SPC 2000Z Day 1 Outlook
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.
