Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EST Fri Jan 29 2021

Valid 00Z Sat Jan 30 2021 - 00Z Tue Feb 02 2021


...Midwest...
Days 1/2...

The trough ejecting east from CA becomes negatively tilted as it
crosses the southern Rockies tonight and closes into a mid-level
low by the time it reaches MO Saturday. This primary low slows
over the Midwest through Sunday due to a blocking pattern
downstream over eastern Canada. A reinforcing shortwave trough
catches up to the main low Sunday and refocuses the surface low to
a coastal low by Sunday evening (more on this eastern portion in
the next section).

Rain ahead of the lee trough/low is pretty anemic right now along
the NM/TX border, but this will blossom overnight as the surface
low develops and Gulf moisture is introduced. Plain rain will
expand and lift northward in warm air advection ahead of the
surface low. A developing warm nose and cold surface conditions by
Saturday morning will allow some freezing rain to develop on the
leading edge of the precipitation shield over both central KS/NE
and eastern IA/MO/western IL Saturday morning. The highest
probabilities for this freezing rain remain over east-central Iowa
with about a 20 percent chance of a tenth inch. Deepening moisture
and antecedent cold air along with a developing TROWAL north and
west of the low center allows locally heavy snow to develop late
Saturday afternoon over northern IL/IN and northeast IA as well as
southern WI. The low shifts east Saturday night, keeping areas
such as the Chicago metro in the cold conveyor belt through the
event with increasing easterly flow allowing enhancement from Lake
Michigan over the Chicago to Milwaukee metro areas through Sunday.
Day 2 snow probabilities for 6 or more inches are 40 to 70 percent
for the Chicago-Milwaukee metro corridor west to northeast IA and
also over northern IN into northwest OH. Further local
enhancements are possible north of the 700mb low center which is
forecast to track just south of Chicago Sunday morning. 


...Southern and Central Appalachians through the Mid-Atlantic and
into New England...
Days 2/3...

As mentioned in the previous section, the main system over the
Midwest Saturday night translates energy to a coastal low in the
Carolinas through Sunday with further development expected just
offshore through Monday. Flow between these two features will be
over a cold air damming wedge extending down the eastern slopes of
the Appalachians from a 1034mb surface high centered over northern
Quebec Saturday night into Monday. This sets up a classic case of
a front end wave of mostly snow with the warm nose extending
across the surface boundary over the NC/VA Piedmont causing mixed
precip with some ice accumulation and snow farther north. The
erosion of this cold air damming wedge is likely to be rather slow
as it will be reinforced by coastal low development Sunday with
precipitation expanding up the Mid-Atlantic coast to southern New
England Sunday night through Monday. Day 2 snow probabilities for
6 or more inches are moderately high over the south-central
Appalachians of VA/WV with decreasing probabilities through the
Potomac Highlands (with Day 2.5 probabilities capturing the whole
front end precip with moderately high probabilities for 4 or more
inches across the central Mid-Atlantic from central VA to
south-central PA. As the low develops and intensifies off the
Mid-Atlantic coast Sunday night/Monday a dry slot lifts through
the southern Mid-Atlantic as an intense band of precipitation
develops along a coastal front resulting in a second heavy snow
threat area from the Baltimore/Washington metro area northeast to
southern New England. Day 3 snow probabilities for 6 or more
inches are moderate from south-central PA across the northern half
of NJ and the NYC metro with low probabilities into southern New
England (though the bulk of the storm will occur in New England
Monday night into Tuesday).

A typical wedge pattern to the freezing rain threat area sets up
with Day 2 ice probabilities for more than a tenth inch across
south-central VA, particularly along the western NC/VA border,
into the foothills of western NC and south onto the northern NC
Piedmont then north up the western slopes of the Allegheny
Mountains through east-central WV toward Pittsburgh. Probabilities
for a quarter or more of an inch of ice are low with areas of 30
percent along the western NC/VA border and in southern
east-central WV, also for Day 2.


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

The full-latitude trough is ejecting east from CA bringing an end
to the 3+ day heavy precip event across CA with ongoing precip
quickly tapering off by this evening in CA as the focus quickly
moves through the Desert SW this evening and the southern Rockies
overnight. Day 1 snow probabilities for 6 or more inches are
moderately high for the White Mtns of AZ, the southern Wasatch in
UT, and the San Juans of CO as well as western CO ranges.

The next wave is a frontal system wrapping around a Gulf of Alaska
low and precip arrives into the WA to northern CA coast this
evening though amplification of the offshore trough will keep
inland progression to a minimum through Monday before another
full-latitude trough pushes through CA Tuesday night. Day 1 snow
probabilities for 6 or more inches are moderate to high over the
Shasta/Siskiyou Mtns as well the highest OR and WA Cascades and
the Olympics. Increasing moisture and lowering snow levels expands
snow over these same areas on Day 2 with snow probabilities
generally moderate to high for 8 or more inches. Then this pattern
continues for northern CA Day 3 with moderate to high
probabilities for 12 or more inches over the CA and southern OR
Cascades as lighter precip occurs farther north in OR and WA.


Jackson