Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 AM EST Wed Jan 06 2021

Valid 12Z Wed Jan 06 2021 - 12Z Sat Jan 09 2021


...Pacific Northwest to the Northern Rockies...
Days 1-3...
Two significant mid-level impulses embedded within persistent
confluent and moist flow will spread precipitation and snowfall
across the Pacific Northwest through the weekend. The first wave
will move onshore this aftn, accompanied by a surface front, and
preceded by briefly backed 700-500mb flow to drive moisture
inland. Favorable ascent within the upper jet will help support
heavy precipitation from the OR Cascades eastward into the
Northern Rockies, with snow likely above 3000 ft. WPC
probabilities on D1 are high for 8 inches in many of these ranges.
After a brief respite D2 due to shortwave ridging, the second
impulse will push onshore following a similar pattern to the
first. Heavy precipitation will once again overspread the region
from SW to NE, with heavy snow likely in the Olympics, Cascades,
and northern CA ranges. WPC probabilities on D3 are moderate for 8
inches. 3-day event total snowfall could exceed 2 ft in the higher
terrain of the WA Cascades.


...Ozark Region to the Southern Appalachians...
Days 1-3...

A mid-level low moving across the Southern Plains this morning
will amplify/close off as it moves over AR Thursday morning, and
then continue eastward to NC by Friday. This impulse will be
accompanied by a modest subtropical jet streaking to its south,
and the combination of height falls, mid-level divergence, and LFQ
diffluence will provide ascent across the region. The antecedent
airmass is marginal, so most areas from AR to NC will likely
receive a mixture of rain and snow through the forecast period.
However, there are likely to be two areas of enhance snow
potential.

The first is Wednesday night into Thursday across northern AR and
southern MO. In the higher terrain, the overall column will be
colder, and precipitation overspreading this region will produce
snowfall across the Ozarks. WPC probabilities across this region
have increased for 4 inches and are now 10-20%, but the general
snowfall is expected to be less than that. However, across S MO,
also in the higher terrain, some enhanced forcing beneath the
upper low could yield better snowfall rates which the HREF
indicates could exceed 1"/hr at times. This could briefly spread
eastward into SE MO Thursday, but WPC probabilities for
accumulations over 2 inches are low.

The surface low beneath the synoptic ascent will deepen slowly as
it moves east across the Deep South. Increasing isentropic ascent
and WAA ahead of this low will spread precipitation into the
Southern Appalachians and eastward into the Piedmont. Once again,
the antecedent column is marginal, but the mountains of NC, far
northern GA, and SW VA should see periods of heavy snowfall
Thursday evening into Friday morning. WPC probabilities are
moderate for 4 inches, with highest chances near the Pisgah
National Forest. Further east into the Piedmont a deformation axis
may develop as the upper low pivots eastward and the main surface
energy transfers to the coast. This could produce a band of
moderate to heavy snowfall across central NC on Friday morning,
with enough dynamic cooling possible to overcome the marginal low
level thermal structure. However, confidence is low and model
spread is high, so current WPC probabilities even for 1" are less
than 50%.


The probability of significant icing (0.25" or more) is less than
10 percent all 3 days.

Weiss