textproduct: Chicago

KEY MESSAGES

- Chance of scattered showers and thunderstorms tonight.

- Gradually turning cooler and less humid this weekend, though remaining seasonably warm.

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

UPDATE

Issued at 835 PM CDT Fri Sep 4 2026

Low-confidence convective forecast tonight, though the atmosphere may be starting to show it's hand with new cells developing near the IL/WI border northwest of Chicago at mid- evening. Have cancelled the Heat Advisory as temps/heat index values lower after sunrise, and also allowed the Beach Hazard Statement to expire with winds/waves decreasing significantly from earlier today.

GOES vapor imagery shows a minor mid-level short wave topping the ridge axis over northeast IA/southwest WI early this evening. A cluster of thunderstorms over northeast IA (likely aided by ascent from the short wave) had weakened significantly with sunset, though recent radar imagery indicates new convective cells developing along the IL/WI northwest of Rockford. DVN 00Z RAOB indicates ~4800 J/kg of MUCAPE (~5000 J/kg in SPC RAP mesoanalysis downstream into northern IL/northwest IN). Despite this, warm profiles/weak capping in low levels and otherwise weak low-level forcing had not been supporting even much in the way of low-level cumulus across the forecast area late this afternoon. However, the mid-level wave appears to be enhancing ascent, increasing effective shear and moistening the profile sufficiently for some threat of convective development into the area through the remainder of the evening and early overnight hours. Though CAM guidance has struggled with the details of development/placement of storms, there has been a somewhat consistent signal for an uptick in thunderstorm coverage from far north/northeast IL into northwest IN tonight. Despite some shallow stabilization of the boundary layer after sunset, the degree of available instability, ~30 kts of effective shear and RAP forecasts DCAPE of ~1000 J/kg would suggest these storms could become locally strong/severe with a damaging wind threat especially if any organized cold pool can develop. Confidence is low in just how current cells northwest of Rockford will evolve, but they'll bear watching. Thunderstorm development has been a little slower than some of the guidance, though it still looks as if storms should exit the cwa to the southeast prior to daybreak.

Otherwise, A weak lake-assisted cold front was pushing slowly southwest across the northern half of the Chicago metro at this hour, and this may further slightly destabilize the boundary layer as slightly cooler (though still humid) air oozes inland. Even south of the front, heat indices have fallen into the 90s with sunset and thus allows cancellation of the Heat Advisory a little early.

Ratzer

DISCUSSION

Issued at 314 PM CDT Fri Sep 4 2026

Breezy east-northeasterly winds this morning in the wake of the outflow boundary have significantly impacted the ability to heat up through early afternoon. This has particularly been true across far northern/northeastern IL, where temperatures as of 1 pm ranged from the mid to upper 70s near the lake, to the mid 80s farther inland. This prompted us to cancel the Heat Advisory earlier for Cook, Lake (IL), McHenry and Boone counties. In spite of this, expect temperatures to spike into the upper 80s to low 90s late this afternoon as the low-level flow gradually veers back to the south-southwest. This could result in a couple hour period of heat indices reaching 95-100 outside the current Heat Advisory. Conversely, slightly warmer temperatures and higher humidity within the Heat Advisory area will support peak heat indices around (or a bit above) 105 degrees through early evening.

In addition to stunting the heat across northeastern IL, The breezy east-northeasterly outflow winds this morning have created rather choppy continues along the southern Lake Michigan shore. Wave heights as high as 4.5' were reported from a buoy off the northern Cook county shore. Waves of this height are hazardous to small craft and swimmers at area beaches across northeastern IL. For this reason, we hoisted a Small Craft advisory and Beach Hazard statement through 7 PM this evening for the northeastern IL Lake Michigan shore. Easing and veering flow should allow these conditions to subside into this evening.

The outflow boundary from this morning's convection across southern WI has settled into areas south of the Kankakee river valley across eastern IL and northwestern IN early this afternoon. However, it bends back northward across far northwestern IL. The airmass south and west of this boundary remains quite unstable (per the 18Z DVN RAOB). This has recently supported the development of some scattered strong storms west- northwest of the area in closer proximity to a surface cold front and approaching mid-level impulse. It is currently unclear as to how long this activity will hold together, owing to suboptimal deep-layer shear. Nevertheless, if this activity is able to develop a formidable cold pool, a cluster of southeastward shift storms could approach parts of far northern IL this evening after 7 pm. Confidence in this holding together remains low at this time.

If the activity to northwest dissipates, there remains a play in which we experience renewed development with the front mid to late this evening into the overnight as falling heights aloft overspread the frontal boundary in response to the main mid/upper-level ridge axis retrograding westward and building northward across the Missouri Valley. If this occurs, the favored placement would be across northeastern IL into northwestern IN from 9pm through 3 am. While not looking to be much of a severe threat, some locally strong wind gusts could accompany the storms, particularly if they form into a southeastward shifting cluster.

The progression of the front is expected to continue into central IL and IN on Saturday. Very warm and humid conditions ahead of this front could result in temperatures (heat indices) topping 90 (100) again across my far southern counties, but at this time another Heat Advisory is not planned for the area. Otherwise, behind the front, it will be turn somewhat cooler and a bit less humid Saturday afternoon, but a period of low stratus may be slow to improve during the afternoon. Temperatures and humidity will continue to improve into Sunday.

A period of moderately strong northerly winds down the full fetch of the lake should result in an increase in waves later Saturday through early Sunday. It is possible that a beach hazard statement may be needed for dangerous swim conditions for a part of this weekend, which is the closing weekend of the beach season.

Dry and seasonably warm weather is expected Labor Day. Temperatures will warm some and humidity will be on the increase Tuesday into Wednesday in the warm air advection regime ahead of the next shortwave. An associated surface cold front could bring some showers and thunderstorms with it midweek before another bout of somewhat cooler, more seasonable, temperatures filter in behind the front to close out the short work week.

KJB

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 636 PM CDT Fri Sep 4 2026

Forecast concerns include...

Chance of thunderstorms this evening into the early overnight. Wind shift to northeast this evening. Low mvfr cigs, possible ifr cigs Saturday morning. Possible fog early Saturday morning.

Convective trends remain uncertain this evening. Showers across far southern WI continue to weaken as do thunderstorms across far northeast IA. Adjusted prob thunder timing early based on some of the latest short term guidance which shows potential new development across northeast IL this evening but confidence remains low and further refinement/changes can be expected as trends emerge. The end time of any thunderstorms also remains uncertain. The later they develop, they later they may persist.

There is a lake boundary moving inland across eastern WI and into very far northeast Lake County IL currently. This is expected to continue moving southwest this evening, with a wind shift to northeast expected at ORD/MDW but only medium confidence for timing. Maintained the previous 03z timing, but will also need to monitor trends. Winds may become light and variable for a time overnight, before turning back northeast and then remaining northeast for the rest of the period with speeds 10-15kt likely Saturday afternoon.

A period of low mvfr cigs is expected in the predawn hours lasting through midday/early afternoon Saturday. There may also be ifr cigs, especially for areas north of the terminals but confidence is low. Areas of fog will also be possible overnight into Saturday morning, mainly for areas north and northwest of the Chicago terminals. cms

LOT WATCHES/WARNINGS/ADVISORIES

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