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

Valid 00Z Sun Jan 31 2021 - 00Z Wed Feb 03 2021


...Midwest/Ohio Valley
Day 1...

A closed low is apparent along the eastern KS/OK border this
afternoon in mid-level water vapor from GOES-East. This low will
shift east-northeast across the Midwest through Sunday when a
reinforcing shortwave trough (currently over western WY) will
shift south of the low, opening it and amplifying the trough over
the Southeast US which in turn promoted translation of energy to
the Carolina coast Sunday/Sunday night (more on this in the next
section). This low is already vertically stacked with the surface
low slowly filling as it moves into the Midwest. Plain Rain is
mainly seen in the precip shield with some freezing rain in
east-central IA. This ice area on the northern periphery should
expand a bit into the evening as air temperatures drop to the
subfreezing wet bulb temp with saturation. Farther northeast, over
eastern IA, northern IL, southern WI and northern IN, snow is
expected to begin soon as marginal antecedent conditions are
further dynamically cooled as the TROWAL further develops. 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. 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
for Day 1 are 70 to 80 percent for 6 or more inches in northeast
IL across northern IN into northern OH with moderate probabilities
now across OH through Day 1.5. Day 1 probabilities for 8 or more
inches are up the Chicago/Milwaukee metro corridor due to some
lake Michigan enhancement in easterly flow.

On the south edge of the snow line, over north-central IL/IL into
south-central OH a narrow stripe of light freezing rain is likely,
though expected to result in less than 0.1" ice to the southwest.


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

The reinforcing shortwave trough mentioned above allows surface
low development to begin late Sunday over coastal Carolina, east
of a cold air damming wedge extending from a 1034mb surface high
over northern Quebec. This low will become the dominant feature
Sunday night and then move very slowly off the Mid-Atlantic coast
or even stall Monday/Monday night as the parent upper trough
closes again with further reinforcing waves making the surrounding
trough negatively tilted. The low then shifts north past Maine
Tuesday/Tuesday night.

As the cold air damming wedge sets up tonight, broad WAA from the
Gulf of Mexico will overspread the region ahead of the main trough
bringing a gradient of rain south then mix then snow to the north.
As usual WAA precip is outpacing most guidance for onset timing
with greater confidence in freezing rain over southwest VA and the
northern NC Piedmont and Blue Ridge as well as up east-central WV,
a typical freezing rain pattern for a CAD wedge that extends into
northern GA. Day 1 ice probabilities for a quarter inch or more
are 40 to 50 percent along the west-central NC/VA border through
the northern NC Blue Ridge.

Farther 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 still appears to be modest such
that snowfall rates of greater than 1"/hr should be the exception
rather than the rule with 12Z HREF 1"/hr probabilities limited to
the south-central Appalachians in two rounds late tonight and
Sunday morning. WPC snow probabilities for 6 or more inches on Day
1 are 70 to 80 percent for the south-central Appalachians of WV/VA
up through the north-central VA Blue Ridge (Shenandoah NP) with
moderately high probabilities for 4 or more inches through the
Washington DC metro area.

Most of the front end precip then shifts offshore ahead of the
developing low Sunday night, returning onshore with a vengeance
across the northern Mid-Atlantic (well west into PA) Monday as the
TROWAL really gets going. The main shift with QPF/snow is on Day 2
where the 12Z consensus is for a pivoting stripe of moderate to
heavy snow across PA/NJ than then shifts up the coast through
Tuesday. Day 2/3 snow probabilities focus this second wave of the
storm with Day 2 moderate probabilities for 8 inches from central
PA to northern NJ/the NYC metro and Day 3 over the Adirondacks and
across south-central New England then up the Maine coast. Given
the closing off of the upper low and associated axis', certainty
with Day 3 is less at this time, particularly with where the
coastal front/surface low track sets up in Maine. 


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

Low pressure lingers just south of the Gulf of Alaska as troughing
amplifies off the West Coast. Persistent precip occurs over far
northern CA and up the OR/WA Cascades and west through Monday
night before the focus shifts south through CA through Wednesday.
Periods of moderate to heavy mountain snow snow will occur in this
precip axis with snow levels will remaining around 5000ft in
northern CA, decreasing to about 4000ft in WA. WPC probabilities
are moderate to high for 12" on Day 1/2 in the
Trinity/Shasta/Siskiyou ranges, where 3-day snowfall may reach 5
feet on mountain peaks. 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.
Moderate to high Day 3 probabilities for 8 or more inches extend
down the CA Cascades and northern Sierra Nevada.


Jackson