textproduct: Chicago

KEY MESSAGES

- Threat for severe thunderstorms and flash flooding Tuesday afternoon/night.

DISCUSSION

Issued at 325 AM CDT Mon Aug 17 2026

Any lingering showers have since shifted southeast of the area in the wake of a cold front which will bring a brief break today from the active weather pattern. A reinforcing lake enhanced front is currently moving down the western side of Lake Michigan and will drift into the area this morning. A low-level convergence axis is expected to set up where this boundary stalls out, near and just west of Lake Michigan this morning into the afternoon. Modeled soundings suggest there is a window when cloud depths may be just deep enough (paired with a ribbon of higher low-level instability) to support the development of a narrow corridor of showers and/or sprinkles late morning into the afternoon. Opted to add slight chances (15-20%) for showers in the gridded forecast to account for this near Lake Michigan and as far west as I-355. However, most areas are expected to remain dry today even if these showers develop.

Temperatures today will be largely dependent upon which side of the aforementioned convergence axis sets up, with upper 70s to lower 80s farther inland and low to mid 70s closer to the lake. Diurnal mixing to the west of this axis should also help keep heat indices in check compared to yesterday. Temperatures tonight are forecast to drop into the low to mid 60s for most of the area. If high clouds are slower to drift into the area overnight, temperatures may manage to drop into the upper 50s across portions of interior northern Illinois.

Attention then turns to Tuesday when a series of subtle waves/vorts are forecast to move within the broader northwest flow aloft across the area in the afternoon and night. This may lead to the development of thunderstorms across southern Wisconsin and northern Illinois in the afternoon that moves southeast across the rest of the area into the evening and overnight hours. While there may be some capping initially to limit coverage, any storms that develop will have the potential to become severe, especially near and north of I-80 where deep layer shear will be the highest (upwards of 40 to 50 kts). The flooding threat may be more localized initially with a more discrete mode, however, if storms transition toward multicell clusters or redevelop over the same areas into the evening and overnight hours, flash flooding would become a greater concern.

There continues to be variability amongst the available model guidance with respect to shower/storm coverage here locally, likely owing to differences in the strength of the capping and position and strength of the low-level jet. Given some of these uncertainties, SPC and WPC have maintained a level 1 of 5 threat for severe weather and level 1 of 4 threat for flash flooding respectively. However, am becoming a bit more concerned that a conditionally higher severe and flash flooding threat may unfold over at least portions of the area and will continue to message this potential. Will have to monitor trends closely over the next 24 hours.

Another break in this active pattern is in store Wednesday into Thursday as a cooler and less humid airmass settles into the area in the wake of a cold front late Tuesday night/early Wednesday AM. Temperatures will be cooler (typically lower 80s for mid August) with highs in the mid to upper 70s each day amidst cooler onshore flow. Confidence in the details then decreases late week into the upcoming weekend as additional waves move within the upper level northwest flow pattern, with a potentially more robust and slower moving wave bringing increasing shower/storm chances for the weekend.

Petr

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 633 AM CDT Mon Aug 17 2026

A narrow convergent axis and associated higher moisture streaming SSW over Lake Michigan is beginning to settle over northeast Illinois as of TAF issuance. A NE wind shift is about to cross ORD/MDW/GYY with winds up to around 15 knots. BKN MVFR ceilings as well as a few SHRA are accompanying this axis. Diurnal heating this morning will support continued BKN MVFR stratocumulus, but steadily lift bases into low-end VFR levels by early afternoon. While a strong inversion around 7 to 8kft supports only marginally sufficient cloud depths for the generation of SHRA, added low-level convergence in the main axis should aid in the redevelopment of at least isolated light SHRA for the Chicago metro terminals late morning to mid afternoon. TEMPO SHRA may be required for a site or two if the main axis settles over an airfield.

Cloud depths will decrease late this afternoon as bases rise well into VFR levels, ending the chance for lingering SHRA. Winds should slowly veer SE at 5 knots or less this evening to SW by around midnight, though there undoubtedly will be a period of VRB or even calm conditions during this time. Expect SW winds of 10 to 15 knots at the end of the TAF period late Tuesday morning.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

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