Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
102 PM EST Sun Jan 21 2024

Valid 00Z Mon Jan 22 2024 - 00Z Thu Jan 25 2024

...The West...
Days 1-3...

Wave of precipitation will continue to shift onshore through
mid-week and generally zonal flow over the Pacific arcs into a
mean trough developing across the Inter-Mountain west by
Wednesday. Embedded within this zonal flow, lobes of vorticity
associated with modest shortwaves will periodically surge onshore,
with a more significant shortwave progged to approach the Pacific
Northwest coast late in the forecast period. At the same time, a
potent 150+ kt Pacific jet streak angled into southern CA early in
the period will gradually expand but weaken into Wednesday,
placing at least modest LFQ diffluence into the West, overlapping
the waves of PVA/height falls and broad region of instability
beneath the longwave trough axis.

This pattern will maintain generally elevated snow levels of
6000-7000 ft or more, so much of the west will experience showers
or rain as precipitation expands onshore each day. However, above
these snow levels, heavy snowfall is likely, especially in the
Sierra, Shasta/Trinity/Siskiyou region, and the Blue Mountains on
D1 where upslope flow will enhance ascent, and WPC probabilities
for more than 6 inches of snow exceed 80%, with locally 12 inches
likely in the higher terrain. By D2, better ascent shifts
southeast towards the Mogollon Rim and White Mountains of AZ where
WPC probabilities for more than 6 inches are 30-40%. Then later D2
into D3, increasing divergence and moisture advection downstream
of the more robust shortwave approaching the Pacific Northwest
will reinvigorate heavy snow in the WA Cascades above 6000 ft, but
even WPC probabilities for more than 4 inches peak at just 20%.

For the Pacific Northwest, a continuation of steady moisture
overrunning a cold air mass will keep the threat of ice
accumulation going in the Columbia River Gorge one more day, where
an additional 0.1+" of ice will be possible as noted by WPC
probabilities of 20-30%. While this additional ice accretion is
expected to be modest, it will contribute to the ongoing
considerable impacts from a week with several rounds of
significant icing.


...Southern Plains into the Ohio Valley...
Days 1-2

A shortwave advecting eastward out of the Pacific will begin to
amplify over the Great Basin and sharpen into a negative tilt as
it approaches the Southern Plains on Monday. Downstream divergence
from this feature will combine with lobes of vorticity shedding
through the and an intensifying, poleward-arcing jet streak
downstream of the primary trough axis to enhance ascent from the
Southern Plains through the Ohio Valley. Additionally, moisture
will begin to surge into the region both due to the 700-500mb SW
flow transporting Pacific air northeastward, but also in response
to locally backing 850mb flow out of the Gulf of Mexico to
isentropically ascend into the region. PW anomalies as progged by
the NAEFS ensembles are progged to reach +2 to +3 sigma,
supporting a heavy precipitation event.

As the most intense ascent and higher moisture begin to overlap
late Sunday and through Monday, it will produce an expanding area
of precipitation from near the Red River Valley of the South by
00Z Monday through the Ohio Valley after 00Z Tuesday. Initially,
precipitation should be of the freezing rain variety in most areas
due to cold surface temperatures within the expansive high
pressure. Wet-bulb temperatures in the mid to upper 20s suggest
efficient ice accretion Sunday night into Monday morning as well,
and the p-type should be almost exclusively freezing rain in many
areas due to an exceptionally deep warm nose depth. However, with
the high retreating to the east, and WAA intensifying through the
period, the warm air will eventually win out and cause a p-type
transition to plain rain, likely as far north as Indiana and Ohio.
Before this occurs, however, significant accretions of ice are
likely, and the models are possibly under-doing the duration of
freezing rain due to the reinforcement of surface cold air that
can occur with precipitation onset. Still, a lack of dry
advection, the impressive WAA, and a deep warm layer which will
help to more rapidly warm the surface with precip-loading,
suggests all areas will eventually transition to rain. The
greatest risk for significant icing is in the higher terrain of
the Ozarks where WPC probabilities for more than 0.25" of ice are
as high as 50-70%, but surrounding this area, high probabilities
for more than 0.1" of ice extend from south-central OK northeast
to the Lake Michigan shore.


...Great Lakes and Northeast...
Days 2-3...

An amplifying trough over the Southern Great Basin/Four Corners
will shed a lobe of vorticity and an accompanying shortwave
northeast across the Missouri Valley and into the southern Great
Lakes Tuesday, with this feature then shearing out into the
confluent flow over the Northeast during Wednesday. A strong, but
weakening, upper jet positioned over southeast Canada during this
time will retreat to the east, but maintain its modest RRQ over
the best PVA to help drive ascent. Expanding high pressure to the
east will slowly shift off the Atlantic coast during Tuesday, and
the resultant return flow  will become increasingly pinched and
more robust from the Gulf of Mexico. This surges moisture
northward noted by NAEFS PW anomalies reaching as high as +3 sigma
across a large portion of the eastern CONUS, which will be wrung
out as precipitation through the overlapping synoptic lift and
concurrent isentropic upglide. The isentropic ascent will be
driven by intensifying WAA, and for the Great Lakes and Northeast,
this will manifest as frozen/freezing precipitation, at least at
onset, before changeover to rain occurs, possible as far north as
the southern L.P. of MI and as far east as Upstate NY. North of
there, however, a burst of heavy snow is likely on the WAA-driven
fgen, and across interior New England the confluent flow aloft
should help reinforce the surface high pressure enough to lock in
cold air, preventing a changeover from snow. This will cause a
stripe of moderate to heavy snow from Michigan through Maine, but
the WPC probabilities for exceeding 4" of snow are confined to the
higher terrain of NH/ME where they reach 10-20%. South of the
heaviest snow, an axis of mixed precipitation could cause
significant icing stretching from the L.P. of MI, across northern
PA, and into the Catskills/Berkshires. WPC probabilities D2 for
0.1" of ice or more are highest near Detroit, MI where they are
50-80%, and this stripe extends eastward D3 peaking at 20-40% in
the Poconos and Catskills.


Weiss