Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
447 AM EST Tue Feb 21 2023

Valid 12Z Tue Feb 21 2023 - 12Z Fri Feb 24 2023

...Long-duration, widespread, major winter storm expands from the
Northwest Monday night through much of the West and northern tier
states through next week...

...Upstate New York and New England...
Day 1...

A shortwave rotating through the broad trough enveloping most of
the east will drag a modest clipper type surface low eastward
along the Canadian/United States border over New England. This
surface low will weaken through D1, while secondary low
development occurs south of New England in response to more
pronounced shortwave troughing moving off the Mid-Atlantic coast.
This secondary low will act to both increase downstream moisture
advection into the Northeast, while also maintaining better cold
air despite high pressure departing to the east. The combination
of ascent in the vicinity of these two lows combined with enhanced
moistured noted by PW anomalies of +0.5 standard deviations will
result in a period of moderate to heavy snow with snow rates
reaching 1"/hr at times. There remains some uncertainty into the
placement of the heaviest snow, and the guidance appears to have
shifted southward just a bit placing the heavier snow into the
better moisture supported by more impressive upper dynamics, but
total accumulations should still remain modest, and WPC
probabilities are less than 10 for 4 inches, but local amounts
reaching 4 are possible in the Adirondacks.


...Western United States through the Northern Plains, Upper
Midwest, into the Northeast...
Days 1-3...

A complex evolution of the mid-level pattern, generally featuring
an amplifying trough across the West with subsequent shortwave
shedding into the Northern Plains will result in lowering snow
levels and strong synoptic/mesoscale ascent, into well above
normal moisture, to produce a major winter storm with far-ranging
and long-lasting impacts for much of the region.

The root of this event will be an amplifying 700-500mb trough
which will dig across the Intermountain West today into Wednesday,
peaking with amplitude of -4 standard deviations below the climo
mean with respect to heights according to the NAEFS ensemble
tables. The core of this upper low will pivot into the northern
Great Basin Wednesday morning before slowly retreating back to the
north and 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, dual amplified shortwaves, one upstream
across the Pacific Northwest, and a second one downstream into the
Northern Plains 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 central Great Basin to start
D2, and then even lower through D2 before leveling off for D3 at
generally 0-1000 ft across the entire West. With steep lapse rates
beneath the upper low, periodic PVA and height falls around the
upstream shortwave, and increasing upper diffluence as the jet
streak digging along the Pacific coast attempts to couple 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. Within this regime,
moisture will be impressive noted by IVT reaching +2 sigma ahead
of the front, and sinking slowly southward with time such that the
heaviest snow will follow suit - from the Northwest and Northern
Rockies D1, through the Sierra and Central/Southern Rockies D2,
and then finally the southern CA terrain and southern Great Basin
D3. WPC probabilities for more than 6 inches across the West are
quite expansive D1 and D2 from the Olympics and Cascades through
the coastal ranges of northern CA, into the Sierra, and across
much of the Rockies from the Northern Rockies through the Mogollon
Rim. For D3, the greatest probabilities become a bit more confined
and focus across much of CA from the Shasta/Trinity/Siskiyou
ranges, along the Sierra, and into the San Gabriels/San
Bernadinos, and then into the Southern Great Basin as well. 3-day
snowfall will likely exceed 2-3 ft in much of the higher terrain.
With snow levels crashing through the period, light accumulations
are expected even down into the lowlands and valleys, including
the central CA valley by D3, with significant mountain pass
impacts also likely across much of the West.

As impressive as the snowfall event will be across the West,
potentially even more impressive and impactful will be the
blizzard that is expected to develop from the High Plains through
the Upper Midwest, especially Wednesday and Thursday. Before this
blizzard begins, a leading wave of snowfall will likely surge
northeastward ahead of the shedding vorticity lobe and associated
with strong WAA along a northeastward advancing cold front. Fgen
along this boundary is robust, collocated with a narrow corridor
of +2 standard deviations PW anomalies. This will result in a
translating band of heavy snow with snowfall rates potentially
eclipsing 1"/hr, but the overall progressive nature and somewhat
more modest DGZ depth being forecast where moisture is best (a
disconnect between the available moisture and deepest DGZ) should
at least limit total snowfall from this first wave. Still,
impactful snow is likely with WPC probabilities for more than 6
inches reaching 30-50% in a stripe from the Coteau des Prairies
through southern MN near the Twin Cities and into western WI.
Locally this first band could produce up to 10 inches in isolated
locations.

By late Tuesday night and more exceptionally on Wednesday, the
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 D2 into D3 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 D3, which would
temporally extend the duration of this heavy snowfall. While
uncertainty still exists in the exactly placement and intensity of
this band, WPC probabilities from near the Black Hills eastward
through southern MN and into the western U.P. of MI are above 90%
for 6 inches, and above 50% for 12 inches in a more narrow
corridor. Where the heaviest snow occurs, potentially near the
Twin Cities, MN, more than 20 inches is likely, and it is possible
a historic top-3 all-time snowfall event is about to unfold for
Minnesota.

Farther downstream, increasingly impressive meridional moisture
advection within the amplifying theta-e ridge on WAA out of the
Gulf of Mexico will yield an expanding precipitation shield into
the Great Lakes and Northeast late D2 into D3. 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. Uncertainty into the exact thermal
evolution create lowered confidence this far out into the forecast
period, but it is likely that heavy snow, freezing rain, and sleet
will spread across the L.P. of MI, northern PA and Upstate NY, and
into New England by Thursday. Models continue to shift the
low-level thermal boundary in response to the strength of the
upstream parent low, but it is likely that significant ice and
sleet accumulation will occur south of the heavier snow, and it is
possible that more than 1" of sleet will occur as cold-layer
thicknesses beneath the warm nose remain quite deep, which may be
under-forecast by some of the guidance. Where the column remains
cold enough for all snow, generally the northern L.P. of MI,
northern Upstate NY, and central/northern New England, a WAA thump
of snow will likely result in heavy rates and accumulations,
especially late Wednesday into Thursday aftn. As this lead wave of
WAA precipitation ejects eastward late Thursday out of New
England, the parent low pressure moving quickly eastward from the
Upper Midwest will likely result in an additional area of heavy
snowfall from MI to New England, moving offshore only at the tail
end of this forecast period. High WPC probabilities for more than
6 inches spread into Upstate NY and VT late D2, and then become
more widespread across northern New England, except far northern
ME, on D3. For mixed precipitation, WPC probabilities for more
than 0.25" of ice are 30-50% from near Quad Cities, IA eastward
across southern MI and into far western NY, but confidence in
amounts is somewhat lowered due to the possible sleet mixing in,
and the likelihood that rates will lead to very inefficient
accretion.

Weiss


...Key Messages for Major Coast-to-Coast Winter Storm...

--A prolonged major winter storm will spread a large swath of
heavy snow from the West Coast to the Northeast this week.
Confidence is high that this winter storm will be extremely
disruptive to travel, infrastructure, livestock, and recreation in
affected areas.

--For portions of the High Plains into the Midwest, the
combination of heavy snow rates of 1-2"/hr and strong wind gusts
of 40-50 mph will create blizzard conditions. This will result in
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. The potential also exists for a flash
freeze in portions of the northern Rockies on Tuesday.

--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.

--Record lows and dangerous wind chills are possible behind this
system as very cold temperatures spread across West Coast and
Northern Plains. These very cold temperatures could also result in
impactful snow accumulations into less typical foothills and
valleys.