textproduct: Chicago

KEY MESSAGES

- High waves and dangerous swimming conditions will continue at Lake Michigan beaches this morning.

- Chance of thunderstorms with possible heavy rain and localized flooding Wednesday into early Thursday.

DISCUSSION

Issued at 234 AM CDT Mon Aug 3 2026

A surface ridge of high pressure continues to foster light surface winds as it slides eastward into the western Great Lakes region early this morning. The main concern with this is that the lingering low-level moisture (noted by current dewpoint depressions of 1-3 degrees F) will favor the development of some locally dense fog/and or low clouds prior to daybreak this morning. Currently, it appears the best potential for fog will be primarily in areas along and west of the Fox Valley in northern IL through about 8 to 9 am. Farther east, slightly greater northeast flow off the surface may favor more of a low stratus deck verses true surface fog. While there have been some reports of shallow fog in parts of northwestern IL, nothing widespread has yet developed. This may change during the predawn hours, so we will continue to monitor observation trends.

Any fog or low clouds that develop early this morning will quickly diurnally improve through mid morning. Once it does, we will end up with a rather nice weather day across the region. Temperatures are forecast to top out in the low 80s across interior sections of IL under partly cloudy skies. Closer to the lake, light northeasterly synoptic flow under the surface high will be reinforced by an afternoon lake breeze, which will hold highs generally in the mid 70s this afternoon.

Dry weather continues Tuesday, though temperatures will be a bit warmer (low to mid 80s) as the flow turns southwesterly along the western periphery of the eastward exiting surface ridge, and in advance of a cold front shifting southward across the Upper Midwest. While dry locally Tuesday, forecast attention during the day will be focused on the approaching upstream cold front, and the large upper-level low shifting over southern Hudson Bay. By midweek, seasonably strong southwesterly mid and upper-level flow developing along the southern periphery of the Hudson Bay low will steer several smaller scale impulses east- northeastward along the surface front as its shifts into our area. Accordingly, it appears that we are heading into a period of active weather midweek, with good chances (50%+) for showers and thunderstorms (some possible strong/severe), particularly Wednesday into Thursday. Instances of very heavy rainfall and renewed flash flood potential look to be of concern with these midweek storms. This is particularly the case given the presence of a very moist prefrontal airmass (precipitable water values ~2"), as well as the possibility for repeated rounds of storms moving over the same areas.

A period of quieter weather looks likely late Thursday into Friday as the surface cold front gets shunted south. However, low- level WAA and moisture advection will quickly return by early next weekend while a broad low-amplitude ridge drifts across the western CONUS. Expect summerlike warmth and increasing humidity for the weekend. Building instability and modest WNW flow aloft will also support increasing chances for convective episodes into the Great Lakes region by Sunday.

KJB

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 1207 PM CDT Mon Aug 3 2026

A large surface high pressure system will continue to meander through the region today leading to generally light and variable winds. The weak surface pressure gradient is allowing for a lake breeze to push inland, which will lead to a northeasterly wind shift at ORD/MDW within the next hour or two and DPA later this afternoon. Otherwise, winds will return to a light and variable regime overnight. After daybreak, winds will trend southwesterly and increase in magnitude toward 10kt by late morning.

Scattered MVFR cigs based near 2500ft will rise to 3500ft this afternoon and scatter behind the aforementioned lake breeze. Pockets of fog may develop near RFD/DPA/GYY overnight, but will keep mention out of the TAF for now given low confidence in timing/placement.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

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