FOUS11 KWBC 030900
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 AM EST Fri Jan 3 2025

Valid 12Z Fri Jan 03 2025 - 12Z Mon Jan 06 2025

 ...Part 2/2...

...Central Plains through the Mid-Atlantic... Days 2-3...

Guidance remains aligned on a significant winter storm to impact a
large area spanning from the Central Plains across the Ohio Valley
and to the Mid-Atlantic Saturday night through Monday. While there
continues to be considerable temporal and some latitudinal spread 
in the models, the overall evolution and footprint is quite 
similar, with impactful snow, sleet, and freezing rain expected.

Starting Saturday afternoon, a deepening shortwave will eject from
the Central Rockies and move into the Southern Plains by Sunday 
morning while taking on a negative tilt. At this point there is 
already considerable differences in the timing and intensity of the
upper low, with the EC/UKMET and accompanying ensemble the 
fastest, the GFS/GEFS making up the slow/strong end of the 
envelope, and the CMC/CMCE in the middle. That being said, the
answer likely lies somewhere in between but could lean towards
either camp. The ensemble sensitivity analysis from 12z 1/2 would
suggest the differences begin this afternoon as the trough moves
ashore the West Coast, so these differences should be brought
closer together in a few more forecast cycles. The WAA regime in
place ahead of the storm without a strong surface high to the 
north- northeast would support some of the more northern solutions,
but the upper- level configuration in the western Atlantic limits 
this potential. Regardless, significant spread in timing and 
intensity results in lower than typical confidence for D3.

Despite uncertainty in timing and placement, confidence remains in
an impactful winter storm. Regardless of the exact placement, the 
setup is favorable for a widespread wintry precipitation from the
Central Plains/Mid-Mississippi Valley by the end of D2 through the
Ohio Valley D3 and eventually Mid-Atlantic D3-D4. As the upper low
tilts negatively over the central U.S. and then closes off, a 
subtropical jet streak will sharpen and rotate around the base of 
this trough leading to coupled downstream ascent. At the same time,
increasing downstream moist advection on low-level flow from the 
Gulf of Mexico will rapidly saturate the column as isentropic 
ascent along the 295K surface lifts northward and then pivots 
cyclonically into a TROWAL around the developing cyclone. 

As this low intensifies, in conjunction with the strengthening 
WAA, the setup seems to conceptually support a laterally 
translating band of heavy precipitation from west to east. With 
DGZ depth probabilities rising to above 30% for 100mb of depth, and
a an overlap of folding theta-e with -EPV in cross sections
supporting CSI/CI, an impressive band with snowfall rates at least
1-2"/hr appears likely. Additionally, a secondary axis of 
deformation could lengthen the heavy snow in some areas with 
more impressive snowfall rates.

There is still uncertainty as to where the transition zone between
heavy freezing rain, sleet, and snow will occur but the EFI is 
highlighting values exceeding 0.9 with an SoT of 2 from eastern KS
through southern IN, and the WSSI-P is suggesting a greater than 
70% chance for moderate impacts in this same geographic window.
Given the strong WAA and deep low-level cold airmass in place,
sleet could become a dominant ptype from parts of eastern KS
through southern IN/northern KY, with over an inch of sleet
accumulation possible for some areas.

In addition to impacts, although confidence in exact placement is
uncertain, WPC probabilities current reflect a high risk (>70%) for
8+ inches near the NE/KS border through northern MO and into
central IL, with a stripe of high probabilities for 6+ inches 
extending east along I-70 through IN and west-central OH. On the 
south side of the system, WPC probabilities indicate high chances
(70-90%) for at least 0.1" ice accretion in a stripe from eastern 
KS through southern MO and into southern IL, southern IN and much
of KY. 0.25" ice probs of 40-60% are found from southern MO to KY.

This system has prompted the issuance of key messages which are 
linked below.


Pereira/Snell

...Winter Storm Key Messages are in effect. Please see current 
 Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/La...

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