textproduct: Chicago

KEY MESSAGES

- Hot and humid conditions will persist through Thursday afternoon, especially south of I-80, with peak heat indices around 100 to 105F.

- Several rounds of thunderstorms will move through the general area Thursday through Saturday. Some of these may be severe.

- A break in the hot and humid pattern may arrive by early next week.

UPDATE

Issued at 922 PM CDT Wed Sep 2 2026

Have allowed the Heat Advisory for portions of far northern and northeast IL (including the City of Chicago) to expire as scheduled at 9 PM CDT. While it will remain warm and muggy across the region overnight, heat indices have dropped into the low-mid 90s and even upper 80s in spots across the area. Thinking has not changed regarding heat trends for Thursday at this time. Areas from Rockford east/southeast to Chicago appear to be at the greatest risk for convective outflow from the north Thursday morning and/or convective development along outflow and/or lake breeze boundary into Thursday afternoon. This reduces confidence in these locations reaching heat headline criteria, with peak forecast heat indices in these locales for Thursday just shy of 100 (mid-upper 90s). Midnight shift can re- evaluate this as convective trends become more clear toward morning. Confidence is higher in another day of 100-105 head index values across the remainder of the area, thus the Heat Advisory remains in effect through 9 PM CDT Thursday evening roughly south of the aforementioned line.

Speaking of convective trends, elevated showers and isolated thunderstorms continue to fester over northeast IA as of 9 PM CDT. Surface analysis indicates a nearly stationary frontal zone from near the IA/MN, border east across southern WI and Lake Michigan. GOES vapor imagery indicates a subtle mid-level short wave propagating slowly east across this region, with a modest 25-30 kt southwest low-level jet oriented into the frontal zone over northeast IA. The nose of this jet is progged to veer into southern WI overnight, likely maintaining the threat of scattered elevated showers/storms primarily north of the IL/WI state line into Thursday morning. Can't completely rule out some of this activity scraping far northern IL, though mid-level moisture decreases with southward extent, decreasing convective potential the farther south into our cwa you go. Have maintained slight/low (20-30%) chance pops north of about the I-88 corridor, though expect most (if not all) of the area will be dry through daybreak.

Did raise hourly temps/dew points through tonight based on obs trends, but otherwise made no significant changes to the forecast into Thursday with afternoon thunderstorm chances still on track.

Ratzer

DISCUSSION

Issued at 246 PM CDT Wed Sep 2 2026

Broad upper ridging continues to reside across the Ohio and Tennessee River Valleys and southern Great Lakes which is resulting in another hot and humid afternoon. Temperatures have continued to warm into the mid-90s and with dew points in the lower to mid-70s, heat indices are once again peaking in the 100-105F range this afternoon. While the upper ridge is expected to gradually drift south and east into the Mid-Atlantic region tonight into Thursday, the hot and humid conditions are forecast to persist through Thursday afternoon. However, with more cloud cover expected Thursday temperatures should be a few degrees cooler which in turn should limit peak heat indices into the 95-105F range with the highest readings forecast to occur south of I-80. That said, where heat indices due break the 100 degree mark there will be the danger for heat related illnesses which is why the Heat Advisory has been extended through 9 PM CDT Thursday evening for our north-central IL counties and all of our northwest IN counties. Regardless, anyone with outdoor plans and/or activities Thursday should use caution and remain hydrated.

As the aforementioned ridging pushes south and east, the parade of shortwave disturbances that are currently ongoing in IA and WI will begin to pivot into northern IL and northwest IN and result in periods of showers and thunderstorms. The first period of shower/storm chances is late tonight into Thursday morning as the disturbance currently in IA pivots across southern WI and northern IL and interacts with a sagging surface boundary. Given that the ridge will still mainly be in control it looks as if the bulk of any showers/storms tonight will stay north of the IL-WI line. Though, if sufficient storm coverage can occur in WI and push an outflow into northern IL tonight then some isolated to widely scattered showers with embedded lightning could drift into portions of northern and northeast IL towards daybreak Thursday morning.

Another shortwave disturbance is expected to pivot across northern IL and Northwest IN Thursday afternoon and evening which should result in another period of scattered showers and thunderstorms. While the environment looks primed with MLCAPE values around 2500 J/kg and dew points in the low to mid-70s, storm coverage continues to be a point of uncertainty given the weaker forcing and lingering effects of the ridge. That said, the greatest coverage (20-30%) is currently forecast to occur north of I-80 closer to the center of the disturbance which should further help to limit the extent of the aforementioned heat in these areas but if storms fail to materialize then higher heat indices may be seen further northward. Any storms that do develop will also have the potential to become severe given the 35-40 kts of shear present. Since most of the instability should be elevated it seems the main threats with any severe storms should be damaging winds and large hail, but will need to monitor the potential for supercell structures which could result in a localized tornado threat especially near any remnant outflow boundaries. Because of this severe threat the Storm Prediction Center has maintained the marginal (level 1 of 5) risk for our entire area for Thursday afternoon and evening.

Heading into Thursday night, yet another thunderstorm complex is forecast to develop in southern WI and begin to drift southward towards northern IL. If the environment is not fully disrupted by any afternoon storms and/or can recover, then this complex should be able to move into northern IL and eventually northwest IN after midnight Thursday night and linger into early Friday morning. Despite the unfavorable diurnal time the complex will still have decent instability and wind shear to work with which may allow a threat for damaging winds and hail to persist into the night. Additionally, the complex will also be moving into a veering low- level jet over northern IL which may support an axis of training thunderstorms. Given the humid air mass these storms will be capable of torrential downpours which may result in instances of flooding especially if the storms train over urban areas. Right now it is unclear where exactly this complex will track and thus where the heaviest rainfall will occur, but it seems that somewhere in northern IL and/or northwest IN could see an area of 1-2+ inch rain amounts.

By Friday, another upper ridge is forecast to begin to build into the central CONUS which will generate northwest flow aloft over the Great Lakes. While this pattern will keep the periodic shower and thunderstorm chances in play through much of the upcoming holiday weekend, it does appear that the aforementioned storm complex should work over the atmosphere enough to result in mostly dry conditions for at least the daylight hours Friday. Our next period of showers and thunderstorms looks to develop Friday night into Saturday as another storm cluster takes shape beneath the northwest flow but confidence on coverage and track of this complex is low. Rain chance are forecast to diminish as we head into the early part of next week as the central CONUS ridge pivots east and pushes the favored storm track out of our area. While temperatures from Friday onward will be cooler than the past few days, highs are still expected to remain above average in the low to mid-80s through middle of next week.

Yack

AVIATION /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 635 PM CDT Wed Sep 2 2026

Main aviation forecast concerns are the probability/timing of thunderstorm chances for the terminals Thursday afternoon and again later Thursday evening/overnight. In addition, wind directions by Thursday evening are of low confidence for the Chicago terminals due to possible thunderstorm outflow and lake breeze impacts. Based on output of several high-res models, have indicated an easterly wind Thursday evening for ORD/MDW.

Terminals remain along the northern periphery of mid-level and surface high pressure ridges early this evening, maintaining modest southwest surface winds. Any lingering gusts should dissipate quickly as sunset approaches, then develop again near 20 kts later Thursday morning. Southwest to west-southwest winds are expected into the afternoon, prior to any TS outflow or lake breeze complications.

Farther to the north and northwest, scattered SHRA/TSRA have been focused mainly across northeast IA this afternoon, in association with a weak mid-level wave in the vicinity of a nearly stationary frontal zone. The general model consensus is that this activity will eventually spread into southern WI/LM later tonight into early Thursday as the wave moves east, but remain mainly north of the IL/WI state line. Have held on to the inherited PROB30 for SHRA at KRFD toward morning, given the closer proximity to frontal zone and better elevated SHRA/TS potential. If convection to the north of the Chicago area were to become organized enough to push an outflow boundary southward toward the terminals after midnight, would need to keep an eye on any potential SHRA/TSRA development farther south, but that is not expected at this time.

TSRA chances do increase Thursday afternoon however, as capping weakens over northern IL and the environment becomes more susceptible to any low-level convergent sources such as outflow or lake breeze boundaries. Expectations continue to be for isolated to scattered TSRA to develop on one of these boundaries across northeast Illinois, possibly as early as mid-afternoon. Any storms which develop Thursday afternoon could be strong to severe and produce strong and gusty winds. Have maintained the PROB30 for TSRA from 21-00Z for all sites. Additional periods of storms appear possible into Thursday night/Friday. Have indicated another PROB30 period for ORD/MDW after 04Z, with decent model and ensemble guidance indicating additional storms spreading south/southeast from WI by midnight.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory until 9 PM CDT Thursday for ILZ008-ILZ010-ILZ011- ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106- ILZ107-ILZ108.

IN...Heat Advisory until 9 PM CDT /10 PM EDT/ Thursday for INZ001- INZ002-INZ010-INZ011-INZ019.

LM...None.