Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
230 PM EST Tue Jan 24 2023

Valid 00Z Wed Jan 25 2023 - 00Z Sat Jan 28 2023

...Southern Plains through the Great Lakes and into the
Northeast...
Days 1-3...

...Expansive winter storm to spread heavy snow from the Southern
Plains through New England this week...

An anomalous closed mid-level low characterized by 500mb heights
around -3 standard deviations from the climo mean over the Four
Corners according the NAEFS ensemble tables will slowly fill
tonight as it shifts eastward into the lower MS River Valley by
Wednesday morning. This trough will then continue to lose some
amplitude but remain negatively tilted as it shifts into the Great
Lakes Wednesday night, with additional vorticity lobes dropping
southward in its wake, while more potent shortwave energy shears
into New England late on Thursday, resulting in a broad longwave
trough enveloping the eastern CONUS. As this longwave trough
amplifies across the CONUS, coupling of downstream jet streaks
will become more impressive as the LFQ of a subtropical jet streak
pairs with the RRQ of a poleward arcing northern stream. The
overlap of this diffluence with the best PVA/height falls will
result in a surface low developing across TX today, and then
shifting progressively northeast into the Great Lakes by Thursday
morning while deepening slowly. This low is likely to occlude
Thursday morning to a triple point near the New England coast,
with secondary low pressure development occurring, and then this
second low shifting into the Canadian Maritimes by Friday morning.
This evolution will result in widespread moderate to heavy snow
from eastern OK/northwest AR tonight through the Upper Midwest and
lower Great Lakes on Wednesday, and eventually into the Northeast
and New England on Thursday.

For D1, the impressive overlap of synoptic ascent resulting in the
deepening of the surface low will yield heavy snow from eastern OK
through the lower Great Lakes. Strong warm/moist advection surging
meridionally will lift isentropically along the 290-295K surfaces
with impressive mixing ratios near 5g/kg to spread anomalous
moisture northward and into the system. This will manifest as a
strengthening TROWAL, which will overlap with the strong WAA and a
pivoting deformation axis on the back side of the low to produce
what could be two distinct areas of heavy banded snowfall. The
first will be within a translating WAA band lifting SW to NE ahead
of the surface low, and although SLRs will likely be modest due to
more marginal thermals and some moisture will be lost to
saturating the column, intense snowfall rates of 1-2"/hr are
likely as these bands lift northward. There is also likely to be a
secondary band, driven by the pivoting deformation, following the
WAA, which may move more slowly from AR through IN. Snowfall rates
within this secondary band could be even more impressive,
exceeding 2"/hr at times as -EPV and theta-e lapse rates <0C/km
overlap to drive the potential for upright convection in a region
of a deepening DGZ and fluffier SLRs. While the exact placement of
these bands is still uncertain, the heaviest snowfall
accumulations are likely in areas that receive both bands. WPC
probabilities for more than 6 inches on D1 are above 50% in the
Ozarks, southeast MO, and from far eastern IL, thought
Indianapolis, and towards Detroit. Locally, more than 10 inches is
possible as shown by WSE plumes and NBM percentiles. This heaviest
axis is surrounded by a broad area of moderate snowfall, generally
to the north of these bands, as the gradient to the south will be
quite extreme.

Late D1 into D2 as the primary low occludes over the Great Lakes
and the secondary low begins to develop south of New England, the
best moisture advection will translate eastward into the
Mid-Atlantic and eventually Northeast. Impressive WAA will collide
with a cold airmass, at least initially, to produce a burst of
excessive snow rates, first across OH and PA Wednesday aftn, and
then lifting rapidly into NY and New England Wednesday night.
Snowfall rates in excess of 1"/hr are likely as shown by the WPC
snow band tool and supported by regional soundings showing
impressive fgen-driven ascent into the DGZ above an increasingly
isothermal layer. This will support large aggregates and snowfall
should accumulate efficiently. However, the continued strong WAA
and advection of a robust dry slot surging northward will cause
snow to changeover to sleet/freezing rain and freezing drizzle
after several inches of snowfall. The exception to this is likely
to be generally NH and ME as the secondary low strengthens east of
MA with enhanced mesoscale lift surging into ME and pivoting back
to the NW. This will slow and suppress the dry slot, and allow
heavy snow rates to continue, especially northwest of the coastal
front and into the terrain where moist upslope flow will occur.
The snowfall forecast for Southern New England and coastal Maine
has come down with this iteration due to stronger WAA causing a
p-type transition, but just to the north WPC probabilities are
high for more than 6 inches for the Adirondacks, Greens, and much
of NH/ME where locally more than 12 inches of snow is possible
well inland from the coast. Moderate to high probabilities for
more than 4 inches of snow exist from central PA through the
higher terrain of NY and SNE, including the Catskills and
Berkshires.

Finally, as both low pressures pull off to the northeast by the
end of D2, increasing NW flow will drive an uptick in lake effect
snow in the favored W/NW belts from lower Michigan to the eastern
U.P. and across the Chautauqua Ridge where WPC probabilities for
more than 4 inches reach above 50% D2 and D3. Additionally,
upslope snow could become impressive on NW flow into the Central
Appalachians producing high probabilities for more than 4 inches
both D2 and D3, with storm total snowfall possibly reaching 12
inches in a few areas.


...Northern Rockies and Northern High Plains...
Day 3...

A shortwave embedded within broad cyclonic flow across the eastern
half of the country will drop out of Saskatchewan into MN, while a
second impulse moves southward out of British Columbia into WA/OR
Friday aftn. These features together will expand the broad
mid-level trough to encompass much of the northern tier of the
CONUS, while at the surface a cold front digs southward out of
Canada and elongated across the northern tier from Minnesota to
Washington. Modest upper level jet energy will dig NW to SE along
this front, helping to produce waves of low pressure which will
ripple along this front beginning Friday.

As this front sags southward, impressive fgen will develop,
especially between 850-700mb which lies just below the saturated
DGZ. This should drive impressive omega into the snow growth zone,
with ascent additionally enhanced through upslope flow as the
front becomes banked against the upwind terrain and winds increase
out of the northeast. The general mid-level flow will be parallel
to the banked front, which should result in waves of moderate to
at times heavy snowfall across the region, with heavy snow
accumulating above 1000-1500 ft, and with moderate accumulations
likely even into the valleys/ There is still some uncertainty into
exactly where this front will settle and the resultant axis of
heaviest snowfall, but WPC probabilities are high for more than 6
inches from the Northern Rockies through the Absarokas, NW WY
ranges, Wind Rivers, and into the Black Hills. With snow levels
quite low, impactful snowfall is likely at the passes as well,
including Lolo, Marias, Rogers, and Bozeman.


Weiss

Key Messages for Jan 24-26 Winter Storm...

--An expansive winter storm will move through the Southern Plains
tonight and into the Mid-South by Wednesday morning. Areas of
heavy snow and a wintry mix over Oklahoma and the Ozarks will
expand northeastward into the Ohio Valley tonight.

--As the storm moves toward the Great Lakes on Wednesday, bands of
heavy snow are possible over the Midwest with snowfall rates of
1-2"/hr. Some icing is possible in the central Appalachians into
New York.

--Heavy snow is forecast to spread across the interior Northeast
by Wednesday night into Thursday as a second area of low pressure
develops along the Mid-Atlantic coast and moves into the Gulf of
Maine.

--Snow-covered roads and reduced visibility will make travel
difficult across impacted areas.  Gusty winds and heavy snow may
result in downed trees and power outages.