textproduct: Central Illinois

KEY MESSAGES

- A marginal risk of severe thunderstorms and localized heavy rainfall exists south of Interstate 70 this afternoon, while a stalled front south of Interstate 72 presents a risk for funnel clouds.

- An extended period of dangerous heat and humidity begins Sunday, with a Heat Advisory active and likely requiring extensions through much of the upcoming workweek.

- Dry conditions dominate through midweek before thunderstorm chances return late next week.

DISCUSSION

Issued at 1254 PM CDT Sat Jun 27 2026

Current surface analysis reveals a weak, stalled frontal boundary positioned just south of the Interstate 72 corridor. High-resolution convective guidance indicates an environment characterized by localized low-level vorticity and weak vertical shear along this boundary. The Non-Supercell Tornado Parameter (NSTP) highlights values between 2 and 4, which justifies a mention of funnel clouds stretched along a Jacksonville to Paris corridor through this afternoon (see recently issued Special Weather Statement). In fact, a brief funnel was reported at noon south of Decatur. If enough cloud breaks can be sustained, an increase in low level lapse rates paired with low LCLs would make a few landspout tornadoes possible, though higher boundary layer friction due to crop height make this less likely than a month or two ago.

South of the Interstate 70 corridor, a more unstable airmass is present, characterized by mid-upper 70s dewpoints and MLCAPE values near 2000 J/kg. Effective bulk shear remains low at 25 kt or less, making organized/widespread severe weather unlikely. The Storm Prediction Center has placed this southern tier under a Marginal Risk for severe thunderstorms into early evening. Convective elements within this zone are capable of producing isolated damaging wind gusts due to precip loading and torrential rainfall. HREF hourly probability fields show a 40% chance of localized rainfall amounts exceeding 1 inch, which raises the potential for isolated/localized flash flooding

Sunday morning, a warm front swings north across the CWA, bringing a low (20-30%) chance of an isolated shower or storm. Behind the front, winds veer southeast to south through afternoon and upper-level heights rise rapidly as a sub- tropical ridge expands northward from the Gulf Coast. NBM guidance indicates high temperatures climbing into the lower 90s over the southwest half of the CWA. Paired with high ambient humidity, heat index values are projected to exceed 100 degrees across much of the forecast area, with values up to 105 degrees in southeast and west central Illinois. A Heat Advisory goes into effect Sunday afternoon southwest of a Peoria to Robinson line to address these conditions.

An intense and prolonged heat wave will dominate the weather pattern across central and southeast Illinois through midweek. Strong multi-model consensus among the GEFS and ECMWF ensembles shows a core 594 dam ridge anchoring over the Mississippi and Ohio River Valleys. The ECMWF Extreme Forecast Index (EFI) highlights values between 0.8 and 0.9 for maximum temperatures Monday through Wednesday, signifying that the magnitude of this heat event is unusual in a climatological context for the end of June. Furthermore, given low-mid 70s forecast dewpoints, peak heat index values exceeding 105 degrees are likely during this timeframe. High confidence exists regarding the severity and medium confidence in the longevity of this heat event, and the Heat Advisory will likely require extension through at least Wednesday for the entire CWA.

A forecast challenge emerges late week regarding the breakdown of the ridge and the return of precipitation. Notable disagreements exist among medium range ensemble guidance. The GFS presents a faster and more progressive upper-level trough shifting across the Northern Plains by Thursday, which acts to suppress the ridge and introduce thunderstorm chances ahead of a cold front. In contrast, the ECMWF Ens/ECMWF-AI maintain a stronger ridge over the eastern United States, holding precipitation chances at bay until Friday or Saturday. Due to these discrepancies, confidence is below normal regarding the exact timing of storm chances late next week.

AVIATION /00Z TAFS THROUGH 00Z MONDAY/

Issued at 623 PM CDT Sat Jun 27 2026

A warm front is draped over the region resulting in scattered showers and a few storms, mainly east of I-55. Skies have scattered/lifted to VFR everywhere except BMI, but these improved conditions will be short lived with fog and low stratus expected to set in areawide once again late this evening and overnight. Conditions will improve by around midday Sunday as the warm front starts to lift back north and winds veer to a more southerly direction and pick up.

ILX WATCHES/WARNINGS/ADVISORIES

Heat Advisory from 1 PM to 7 PM CDT Sunday for ILZ027>029-036- 037-040>042-047>054-061-062-066>068-071>073.