to increase in an exceptional swath of heavy
snow stretching from the Black Hills of SD through
southern/central Minnesota and central Wisconsin, although the
exact placement of the heaviest axis still features some
variability among the guidance, with a subtle south and east shift
noted in the axis of heaviest snow with tonight's models. WPC
probabilities for more than 6 inches D1 and D2 are above 80% for
much of SD, southern MN, and central WI, D1, shifting into
northern WI, the U.P. of MI, and the northern L.P on D2. 12-24" is
possible where the best banding occurs, which is most likely
across the Coteau des Prairies and Buffalo Ridge, eastward through
the Twin Cities, and then extending across northern WI, with LES
enhancement leading to locally higher snowfall along the Bayfields
Peninsula.

Farther to the east, and just south of the heavy snow areas across
the Upper Midwest and Great Lakes is the growing signal for an
impactful and significant ice storm. A potent warm nose with 850mb
temps > 0C will lift well northward, and as a piece of the
elongated Canadian high pressure arcs eastward towards northern
Quebec and Ontario, an overrunning/wedge scenario appears to be
unfolding resulting in a mixed precipitation event of snow, sleet,
and freezing rain beginning Wednesday morning then continuing into
Wednesday night. From central IA through southern WI, northern IL
and particularly southern MI, a band of heavy freezing rain is
likely, and guidance has become a bit more aggressive with ice and
also features a more southern push in the heaviest axis, likely
due to the previously noted deep cold layer below the warm nose to
support heavy sleet in some areas. However, elevated instability
and tremendous moisture advection resulting in heavy rain rates
should limit accretion efficiency despite a continued push of
dry-bulb temps on low-level northeast winds. Still, freezing rain
exceeding 0.25" and potentially reaching 0.5" is most likely in a
stripe from eastern IA through norther IL and especially southern
lower Michigan, with Chicago, Grand Rapids, and Detroit all
possibly seeing a significant ice storm. Heavy sleet exceeding 1"
is likely just north of the heaviest freezing rain.

Even farther downstream, on the leading edge of the warm advection
push across the interior Northeast and New England, a rapid
transition from heavy, wet snow to mixed precip is expected. The
best signals for impactful ice accumulations looks to be across
western/interior New York, northern PA and into parts of New
England where probabilities of 0.1" or greater reach 50-80%,
highest near Buffalo, NY. Farther north, heavy snow is expected,
with WPC probabilities featuring a high risk for more than 8
inches in the Adirondacks, northern Greens, and Whites, with
slightly lower probabilities extending across much of Maine.
Lighter snow and ice accumulations are likely across much of
southern New England.


...Western U.S. from Oregon through Southern CA and the
Southwest...
As the primarily and expansive mid-level trough across the
Intermountain West retreats to the northeast late D1, a rapidly
amplifying mid-level low will sharpen off the Pacific Northwest
coast and then dive southward along the length of the Pacific
coast to take up position west of Los Angeles by the end of the
forecast period /Saturday morning./ This impressive system will be
characterized by height anomalies reaching -3 to -4 standard
deviations below the climo mean according to NAEFS ensemble
tables, supporting similar departures from normal of the 500/700mb
temperatures across the West. Impressive height falls along the
coast paired with downstream divergence into the Great Basin, and
steepening lapse rates beneath the extremely lowered heights will
provide plentiful synoptic ascent through the period. This strong
omega will act upon an environment that will become increasingly
saturated fueled by an atmospheric river (AR) with IVT of 250-500
kg/m/s surging onshore according to the CW3E probabilities,
focused most significantly into southern CA late D2 into D3. With
snow levels crashing to 500 ft or less across much of the West,
and around 1000-2000 ft for the southwest/southern CA, even some
of the foothills, valleys, and mountain passes could receive
impactful snowfall with this event. Copious onshore flow in the
presence of this increased moisture will effectively upslope into
the Cascades, the coastal ranges from northern CA through the San
Bernadinos and San Gabriels, with impressive ascent also focusing
into the Sierra. While the heaviest accumulations of snow will
likely be in the ranges, strong synoptic lift within the
downstream divergence overlapped with strong LFQ diffluence as the
subtropical jet streak pivots northeastward will allow for heavy
snowfall across the southern Great Basin within the core of the AR
by D3 as well. WPC probabilities for more than 6 inches of
snowfall sink southward D1 and D2, first reaching 80% or more in
the OR Cascades and OR/northern CA coastal ranges D1, then
shifting into the Sierra and Shasta/Trinity/Siskiyou regions D2.
By D3, the closed low becomes more intense driving the AR directly
into the southern Sierra and Transverse/Peninsular ranges, but the
WAA should drive snow levels back towards 3000 ft. Still,
impressive moisture being acted upon by strong ascent will result
in widespread heavy snowfall, and it is likely that multiple feet
of snow will occur in the terrain, with current forecasts
suggesting upwards of 5-6 feet possible in some areas. Heavy snow
will likely spillover into Nevada as well where WPC probabilities
for more than 8 inches are 50-80% on D3. This event will become
extremely impactful for parts of southern CA, and pWSSI already
indicates a high chance for extreme impacts due to snow amount and
snow load for the San Gabriels.


Weiss


...Key Messages for Major Coast-to-Coast Winter Storm...
-A prolonged major winter storm will continue from the West Coast
to the Northeast through this week. This will be extremely
disruptive to travel, infrastructure, livestock, and recreation.

--The greatest impacts will be over portions of the High Plains
into the Upper Midwest today into Thursday. The combination of
heavy snow rates of 1-2"/hr and wind gusts of 40-50 mph will
create blizzard/whiteout conditions, nearly impossible travel,
power outages, and scattered tree damage.

--Across the Pacific Coast and Intermountain West, heavy snow
rates and strong winds, which in some places will gust above 60
mph, will result in near-blizzard conditions producing dangerous
travel and scattered power outages. In addition, impactful snow is
possible into less typical foothills and valleys.

--A wintry mix of heavy snow, sleet, and freezing rain will impact
the Great Lakes and Northeast from late Wednesday into Friday.
Treacherous travel conditions are likely, with scattered power
outages possible in the region of heaviest freezing rain.

--Record lows and dangerous wind chills are likely across much of
the West into the Plains through the week.