textproduct: Chicago

KEY MESSAGES

- Scattered thunderstorms will affect far northern IL/northwest IN early this morning, with isolated instances of large hail and damaging wind as well as localized flooding are possible.

- Possible line/complex of severe thunderstorms to affect the area by mid/late morning into early this afternoon. Severe wind gusts to 80 mph possible, along with heavy rain and localized flooding.

- Pattern remains active with additional thunderstorm chances through the end of the week/weekend.

DISCUSSION

Issued at 253 AM CDT Tue Aug 11 2026

Forecast remains complex today, with ongoing storms early this morning and the potential evolution of a more significant widespread severe weather/damaging wind threat later this morning and early afternoon.

Early morning surface analysis depicts a rather diffuse warm frontal zone stretching from north-central IA east-southeast across northern IL and northwest IN, roughly from north of the Quad Cities to LOT and VPZ. KLOT VAD wind profile and SPC/RAP mesoanalysis indicates a ~25 kt west-southwesterly low level jet impinging on the frontal zone, with associated isentropic ascent atop the shallow frontal slope resulting in isolated thunderstorms from near ARX over to the Lake Michigan shore near Michigan City IN at this hour. Low-level jet forcing may increase a bit during the predawn hours as a subtle mid-level short wave tracks across the area. There has been a signal in CAM guidance for potential additional development along the front prior to sunrise, and we continue to monitor for that. MUCAPE near 3000 J/kg and ~40 kts of effective shear per SPC mesoanalysis would support storm organization and an associated severe (primarily wind/hail) threat as evidenced by the scattered supercell storms currently over LM/IWX's cwa.

Attention then turns to a developing area of thunderstorms over far southeast South Dakota, which may evolve into a more widespread and potentially significant severe weather threat across our forecast area later this morning and early afternoon. CAM guidance has been consistent in this cluster of storms eventually growing upscale into a forward-propagating MCS across northwest IA/southwest MN over the next few hours and eventually tracking east-southeast along the corridor strong instability along the aforementioned frontal zone into northern IL toward mid-late morning. The combination of unseasonably strong westerly mid-level flow and resulting deep shear(50+ kts at 700 mb), steep mid-level lapse rates (near 8 C/km) MUCAPE values in excess of 3500 J/kg, the potential for significant severe weather appears rather high. Several CAMs are depicting pockets surface wind gusts of 80+ mph with this MCS across portions of the forecast area. A tornado threat would also likely exist given sufficient low-level shear and the proximity of the previously mentioned surface frontal boundary. While the exact evolution of this MCS remains uncertain, SPC's Day 1 enhanced (level 3 of 5) for out forecast area appears warranted. Currently timing of this threat looks to be from perhaps 9 AM in our northwest cwa through around 3 PM in the far east. There may also be some convective development along the front ahead of the MCS, if a strong MCV develops prior to reaching the cwa increases warm-advection ascent ahead of the main cluster as indicated by some of the CAMs.

As if the significant severe/widespread damaging wind threat wasn't enough, we'll also need to monitor the location of heavy rainfall and flooding potential - particularly for the urbanized Chicago metro area as well as locations which saw 3-5"+ of rainfall Sunday and evening. Had contemplated issuing a flood watch, though with current predawn convection slow to fill-in (so far) and some uncertainty remaining with the evolution of the MCS later this morning, elected to hold off at this time pending some further clarity on location of higher rainfall impacts.

In the wake of today's expected MCS, a lull in convective activity is expected from later this afternoon into tonight, though this will likely depend on the size and magnitude of the remnant cold pool and how far south the effective frontal zone ends up along it's southern periphery. The overall pattern some uncertainty is how far south an area of surface high pressure builds across the western Lakes in the Friday/Saturday period. This will have significant impacts on thunderstorm trends/timing for the local area.

Ratzer

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 1253 PM CDT Tue Aug 11 2026

Mainly VFR conditions this afternoon, but do expect redevelopment at some point (already seeing some across IA as of 1740z) and will need to monitor this for the rest of the afternoon. Currently have VCTS in for 23z-2z timeframe for this idea. Winds likely go light overnight with cigs trending down with potential for LIFR conditions esp toward daybreak, whether FG/BR or low stratus...and think this will try to mix to VFR again toward daybreak. Not impossible there could be another round of TSRA between 6-12z but very low confidence in this attm. Winds primarily light and E/NE thru period but could go more S at GYY if lake/land breezes develop.

LOT WATCHES/WARNINGS/ADVISORIES

IL...Lakeshore Flood Warning until 5 PM CDT this afternoon for ILZ104.

IN...Lakeshore Flood Warning until 5 PM CDT this afternoon for INZ001-INZ002.

LM...None.