Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
147 PM EST Fri Jan 27 2023

Valid 00Z Sat Jan 28 2023 - 00Z Tue Jan 31 2023

...Northern Rockies and Intermountain West...
Days 1-3...

Expansive mid-level low will remain anchored near the Hudson Bay
through next week, leading to broad troughing across nearly the
entirety of the CONUS through Saturday before ridging begins to
blossoms across the Southeast on Sunday. Periodic shortwaves and
vorticity lobes shedding from this expansive low will drop
southward out of Western Canada into the Pacific Northwest and
Intermountain west, with the most impressive feature moving into
WA/OR Sunday morning before amplifying into a closed low near
central CA Sunday night and extending a positively tilted longwave
trough towards the Northern Plains. This low will continue to
amplify as it drops towards southern CA by the end of the forecast
period, with resultant downstream divergence and moisture
confluence combining with a phased subtropical jet streak
amplifying over the Southeast and backing into the West during
Monday, producing an impressive overlap of moisture and ascent
into the Southwest/Four Corners on D3.

For D1, a surface cold front dropping southward within the
amplifying trough across the west will bank along the eastern side
of the terrain from the Northern Rockies through the central High
Plains, resulting in a robust overlap of upslope flow on NE winds
behind the front, and strong low-level frontogenesis. While the
fgen appears strongest just beneath the DGZ, it should drive
strong UVVs into the DGZ, which at the same time will begin to
deepen in response to column cooling. This suggests periods of
heavy snow rates are likely in the terrain, but enough ascent
should also result in heavy snow accumulating in the lower
elevations and valleys. WPC probabilities for D1 are high for 6
inches or more
above 1500 ft from the Northern Rockies and Salmon River range
southeast through the Absarokas, NW WY ranges, Northern Wasatch,
and CO Rockies including the Park Range. Locally 1-2 ft is
possible in the highest terrain. In the lower elevations several
inches of snowfall is also likely, creating notable impacts in the
valleys and at pass level.

During D2 as the primary trough axis retrogrades westward in
response to the amplifying shortwave along the Pacific coast,
strong height falls will drive the cold front farther southward
while SW mid-level flow provides overrunning moisture downstream
of the closing mid-level low. This will result in heavy snow from
the OR Cascades through the Sierra, with additional heavy snow
continuing near the Tetons, Northern Wasatch, and into the
Medicine Bow/Park Ranges of WY/CO. By D3, as the closed low
deepens further and sinks south in conjunction with the
intensifying upper diffluence and continues moisture confluence
into the Four Corners region, heavy snow will become more
widespread in the terrain from the Sierra through the Southern
Wasatch, into the San Juans and much of the CO Rockies. The
rapidly cooling airmass should support above normal SLRs in this
area as well, with fluffy/efficient accumulation leading to high
WPC probabilities on D3 for an additional 6+ inches of snow in
these areas.


...North-Central Plains through the Great Lakes and Northeast...
Days 1-3...

Elongated mid-level trough will envelop much of the CONUS D1 as a
shortwave drops across the Intermountain West and into the Central
Plains Saturday night. As this shortwave shifts eastward, it will
begin to encounter slow height rises bulging across the Southeast
in response to more impressive height falls digging into the
Pacific Northwest. The resultant confluent flow into the Ohio
Valley will cause the vort max to shear off to the east, causing
an overall weakening trend in the shortwave amplitude and
associated forcing despite intensifying upper level jet energy
surging into New England during Sunday. Beneath this shortwave, a
wave of surface low pressure will move progressively along a cold
front which will be draped from NW to SE from the Northern Rockies
through the Central Plains, and it is this fgen that will provide
the most robust ascent for snowfall during the period, and the
guidance has trended a farther north and become slightly more
intense to the east, likely due to more pronounced WAA providing
better moisture into the system. An overlap of sharpening low/mid
level fgen in response to this downstream WAA will produce strong
omega into the DGZ, providing a setup favorable for 1"/hr snow
rates as reflected by the WPC prototype snow band tool and
supported by high SREF probabilities for 100mb of DGZ depth. While
truly convective rates much above 1"/hr are not likely as theta-e
lapse rates only fall to around 0C indicating CSI is unlikely, the
strong ascent into the deepening DGZ combined with an isothermal
layer below will support periods of efficient snow growth and snow
accumulation, especially along the SD/NE border and into western
IA. WPC probabilities for more than 6 inches have increased to
50-80%, highest in far northeast NE and southeast SD where locally
more than 10 inches of snowfall is possible. Probabilities for
more than 4 inches above 30% extend as far east as SW WI with
again, locally higher amounts where banding is most pronounced.

As the entire system moves east into the Upper Midwest and Great
Lakes D2, the fgen will begin to ease as the shortwave shears into
the aforementioned confluent flow to the east, suggesting both the
intensity and duration of snowfall should wane to result in lesser
accumulations. By D3, more impressive moist advection will surge
out of the Gulf of Mexico as mid-level heights rise across the
Southeast, and this will allow a more widespread swath of snowfall
to spread into the Northeast, but with the surface low expected to
track across central and northern New England, warm air will flood
northward producing only marginal thermals for snow as the low
continues to move quickly to the east by Monday aftn. WPC
probabilities D2 for more than 4 inches are high from southern WI
through lower MI where the same laterally translating bands will
traverse, and some lake enhancement is possible later D2 into D3
as the low pulls away. Into the Northeast late D2 into D3, WPC
probabilities for more than 4 inches reach 30-50%, generally
confined to the St. Lawrence Valley and Northern New England.


Significant icing is not forecast across the CONUS through Monday.
However, southern Plains/Mid-South icing will become a concern
Monday night.

Weiss