textproduct: Chicago

KEY MESSAGES

- Hot and humid conditions will continue today, though the highest heat indices (up to/around 105F) will likely focus near/south of the Kankakee River. On Independence Day, peak heat indices of ~95-100F are expected near and south of I-80.

- Waves of thunderstorms are expected through the weekend, though there will be many dry hours. Any storms today will be capable of producing damaging winds. There will also be a threat for localized flash flooding with any focused corridors of storms through the weekend.

DISCUSSION

Issued at 333 AM CDT Fri Jul 3 2026

Earlier storms have exited the area to the east with the associated outflow boundary having managed to push as far south as Paxton, IL bringing temperatures into the lower 70s areawide. While a stray shower or storm can't be ruled out overnight, most areas are expected to remain dry.

The residual outflow boundary from storms early this morning is anticipated to lift back to the north some though exactly where remains uncertain. Opted to maintain the Extreme Heat Warning for counties along and south of the Kankakee River Valley, where heat indices may reach 105 degrees this afternoon. Farther north will still be hot and humid, but not to the extent of the past several days with forecast peak heat indices in the 95-100 degree range.

The shower and thunderstorm potential trends for today are also uncertain, as is often the case in these types of patterns. First, we are monitoring storms over northeast Nebraska into central Iowa ongoing early this morning. This feature is expected to progress east with time, likely in a weakening state. However, an MCV appears to be developing across northwest Iowa which, paired with a likely convectively augmented mid- level vort, may serve as a forcing mechanism for shower and thunderstorm development into northern Illinois in the afternoon. Additionally, the aforementioned outflow boundary may serve as a forcing mechanism for shower and thunderstorm development in the afternoon. Again, questions remain as to exactly how far north the boundary will lift this afternoon. It is tough to say whether the late evening timeframe will be shower and storm free given the potential for additional redevelopment along any potential residual outflow boundaries from any storms that develop earlier in the day. While shear is not especially strong, ample instability in place will be supportive of damaging wind gusts with any storms today/tonight. Any training corridors of storms could also lead to flooding.

Going forward into Independence Day, a larger disturbance is forecast to move across the region bringing another round of showers and storms, currently favoring the afternoon to early evening timeframe. The northern periphery of the better instability will likely have shifted south some in response to the Friday/Friday night rounds of storms. Still can't rule out locally strong to damaging wind gusts with any storms that do develop, with perhaps a locally higher threat south of I-80. While it is important to not latch onto any one model run, it is worth noting that the latest HRRR (6Z) attempts to develop more robust storms north of I-80 on Saturday as the wave moves through (likely owing to farther north position of earlier rounds of convection). Accordingly, the message will remain that additional rounds of storms can be expected (some potentially strong to locally severe).

Sunday will feature yet another mid-level wave moving across the region due to the flattening of the upper ridge. This will bring additional rounds of showers and isolated embedded (non-severe) storms with greatest coverage favored in the afternoon-early evening. Temperatures will be closer to normal for early July with highs in the lower to mid 80s.

Beyond the weekend the upper ridge begins to build back north slightly with the embedded waves moving to our north. Meanwhile another wave (or waves) will move within the ridge to our south (and potentially stall out). Overall this favors shower/storm coverage focusing mainly north or south of the area into mid- week. Shower and storm chances return later in the week as the ridge begins to break down again.

Petr

AVIATION /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 624 AM CDT Fri Jul 3 2026

Forecast concerns for the terminals...

- Scattered thunderstorms likely this afternoon and evening. A few of which could be strong with gusty winds in excess of 30 kts and heavy downpours.

- Another period of showers and storms is expected overnight into Saturday morning.

The earlier line of showers and storms has moved away from the terminals giving us a mostly sunny start to our Friday morning. While winds have been varying between 170-190 early this morning, winds will settle into a west-southwest direction this afternoon with speeds around 10-12 kts.

The main forecast concern for today will be timing out the arrival of the thunderstorm complex ongoing in central IA. Current trends suggest the complex arriving in the 19-21z timeframe (earliest at RFD and latest at GYY), but with the outflow boundary from last night's storms still lingering in northeast IL and northwest IN cannot rule out some isolated activity tries to get going a bit sooner. Regardless, once storms arrive they should persist for at least 2-3 hours before exiting the terminals. Given the humid and unstable air mass forecast to be in place, any stronger storms will be capable of gusty winds in excess of 30 kts and heavy downpours that will result in reduced visibilities and some lower ceilings.

A break is expected to develop behind the storms this afternoon/early evening. Though a lot of uncertainty resides on how long the break will last as another thunderstorm complex should be ongoing in IA and working its way across IL and IN overnight. Due to the uncertainty in arrival/development of storms tonight have opted to introduce a lengthy PROB30 in the TAFs for this potential but suspect the actual duration of showers and storms overnight to be in the 3-4 hour range. With the loss of diurnal heating storms tonight should mainly be a heavy rain threat, but some locally gusty winds cannot be ruled out.

Outside of the showers/storms today and tonight, expect VFR conditions to prevail with otherwise southwest winds around 5-10 kts. However, winds may become variable around and behind thunderstorms for a couple of hours before settling back into a southwest direction.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

IL...Extreme Heat Warning until 7 PM CDT this evening for ILZ023- ILZ032-ILZ033-ILZ039.

IN...Extreme Heat Warning until 7 PM CDT /8 PM EDT/ this evening for INZ010-INZ011-INZ019.

LM...None.