Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EST Sat Dec 26 2020

Valid 00Z Sun Dec 27 2020 - 00Z Wed Dec 30 2020


...Great Lakes..
Days 1-3...

Bands of heavy Lake Effect snowfall are likely to persist much of
Saturday night and Sunday across Lake Erie and Ontario. However,
subtle shortwave ridging lifting northward ahead of the next
trough, combined with dry advection within the DGZ, will slowly
shunt the forcing required for LES. After 00Z tonight, additional
heavy snow is likely in the Tug Hill Plateau east of Lake Ontario
where WPC probabilities are high for 4 inches, and locally more
than 8 inches is possible before snow winds down Sunday aftn. Off
Erie, only light additional accumulations are expected on the
eastern lake shore and up towards Buffalo, NY.

Behind this initial LES, a modest wave of low pressure is likely
to move from IL to WI and across the L.P. of Michigan late D1 into
D2. As this low moves northeast it will interact with modest
northern stream energy to expand precipitation across the Midwest
into the Great Lakes, with moderate snow likely NW of the low
center into WI. The system is transient and forcing is modest, but
high SLRs due to very cold temperatures combined with a band of
deformation likely to develop should produce at least pockets of
heavy synoptic snows across WI. WPC probabilities are as high as
30% for 4 inches, greatest across the Door Peninsula.

Behind this low, another round of LES is likely for parts of the
U.P., northern L.P., and extending to off Lake Ontario D3. The
intensity and duration of this next LES round should be less than
ongoing snows, but WPC probabilities indicate a 20-30% chance for
4" in the favored snowbelt regions on W/SW flow.
   

...Pacific Northwest/CA/Great Basin/Rockies...
Day 1...

A 130kt Pacific jet streak will continue to push onshore driving
moist advection into the NW and Central Rockies. Modest and
periodic height falls through 700-500mb impulses embedded within
the flow will aid in ascent, and periods of moderate to heavy snow
are likely in the terrain. WPC probabilities on D1 show a high
risk for 4 inches in the ranges of NW WY and into the CO Rockies
where isolated amounts to 10" before forcing shuts off Sunday
evening.

Days 2-3...
More impressively, an upper low will dive south of shortwave
ridging and move onshore CA and into the Great Basin Monday and
Tuesday. This feature will be accompanied by persistent jet
energy, and impressive mid-level divergence and height falls to
drive ascent. As the cold core center moves into CA, snow levels
will drop significantly as well, and heavy snow is likely in much
of the terrain across central and southern CA including the San
Bernadinos, and the higher terrain outside of San Francisco. The
heaviest snow D2 is expected in the Sierra and also east into the
San Juans which will become aligned orthogonally to increasing
low-level WAA. WPC probabilities on D2 are high for 6 inches in
these ranges with maximum amounts over 12" likely. Lighters snows,
but still significant are likely in the southern CA ranges and
mountains within the Great Basin. As the upper low shifts east
into D3, the lowering snow levels and impressive forcing will
follow, driving the heaviest snow into the CO Rockies San Juans
again, and along the Mogollon Rim.


...Central Plains...
Day 3...

A deepening longwave trough moving across the Great Basin will
shed an amplifying jet streak around its base. As this jet streak
shifts northeast towards the Plains, its LFQ will interact with
the RRQ of a departing more zonal jet streak into the Great Lakes,
driving a coupled structure and surface pressure falls. This will
produce cyclogenesis in the lee of the Rockies early Tuesday.
Ahead of this system, a robust 1036mb High will gradually shift
east, and between the two pinched southerly flow will drive strong
WAA across the Plains and into the Midwest. The combination of
this WAA with the large scale synoptic ascent will spread
precipitation northward while expanding in coverage. There remains
considerable spread in both the placement and timing of this low
and its track, with the GFS/GEFS being fast/north, the UKMET slow,
and the EC/ECENS/NAM somewhere in between. A preferred blend
included the EC/NAM combination despite the NAM likely pushing
moisture too far north into the cold air, as it may be handling
the low-level thermal structure the best. This is important as
there will likely be a transition zone from rain, to freezing
rain, to heavy snow. The system appears to remain progressive into
D4, but a stripe of heavy wintry precip N/NW of the low is likely.
At this point the highest WPC probabilities for 6 inches extend
from central NE through southeast SD and into IA/MN, although the
presence of a dry slot may impact these totals. Southeast of here,
across northeast KS into northern MO, WPC probabilities indicate a
moderate risk for 0.1" of freezing rain accretion.


Weiss