textproduct: Chicago

KEY MESSAGES

- Thunderstorms will move across the area today. Coverage of storms will be highest now through about 8 or 9 PM (ending from west to east).

- Locally damaging winds and very heavy rainfall are the main threats with a pronounced threat for flash flooding particularly in the western and southern Chicago metropolitan area.

- Sunday will feature a few showers and storms largely south of I-80. Coverage and intensity will be far less compared to the past few days.

- Drier and more seasonable conditions expected to start the week before stormier weather returns late week.

DISCUSSION

Issued at 1251 PM CDT Sat Jul 4 2026

Showers and thunderstorms are beginning to expand in coverage across western Illinois ahead of a notable mesoscale convective vortex (MCV). The airmass ahead of the MCV is very moist and unstable with mean 1km mixing ratios in excess of 15 g/kg, PWATs nearing 2", and MUCAPE nearing 3000 J/kg. Unlike yesterday, mid-level flow today is notable weaker with the DVN VWP sampling only around 25 to 30 kt at 4 to 6 km. As a result, the main threats with thunderstorms today will be locally damaging winds in wet microbursts and very heavy rainfall (rain rates >2"/hr) with a threat for flash flooding.

In terms of the evolution of thunderstorms, current expectations are for the roughly north to south-oriented band of storms from Rockford toward Peoria at press time to intensify and shift eastward with time. At the same time, also anticipate storms to develop in a west-to-east oriented band across northern Illinois associated with modest warm-air advection ahead of the MCV atop a remnant frontal boundary and approaching lake cold front. The net effect should be a fairly high coverage of thunderstorms across the area, especially across northern Illinois and the Chicago metropolitan area. In terms of timing, thunderstorms should expand in coverage and cross the I-39 corridor in the next few hours, move across the Chicago metropolitan area generally between 3 and 6 PM, and then move through northwestern Indiana in the 6 to 9 PM time window. Unlike yesterday, we do not anticipate additional storms after 9 PM outside a few lingering cells here or there.

As previously mentioned, thunderstorms today will be capable of producing very heavy rainfall with hourly rain rates in excess of 2 inches per hour (instantaneous rates in excess of 4 inches per hour). Of particular concern is the potential for several hours of such rain rates in the western and southern Chicago metropolitan area where observed two-day rainfall amounts are in the 3 to locally 6 inch range. It is thus no surprise that many rivers and creeks in the aforementioned area are at bankfull or flood stage. With the potential for several more inches of rain today, continue to think there is a localized threat for significant flash flooding today primarily across portions of Kane, DuPage, Cook, Will, and Lake (IN) counties.

A Flood Watch and Severe thunderstorm Watch are in effect for portions of the area through this evening.

Tonight into Sunday:

Winds will turn northeasterly overnight in the wake of the aforementioned backdoor lake front. With cooling temperatures within the residually moist airmass, areas of fog may develop overnight especially near the Lake Michigan shoreline.

Tomorrow, a weak area of low pressure is expected to pass through central Illinois encouraging another backdoor front to move onshore from Lake Michigan. A period of low clouds or fog may follow the secondary front. With forecast soundings depicting largely uncapped profiles by tomorrow afternoon, suspect a few showers and storms will develop along the inland- pushing lake breeze favoring areas south of Interstate 80. Compared to the yesterday and today, coverage of storms tomorrow should be far less (isolated to scattered), and be tied to the peak heating of the day (not lingering after sunset). Highs tomorrow will be in the upper 70s to lower 80s. Finally, will have to watch winds in the wake of the secondary lake front closely as they may cause waves to rise into the 3 to 4 ft range, making swimming dangerous. At this point, we favor waves moreso in the 2 to 3 ft range. Will proactively advertise a Moderate Swim Risk tomorrow just to be safe.

Borchardt

Monday onward:

By Monday the upper trough will be ejecting eastward and taking with it the chances for showers and thunderstorms as a surface high settles into the western Great Lakes. It should be noted that our official forecast does have some 20-30% POPs in our far southern CWA Monday afternoon where some slower guidance has a stray shower/storms trying to develop, but the overall environment locally should support dry conditions areawide so suspect these POPs will get removed in future updates. Nevertheless, temperatures on Monday will be on the seasonable side with highs once again in the lower to mid-80s inland with 70s expected near the lake due to onshore winds. Tuesday will feature similarly dry conditions, but with slightly warmer conditions (highs in the mid to upper 80s) as winds turn back southerly.

Unfortunately the tranquil weather is not forecast to last long as guidance continues to show the return of west-southwest flow aloft for the middle and latter half of next week. While temperatures look to remain seasonable, humidity will be on the increase which will also bring the return of shower and thunderstorm chances.

Yack

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1200 PM CDT Sat Jul 4 2026

An approaching MCV interacting with the residually moist and unstable environment will support a fairly high coverage of thunderstorms across the airspace this afternoon. Current expectations are for a west-to-east oriented band of storms to develop near the vicinity of DPA/MDW/ORD/GYY (near developing cu) as early as 19Z and then a north-to-south oriented band of storms (first signs of which already west of I-39) to sweep across the remainder of the area this afternoon. For this reason, will continue to advertised a relatively prolonged 4-hour TEMPO group for DPA/MDW/ORD/GYY to cover both "regimes" of thunderstorms. The highest coverage of storms may end up just south of RFD. Any storm will be prone to producing wet microbursts with gusty to erratic wind gusts in excess of 30 kt and markedly reduced visibility below 1 mile.

Outside of the thunderstorms, a backdoor front coming off Lake Michigan will cause current variable wind directions to turn northeasterly (though actual directions this afternoon will be modulated by thunderstorm outflow).

This evening, northeasterly winds advecting cool air off Lake Michigan may encourage the development of BR or LIFR cigs. Will advertise SCT005 as early as 03 to 04Z at GYY/ORD/MDW for the overall low confidence scenario. Winds may tend to veer northerly through the night before a reinforcing push of cooler air arrives after daybreak (which may be accompanied by another push of BR/low cigs).

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch until 10 PM CDT this evening for ILZ010-ILZ011- ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105- ILZ106-ILZ107-ILZ108.

IN...Flood Watch until 10 PM CDT this evening for INZ001.

LM...None.