textproduct: Chicago

KEY MESSAGES

- Another round of heavy rain producing showers and thunderstorms late this evening into Sunday morning.

- A brief break in the stormy pattern is expected Monday into early Tuesday before storm chances return ahead of the next cold front late Tuesday into Tuesday night.

DISCUSSION

Issued at 236 PM CDT Sat Aug 15 2026

The remnant MCV responsible for this morning's heavy rain across central Illinois is shifting into northern Indiana this afternoon. In its wake, widely scattered showers and drizzle persist within a broad warm air advection pattern. While locally heavy downpours remain possible, limited coverage should mitigate renewed flooding concerns through late afternoon. Expect muggy conditions with temperatures near 80 degrees under mostly cloudy skies.

Another round of showers and thunderstorms is likely (60%+) tonight ahead of an approaching cold front and mid-level impulse. Timing favors an arrival into western Illinois around and after 10 pm, reaching eastern Illinois and northwest Indiana after midnight. Poor lapse rates and marginal deep-layer shear should limit the severe threat, but very heavy rainfall and localized flash flooding remain concerns given precipitable water values exceeding 2 inches. While most activity appears progressive, we are monitoring the potential for backbuilding or training storms, particularly along the southern periphery of the MCS as the low-level jet veers overnight. Central Illinois and Indiana are most at risk, encompassing areas saturated by this morning's rainfall. A Flash Flood Watch was considered, but we opted to hold off due to low confidence in the exact placement of training cells. The evening shift will continue to monitor trends to identify high-risk areas.

A cold front will move southeast across the region Sunday, maintaining a 20-30% chance for scattered showers and thunderstorms, primarily southeast of I-55. Humidity will be slow to recover following the frontal passage; expect warm, humid conditions with highs in the low to mid-80s.

Mainly dry weather is expected Monday and Tuesday as surface high pressure settles over the Mid-Mississippi Valley. Lower humidity and slightly cooler temperatures will provide a brief reprieve before another round of convection arrives late Tuesday. This next system, driven by a cold front and shortwave digging into the Great Lakes, brings a renewed threat of strong storms and heavy rain Tuesday night. Quieter, cooler weather returns Wednesday and Thursday.

KJB

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 637 PM CDT Sat Aug 15 2026

Forecast concerns include...

Chance of thunderstorms late this evening and overnight. Ifr/lifr cigs/vis overnight/Sunday morning. West/northwest winds gusting to 20kt Sunday.

A few showers will be possible this evening but the focus for new shower and thunderstorm development is across southeast IA and western IL. With at least scattered coverage expected to develop late this evening and continue into the overnight hours. Current tempo mention has this potential handled well and made no changes for now, but some timing tweaks can be expected once trends emerge. A few lingering showers may persist through daybreak.

While cigs have lifted to low vfr in many areas south of the terminals early this evening, mvfr cigs continue to redevelop across northern IL and confidence is low for cigs trends this evening and opted to continue prevailing mvfr cigs which are expected to persist into the overnight hours. Cigs are then expected to lower into ifr and likely lifr for at least parts of the area in the predawn hours and continue through mid morning. Cigs will slowly lift back through mvfr by late morning with vfr expected by early Sunday afternoon. Visibilities may drop as low as 1sm across parts of northern IL by daybreak but overall confidence is low for how low visibilities may drop.

Light southerly winds currently may become light southeast over the next few hours and then light southwest overnight. A cold front will move across the area Sunday morning, shifting winds to the west/northwest with speeds increasing into the 10-12kt range and gusts in the 15-20kt range Sunday afternoon, which will diminish with sunset Sunday evening. Winds may turn more to the north/northwest Sunday evening. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.