Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 AM EST Sat Jan 30 2021

Valid 12Z Sat Jan 30 2021 - 12Z Tue Feb 02 2021


...Midwest/Ohio Valley
Days 1-2...

Shortwave energy ejecting out of the Four Corners will sharpen
into a negatively tilted trough and close off over Missouri this
evening before lifting towards lower Michigan by Sunday aftn. This
feature will be accompanied by modest but coupled jet streaks, and
the combination of these features will drive surface cyclogenesis
from the TX Panhandle northeast towards the lower Ohio Valley
through Sunday. Downstream of this trough, mid-level divergence
will rapidly increase, which combined with low-level WAA will
drive an expanding precipitation shield across the Mid-MS Valley
northeastward into the Great Lakes and Ohio Valley. While the
southern half of this region will be warm enough for all rain,
there is likely to be a stripe of heavy snow on the northern edge
of this precip shield north of the 700mb low track, with a narrow
band of modest freezing rain in between.

As the precip lifts northward, it will encounter slowly retreating
cold high pressure. The isentropic ascent of the moist air atop
this high will spread snowfall from eastern IA into southern WI,
northern IL, northern IN, and into OH. The lift at 290-300K is
quite moist noted by mixing ratios of 3g/kg, which will ascent
into the DGZ to drive heavy snow rates of 1"/hr or greater as
noted by HREF probabilities. While the developing TROWAL may
remain modest and somewhat south of the axis of heaviest snow, a
prolonged period of moderate to heavy snow north of the eastward
shifting 700mb low will likely produce heavy accumulations,
although some moderation is likely due to the heavy wet nature of
the snow with SLRs less than 10:1. WPC probabilities on D1 are
high for 6 inches in northeast IL into northwest IN, shifting east
across northern IN into western OH D1.5. There is likely to be
some isolated totals approaching 10", with the highest amounts
possibly in the Chicago vicinity due to some lake enhancement as
easterly flow moves across Lake Michigan.

Further to the southwest, a zone of moderate freezing rain is
possible where the warm nose reaches above 0C but surface
temperatures remain cold within the surface high pressure regime.
Most of the freezing rain is likely to be light as p-type
transitions from rain to snow due to dynamic cooling, but WPC
probabilities indicate a 40-50% chance for 0.1" of ice across
parts of Iowa.


...Southern and Central Appalachians through the Mid-Atlantic and
into New England...
Days 1-3...

The low pressure mentioned above affecting the Midwest will become
vertically stacked and occlude on Sunday, with secondary low
pressure development occurring off the VA coast Sunday night.
There remains some spread in the placement and timing of this
secondary low development, but it is likely that this low will
become the dominant feature by Monday and then move very slowly
off the coast as it interacts with the broader upper low and lobes
of PVA rotating through the main trough. As this secondary low
meanders northeast through D3, it is likely to strengthen as upper
divergence intensifies within an increasingly coupled jet
structure.

Before secondary low development begins, broad WAA from the Gulf
of Mexico will overspread the region from the southwest bringing
periods of rain, snow, sleet, and freezing rain to the region. A
1030mb surface high over Quebec will wedge down along the eastern
slopes of the Appalachians as far as SC, and as WAA commences, it
will lead to an overrunning situation with wet-bulb surface
temperatures below freezing. Guidance has slightly sped up the
timing of precipitation for the Appalachians and adjacent
foothills, while also showing a subtly stronger warm nose. This
has led to an increase in freezing rain across western and central
NC and into southern VA, where WPC probabilities are now as high
as 30% for 0.25" of accretion. However, slowly warming sfc temps
and expected periods of heavier rain rates suggests accretion will
be limited at times by runoff, so widespread warning criteria
freezing rain is not expected. Lighter freezing rain amounting to
0.1" or less is likely further northeast into WV and the I-95
corridor from DC to Philadelphia where a dry slot moving overhead
Sunday night will lead to a loss of cloud ice and a prolonged
period of light freezing rain or freezing drizzle.

Further north, the leading WAA/isentropic ascent will lead to a
thump of snowfall late Saturday into Sunday from the terrain of VA
northward into PA and NJ. This WAA appears to be modest such that
snowfall rates of greater than 1"/hr should be the exception
rather than the rule, but very moist ascent at 300K noted by
mixing ratios of 4g/kg lifting into the DGZ should support
widespread moderate to at times heavy snowfall producing several
inches of accumulation. WPC probabilities through D2 of 4" are as
high as 60%, including the I-95 corridor from DC to PHL.
Uncertainty increases considerably on D3 due to the secondary low
development and its impact. As the low deepens offshore, a robust
CCB is likely to develop just north of the dry conveyor/slot,
which will be aligned with intensifying NE to SW deformation.
Exactly where this occurs is still very much in question, and
there has been a notable shift northward in the 00z suite of
guidance overnight. While easterly low-level flow overrunning a
coastal boundary will lead to enhanced snowfall just NW of that
boundary, likely from Boston southwest to NYC and then just inland
of PHL, the location of this CCB and deformation band could
produce local maxima of snowfall much greater than the areal mean.
This will become better resolved with time, but for D2.5-D3, WPC
probabilities suggest a greater than 50% chance of 6" from
south-central PA northeast to extreme southeast NH, with local
amounts more than double this likely.


...Western U.S....
Days 1-3...

A slow moving trough will drop out of the Gulf of Alaska beginning
this evening, moving onshore the Pacific Northwest only by Tuesday
morning. Ahead of this trough, mid-level winds become confluent
and backed to the SW, driving warm and moist advection from
northern CA into the Pacific Northwest and Northern Rockies all 3
days of the forecast period. Within this flow, weak impulses will
repeatedly advect onshore, driving enhanced ascent. This suggests
that periods of moderate to heavy snow will occur through early
next week, with the heaviest snow in the favored high terrain of
the Sierra and northern CA ranges which will be somewhat
orthogonal to the 700mb flow for upslope. Snow levels will rise on
this warm advection, becoming 6000 ft or more in CA, and 3000-4000
ft further north. WPC probabilities are high for 8" all 3 days in
the Trinity/Shasta/Siskiyou ranges, where 3-day snowfall may
exceed 5 feet in some places. Further north, WPC probabilities in
the Cascades and Olympics are moderate all 3 days for 8 inches,
with event total snowfall likely reaching 2-3 ft in the higher
terrain.


Weiss