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FORMAT: flowed
ACUS01 KWNS 281947
SWODY1
SPC AC 281946

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0146 PM CST Sun Dec 28 2025

Valid 282000Z - 291200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
ILLINOIS...WESTERN KENTUCKY AND INDIANA...

...SUMMARY...
Scattered severe thunderstorms are possible over parts of the
Midwest and lower Ohio Valley this afternoon through the evening.
Severe/damaging wind gusts and a couple of tornadoes are the primary
threats.

...20Z Update...
The only change to the outlook for this issuance is to trim the
Marginal Risk area across parts of north-central Illinois behind a
cold front that is moving southeastward across the region.

..Broyles.. 12/28/2025

.PREV DISCUSSION... /ISSUED 1040 AM CST Sun Dec 28 2025/

...Mid Mississippi Valley into the Midwest/Ohio Valley...
An upper trough/low over the northern Plains this morning will
further intensify today as it develops towards the Upper
Midwest/Great Lakes through the period. Pronounced large-scale
ascent associated with a strengthening mid/upper jet (100-120 kt at
500 mb) over the mid MS Valley into IL/IN will encourage rapid
deepening of a surface low as it develops from central/northern IL
into Lower MI by this evening. A rather moist low-level airmass with
generally upper 50s to low 60s surface dewpoints will continue to
spread northward across the mid MS Valley into parts of the
Midwest/OH Valley and southern Great Lakes regions in tandem with a
northward-shifting surface warm front. A cold front is also expected
to sweep east-southeastward across these regions this
afternoon/evening as the surface low tracks northeastward. This cold
front should serve as a focus for strong to severe convection later
today.

The 12Z ILX sounding showed a moist but saturated profile through
much of the troposphere, with strong west-southwesterly
mid/upper-level winds already present over the warm sector. Current
expectations are for ongoing, mostly elevated convection across
central IL to pose a threat for isolated hail and strong to locally
severe gusts given the strength of the mid-level flow. This activity
may linger along/just north the warm front through the afternoon as
it spreads quickly east-northeastward into IN. Additional
thunderstorms are expected to form along/near the cold front across
IL and vicinity by 20-22Z as modest/filtered daytime heating occurs,
with come cloud breaks already noted in visible satellite imagery
across east-central MO/south-central IL. It does appear that surface
temperatures are warming faster/more than forecast by some guidance
across the warm sector in IL, with 16Z observations at some stations
already reaching into the upper 60s/low 70s.

The more aggressive 12Z guidance and recent RAP runs suggest that
around 500-1000 J/kg of MLCAPE may ultimately develop in a narrow
corridor across eastern IL and western IN/KY ahead of the front,
even though modest lapse rates aloft will likely inhibit more robust
destabilization. This weak instability should be sufficient to
support organized updrafts, as both low-level and deep-layer shear
will be quite strong owing to the strengthening wind profiles across
the warm sector this afternoon/evening with the approaching
mid/upper-jet. There is still some uncertainty regarding the
potential for pre-frontal convection. But, any low-topped supercells
that can form and be sustained should pose a threat for a couple of
tornadoes with upwards of 200-250 m2/s2 of effective SRH present.
Eventual development of a thin/forced line of convection along the
cold front appears likely, with a threat for scattered
severe/damaging winds and perhaps a couple of embedded tornadoes. At
least an isolated severe threat may persist this evening/tonight
across the OH Valley and parts of western PA, where low-level flow
is forecast to remain quite strong even with minimal instability.

$$

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