Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EST Sun Jan 31 2021

Valid 00Z Mon Feb 01 2021 - 00Z Thu Feb 04 2021


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

...Major nor'easter will spread heavy snow from eastern
Pennsylvania through Maine tonight through Tuesday...

The parent closed mid-level low is evident in mid-level GOES-E WV
imagery this afternoon drifting east over northeast OH.
Reinforcing shortwave energy rounding and digging the trough south
of this low is promoting low pressure development along the
Carolina Coast with a sizable dry slot from the Mid-south over the
southern Appalachians to central VA. A CAD wedge is well
established down the eastern lee of the Appalachians from a 1034mb
surface high over northern Quebec.

This evening, the dry slot and developing low pressure pushing
north-northeast from Cape Hatteras focuses the WAA precip shield
toward the coast with an east-west oriented swath of generally
light snow from the parent low over the OH Valley across the CAD
wedge to the Jersey shore. Late tonight, however, the surface low
deepens and lifts up to near the Jersey Shore, enhancing low level
frontogenesis north of the low and back west across much of PA
where increasingly stronger banding will form through Monday. Then
as the coastal low occludes, the surface pressure center remains
nearly stationary as the low level frontogenesis lifts up the
Northeastern Seaboard into New England while the back side just
pivots. This pivoting area of generous forcing and ample moisture,
all the while remaining cold, brings the risk for two feet of snow
to areas such as the Poconos to just west of NYC with locally
heavy snow possible as far south as northeast MD and eastern
PA/much of NJ then up through southern/central New England through
Monday night before the entire storm lifts north, affecting Maine
and northern New England through Tuesday.

Day 1 snow probabilities for 6 or more inches are moderate from
central PA up and just across the NY border and east across
northern NJ and lower NY, onto Long Island and across southern New
England. These probabilities expand NE for Day 1.5 from
south-central PA (to near the MD border) up to southern Maine and
as far inland as Albany. Day 2 snow probabilities are moderate for
6 or more inches from northeast PA to the Adirondacks and for
central/northern New England including all but far eastern Maine
where mixed precip is expected to develop.

Farther south, in the dry slot continued surface temperatures in
the low 30s over central and northeastern MD into southeast PA
brings low to moderate probabilities for a tenth inch of freezing
rain tonight. Low probabilities for a tenth inch of ice are also
located over eastern Maine for the Day 2.5 time frame.

A compromise between the slightly farther west 12Z ECMWF and the
slightly farther east 12Z GFS is generally the preferred solution
for precipitation and thermals with this system.


...Ohio Valley to southern Appalachians...
Days 1/2...

Snow rates will decrease this evening over IN/OH as energy
transfer to the coast continues. However, the parent upper trough
continue to dig while only drifting east, allowing Great Lake
moistened air to be lifted a bit over the eastern OH Valley
resulting in areas of light snow into Monday night. Farther south,
this veering flow (from westerly to north-northwesterly) makes for
good upslope conditions for the western side of the southern
Appalachians, particularly the Great Smokies. Periods of locally
heavy snow will occur in the Smokies into Monday night. The Day
1.5 snow probabilities for 6 or more inches are moderately high
for the crest of the Smokies. Localized enhancement off Lake Erie
could also bring some additional snow to northern OH with Day 1
snow probabilities for 4 or more inches off the western side of
the southern shore of Lake Erie (where the northeasterly flow has
the best fetch over Lake Erie).


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

Low pressure of the BC Coast this afternoon drifts south through
Monday before likely being reinforced and ejected east across the
Pac NW Tuesday night and the northern Rockies Wednesday. Ahead of
this feature, prolonged confluent and moist flow will drive
persistent moisture onshore from northern CA into WA state through
Monday before expanding inland Monday night through Tuesday which
will affect the Sierra Nevada, northeastern Great Basin ranges and
northern Rockies. Low pressure spinning off the tail end well off
southern CA spares areas south of central CA from precip at this
time with the digging trough from the Pac NW on Wednesday reaching
the western CO by late in the day. Snow levels around 5000ft
(lower in WA, higher in CA) gradually decrease on approach of the
trough Monday/Monday night with continued heavy snow for the CA
and WA Cascades where Day 1.5 snow probabilities are moderately
high for an additional foot or more. This probability for a foot
or more shifts to the northern Sierra Nevada and at lower
elevations in the WA Cascades/Olympics, then expands into the OR
Cascades on for Day 3. Low to moderate probabilities for 8 or more
inches are over the northern Rockies of ID/MT/Greater Yellowstone
on Day 3.


Jackson