Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
224 PM EST Thu Jan 26 2023

Valid 00Z Fri Jan 27 2023 - 00Z Mon Jan 30 2023

...Lower Great Lakes, Central Appalachians, and Ohio
Valley/Northeast...
Day 1...

The large storm that brought heavy snow to the region will be well
into the Canadian Maritimes this evening, but the residual
longwave trough will persist across the area, reinforced by a
secondary shortwave digging across the Great Lakes tonight before
pushing into New England on Friday. A modest surface
trough/convergence axis will advect eastward beneath this
shortwave, causing a surge in CAA with 850mb temps falling to -10C
to -15C on NW winds. This will yield efficient lake effect snow
(LES) in the typical W/NW snow belts downstream of Lakes Erie and
Ontario, as well as eastern lake Superior D1, with additional
moderate snow likely through upslope flow into the Central
Appalachians of WV. Although lapse rates both atop the lakes and
into the upslope regions will be modest, impressive ascent near
the surface will drive strong UVVs into the deepening DGZ to
support efficient snow growth and snowfall rates which may exceed
1"/hr at times. WPC probabilities for more than 4 inches of snow
reach 20-40% in the LES bands, highest along the Chautauqua Ridge.
Lower but still notable probabilities of 10-30% for 4+ inches
exist in the WV terrain.

Additionally, along this surface trough and beneath the secondary
shortwave energy embedded within the amplifying longwave trough
across the east, scattered snow showers and snow squalls will
likely persist tonight from Lower Michigan and northeast Ohio
through western PA/NY. This is due in part to an overlap of 0-2km
theta-e lapse rates falling to slightly below 0, with at least
pockets of SBCAPE reaching 100-200 J/kg, where low-level
streamlines indicate additional moisture transport from the Great
Lakes. CAMs continue to depict scattered to widespread snow
showers with potential for 1"/hr snow probabilities across these
areas. While total snowfall within any of these snow showers or
squalls will be minimal, the combination of heavy snow rates with
gusty winds of 20 kts or more could produce brief periods of
severely reduced visibility resulting in hazardous travel.


...Intermountain West...
Days 1-3...

Expansive mid-level trough will engulf nearly the entire CONUS
through the weekend, with repeated shortwave energy diving out of
Canada into the West eventually resulting in ridging downstream
across the east by Sunday. A lead shortwave embedded within this
trough moving across the Great Lakes early D1 will stretch a cold
front southwestward into the Rockies and Pacific Northwest, and
this front will become banked into the eastern side of the terrain
from the Northern Rockies through the Central High Plains Saturday
morning. During this period, a second shortwave will dive out of
British Columbia and into the Pacific Northwest Friday night into
Saturday, before racing eastward along the front into the Central
Plains Saturday aftn, with a more pronounced shortwave following
quickly on its heels into the Pacific Northwest again by Sunday
morning. This third shortwave will be the most impressive, driving
an arctic airmass southward causing the front so surge into the
Great Basin on D3.

As the front drops south D1 and banks into the terrain into D2,
the enhanced fgen associated with this evolution will help to
enhance ascent which will already be impressive through the height
falls and at least periods of upper diffluence. This fgen will
combine with strengthening upslope flow in the wake of the front
on increasing NE winds to result in waves of heavy snowfall from
the Northern Idaho through Colorado and Utah. Regional soundings
indicate that the best ascent will occur within or just below the
deepening DGZ as the column cools, suggesting impressive snow
rates of 1+"/hr at times, which will accumulate rapid as SLRs
climb to as high as 15-18:1, especially on D2 thanks to the cold
column. Snow levels initially around 1500-2000 ft will fall to
ground level, so while the heaviest accumulations are likely to be
in the terrain, lower elevation and valley snowfall is also
expected. For D1 and D2, WPC probabilities for more than 6 inches
of snow are high from the Northern Rockies and Salmon River range
southeast through the Absarokas, NW WY ranges, Wind Rivers, and as
far south as the Park Range of Colorado, extending into the
Northern Wasatch by D2. Snowfall will likely be quite impressive
in the higher terrain, with storm total snowfall exceeding 3 feet
likely. While snow in the valleys and at the passes will likely be
much less, still impactful accumulations are expected.

During the latter half of D2 and D3 as the tertiary shortwave
drops southward and drives the front into the Great Basin with
associated height falls, heavy snow will likely spread along the
Cascades and into the Sierra. There remains uncertainty into how
quickly the cold air will ooze southward which has considerable
implications into the SLR, but it is likely that fluffy SLRs near
the 75th Baxter climatological percentile will develop across the
area, leading to efficient snowfall accumulations that may exceed
6 inches in the terrain. Also on D3, this colder air could allow
some light snow to spread into the lowlands around Portland, OR,
but cold air following the moisture should limit accumulations, if
any, in these areas.


...North-Central Plains and Midwest...
Days 2-3...

A shortwave diving across the Pacific northwest Friday morning
will race southeastward along a cold front which will be slowly
sinking southward across the Intermountain West. As this shortwave
moves into the High Plains, the interaction of these height
falls/PVA with the low-level baroclinic gradient and modest LFQ
diffluence ahead of an upper jet streak to drive surface
cyclogenesis in eastern WY Friday evening. This feature will
remain progressive as it moves eastward, but at the same time will
deepen across SD/NE thanks to the strengthening upper jet streak,
before shearing out into more confluent mid-level flow across the
Midwest Saturday night. Despite this fast evolution, downstream
WAA and the ageostrophic response to the strengthening upper jet
should result in a dynamic stripe of 850-600mb fgen to drive
intense omega into the column. Regional soundings suggest that
this will result in a cross-hair signature with the most intense
UVVs intersecting the deepening DGZ as the column cools
along/behind the surface front, while the low/mid level WAA will
help deepen an isothermal layer beneath the DGZ to support
aggregate maintenance. Guidance indicates at least a modest
reduction in mid-level theta-e lapse rates driven by warm/moist
advection along the 285K-290K surfaces, which could result in
snowfall rates around 1"/hr, but CSI/convective snowfall potential
at this time looks low. Still, the impressive fgen should produce
a period of heavy snow along the SD/NE border late D1 into D2,
where WPC probabilities indicate a high risk for at least 4 inches
of snow, and despite the progressive nature of the system there
will likely be some maxima of 6-8" where the best banding
develops. Late D2 into D3, the overall forcing begins to wane as
the shortwave shears into more confluent flow to the east in
response to height rises across the Southeast, but a stripe of
moderate snow is likely from northern Iowa through southern
Michigan, including Chicago and Detroit, where WPC probabilities
for 4 inches are 20-40%.


Weiss