FOUS11 KWBC 040840
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Sat Jan 4 2025

Valid 12Z Sat Jan 04 2025 - 12Z Tue Jan 07 2025

 ... Part 2/2 ....

...Central Plains through the Mid-Atlantic... Days 1-3...

...Widespread significant winter storm to develop tonight across
the Central Plains today and then move through the Mid-Atlantic 
early next week...

A closed low currently moving over the Northwest will fill as it 
moves towards the interior, and then weaken briefly during its dive
near the Central Rockies. However, as it exits the Central Rockies
tonight it will re-amplify, becoming a closed low once again into 
the Central High Plains. Over the past few days models have
converged on a stronger and slightly north solution regarding the
initial placement of this upper low, with still some differences
regarding it's track and ability to maintain strength as it moves
eastward to the Mid-Atlantic. Downstream confluence is quite strong
and limits to what extent the precipitation shield can lift north. The
GFS/GEFS remains alone in a stronger solution which leads to more 
potent WAA and a farther north heavy snow axis, as well as mixing 
issues spanning farther north than most other guidance. The answer 
likely lies somewhere in between, but may be closer to the ECMWF 
suite given latest trends.

As the upper low shifts almost due east through Monday with 
continued deepening, it will force a strengthening jet streak aloft
to help enhance ascent. This will cause the accompanying surface 
low to deepen slowly as it moves east, leading to a large swath of 
heavy snow, sleet, and freezing rain from eastern Kansas through
Maryland/Virginia. While confidence is high in an impactful winter 
storm, some details are still in question.

Downstream moisture advection will begin to intensify this
afternoon as reflected by intense 290K isentropic ascent 
blossoming across Kansas Saturday afternoon and then continuing 
eastward in advance of the low. This will spread PWs that reach as 
high as +2 sigma across the Central Plains/Ozarks, with subtly 
lower anomalies into the Mid- Atlantic. The accompanying theta-e 
ridge is then progged to lift cyclonically into a TROWAL, 
especially Sunday when it becomes quite strong from KS into IN, 
before weakening in response to the filling of the upper low and 
shunting of the moisture to the east. The overlap of this TROWAL 
with impressive WAA will help deepen the DGZ (SREF probabilities 
for 100mb of depth now reach 70% in KS/MO) to increase the 
potential for heavy snow rates. Additionally, cross sections 
indicate a threat for CSI/CI where folded theta-e surface intersect
with -EPV, resulting in the potential for thundersnow (through CI)
and snowfall rates of 2+"/hr.

The greatest risk for banded snow appears to be within the WAA from
Missouri to Indianapolis as the setup matches the conceptual model
for a laterally translating band, but then behind the low an
extremely impressive deformation axis is likely which would have
higher SLRs and the greater potential for excessive convective snow
rates over northern KS. This deformation will likely weaken 
towards the east as the upper low begins to fill, but still strong 
WAA/fgen ahead of the low within the narrow translating band to the
east will support impressive rates of 1-2"/hr at times. South of 
this band, the WAA will overrun the cold surface temps, but 
forecast soundings indicate an impressive E/NE dry advective flow 
beneath the warm nose to maintain sub-freezing wet bulb 
temperatures. This will support an axis of heavy sleet and freezing
rain, generally from southern MO through northern KY, where sleet
accumulations above 1", and freezing rain accretions of 0.25-0.5" are likely.

As the system moves east and weakens a bit, there becomes more
dichotomy between the northern GFS/NAM camp and other guidance, 
but still a heavy corridor of snow and mixed precipitation is 
likely from Ohio to Delaware. The wave is opening during this time,
but the downstream WAA/fgen remains impressive especially in the 
850-700mb layer, and a continuation of the upstream translating 
band is likely right through the Mid- Atlantic states. The 
intensity and accompanying snowfall is likely to be reduced from 
upstream, but could still feature intense rates and heavy snow from
west to east, reflected by the most recent EFI reaching 0.8-0.9 
with a co- located SoT of +2, highest in northern VA extending to
the Delmarva Peninsula. The placement of this band remains in 
question, but where it sets up just north of the warm nose aloft, 
1"/hr snows are likely at times.

With the event not really ramping up until the second half of D1, 
WPC probabilities for snow on D1 are modest and confined to
northern KS and southern NE, where probs for at least 4 inches are
40-70%. Freezing rain chances however are high (>80%) for at least
0.1" through Sunday morning across parts of central/southern KS 
and southern MO. By D2, snow and ice probabilities light up and 
stretch east across the Midwest and Ohio Valley. WPC probabilities 
for 6+ inches reach above 70% from central KS through just south of
the Indianapolis metro area, and locally 12+ inches of snow is 
likely, especially in far NE KS or close to Kansas City, MO. Gusty 
winds and blowing snow may also lead to near blizzard conditions 
over parts of KS and MO. During the end of D2 and into D3 the 6+ 
inch probabilities race eastward, and Sunday night/Monday features 
a moderate risk (50-70% chance) of 6+ inches from southern OH 
through the Washington, D.C. metro area, central MD, and DE. South 
of the heavy snow, a swath of significant, even isolated damaging 
icing, is likely, with WPC probabilities for 0.25" reaching 50-70% 
from southern MO through western KY, pushing into eastern KY and 
the central Appalachians on D3. Some places, especially in eastern 
MO, southern IL, or western KY, could receive 0.5" of ice resulting
in damage to trees and power lines.

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

Snell/Weiss/Pereira

...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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