textproduct: Central Illinois

KEY MESSAGES

- There is an Enhanced Risk (level 3 of 5) for severe weather northeast of I-74 on Tuesday. The primary hazard will be damaging wind gusts in excess of 70mph: however, isolated tornadoes cannot be ruled out.

- Typical summertime heat and humidity will persist into Tuesday. Locations not impacted by convective outflow will see afternoon heat index readings reach 100-105 degrees. As a result, a Heat Advisory will be issued along/southwest of a Macomb...to Lincoln...to Casey line.

- Several storm clusters will track through the region Tuesday night through Friday night...posing additional risks for severe weather and heavy rainfall.

DISCUSSION

Issued at 232 PM CDT Mon Aug 10 2026

Quiet weather will continue across central Illinois this afternoon and evening as convection develops near a lingering outflow boundary along/north of the I-80 corridor where 19z/2pm MLCAPEs have exceeded 4000J/kg. Given the lack of a clear forcing mechanism and an overall more stable environment due to the presence of a mid-level cloud deck, think CAMs such as the HRRR and RRFS are correct in keeping central Illinois dry and developing storms across northern Illinois, then tracking them generally eastward into the Chicago metro and eventually into southern Michigan/northern Indiana this evening. Additional storm clusters may form overnight as the low-level jet strengthens, but these will largely stay north of the KILX CWA as well.

Confidence is growing that a more significant cluster of thunderstorms will form over northern Iowa late tonight, then race E/SE across northern Illinois/Indiana on Tuesday. The 12z HRRR/RRFS are in relatively good agreement concerning the track/timing of the storms...taking the most significant wind risk along/just north of the northern KILX CWA boundary during the morning into the early afternoon. The latest Day 2 Convective Outlook highlights locations north of I-74 with an Enhanced Risk (level 3 of 5) for severe weather. Wind gusts in excess of 70mph will be the primary hazard: however, a few tornadoes may be embedded within the leading edge of the squall line as well. Model consensus generally focuses the strongest wind gusts along I-80 into north-central Indiana: however, it appears likely that the trailing end of the line will graze the I-74 corridor. Aside from the convection, Tuesday is also shaping up to be a very warm and humid day. The big question will be how far south the storms and/or the resulting outflow boundary will settle...as this will significantly impact the temp/heat index forecast. Based on the 12z HRRR/RRFS, have opted to issue a Heat Advisory along/south of a Macomb...to Lincoln...to Casey line where afternoon heat index values will peak at 100-105. Further N/NE, outflow from the early days storms will keep temps a bit cooler.

Once the initial severe storm cluster exits into Indiana, will need to keep an eye on possible convective development along the trailing outflow during the afternoon/evening. Many CAMs are initially fairly sparse with convective coverage, but then develop activity after dark as the nocturnal jet cranks up. As such, have kept afternoon PoPs confined to 20-30% mostly east of I-55. Areal coverage will increase into the 30-50% range after dark as the outflow boundary serves as the primary focusing mechanism.

W/NW upper flow will keep central Illinois in an active convective pattern through the remainder of the week. As the primary baroclinic zone wavers across the area, several rounds of showers and thunderstorms will occur Wednesday through Saturday. While it will not be a complete wash-out on any of those days, the risk for occasional severe weather/heavy rainfall will be persistent. Given the favorable synoptic pattern, the latest WPC 7-day QPF shows widespread amounts of 1.50 to 2.50 along/north of a Quincy to Lawrenceville line. There will likely be swaths of even heavier rainfall depending on exactly where each convective complex ultimately tracks. Stay tuned to later forecasts for additional details.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 1207 AM CDT Tue Aug 11 2026

Late Tuesday morning, a thunderstorm complex is projected to move eastward across northern IL, though it will likely remain north of the central IL terminals. A PROB30 for TSRA has been maintained for KPIA, KBMI, and KCMI, as the associated outflow boundary is predicted to generate scattered TSRA from midday through the afternoon. Predominantly VFR conditions are anticipated elsewhere through Tuesday afternoon, with winds maintaining a S-SW direction at 7-12 kts.

ILX WATCHES/WARNINGS/ADVISORIES

Heat Advisory from noon today to 8 PM CDT this evening for ILZ036-040>042-047>054-056-061>063-066>068-071>073.