textproduct: Chicago

KEY MESSAGES

- Hot again today away from the lake with afternoon heat indices in the 90s, and as high as 100 to 105 south of the Kankakee and Illinois river valleys.

- Relatively cooler early next week, although remaining seasonably warm.

- Moderate to high swim risk expected late Saturday into early Sunday during the final weekend of beach season.

DISCUSSION

Issued at 211 AM CDT Sat Sep 5 2026

It's an unseasonably warm start to our Saturday with temperatures across most of the area still sitting in the middle to upper 70s as of 2 AM. A cold front is gradually inching south across the area as we speak. The placement of the front isn't terribly defined, but it looks to be draped roughly along the I-88 corridor. There isn't much of a cold advective response immediately behind the boundary, but slightly cooler temps in the lower 70s are found north of the front across our northern CWA and up into southern WI. A few thunderstorms traversed our northern CWA along the boundary late last evening driven by a midlevel shortwave impulse scooting east atop the area. That wave has since moved away to the east and taken the precip coverage with it, but a reservoir of elevated instability remains over the area and it's possible that a light shower or even isolated thunderstorm develops along the boundary early this morning as it progresses south. A general lack of forcing, however, will largely inhibit precip development this morning. We are seeing some areas of fog developing in the wake of the front up in southern WI. Nothing too dense has been observed nor is expected, but it's possible that some visibility reductions spread into northern IL early this morning north of I-80.

Mixing into relatively cooler, northerly BL flow today will keep temperatures a couple to a few degrees cooler than yesterday with highs forecast in the middle 80s to lower 90s across inland areas. The warmest temperatures and highest dewpoints are forecast in our far southern CWA where afternoon heat indices may approach 105. Expectations are for largely closer to 100 though, so opted against issuing any heat headlines for today. Elsewhere across inland areas, forecasting heat indices throughout the 90s today. It'll be a different story near the lakeshore where the onshore flow will bring noticeably cooler conditions than yesterday with afternoon temperatures forecast in the 70s up to a few miles inland from the lake.

The breezy northerly fetch will also generate choppy conditions on the lake this evening into early tomorrow. Will continue to advertise a Moderate Swim Risk for now as waves generally look to top out in the 2 to 4 foot range, but expectations are not far from warranting a High Swim Risk for a period this weekend, especially at northwest Indiana beaches.

The midlevel ridge will center itself over the region early next week shunting any meaningful baroclinicity well away for a few days. Highs in the lower 80s are forecast for tomorrow and Monday with mostly sunny skies. Upper 80s and lower 90s are favored again for the middle part of next week with scattered chances for rain returning as early as Tuesday night.

Doom

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 621 AM CDT Sat Sep 5 2026

IFR CIGS have rapidly developed along the southeast WI south into the far northeast IL shore over the past 90 minutes. Anticipate this IFR to continue to develop southward while expanding westward some. This should result in IFR CIGS into ORD, MDW, and GYY by or just after the start of the 12z TAF cycle. IFR is likely to linger until mid to late morning when there should be some diurnal lifting of bases, likely to MVFR. Should see CIGS scatter out to VFR this afternoon, though timing of each of the category transitions is low.

Farther inland at DPA and RFD, a brief period of IFR is possible early this morning followed by a period of MVFR CIGS. Similar to terminals close to the lake, anticipate a scattering out to VFR conditions early this afternoon.

Finally, some patchy ground fog is possible early Sunday morning at DPA and RFD.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES

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