textproduct: Chicago

KEY MESSAGES

- Heat and humidity will continue on Friday, though the worst conditions (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 thunderstorms on Friday will be capable of producing damaging winds. There will also be a threat for at least localized flash flooding with any focused corridors of thunderstorms through the weekend.

DISCUSSION

Issued at 315 PM CDT Thu Jul 2 2026

Through Early This Evening:

The outflow boundary that served as an impetus for isolated convection (showers and a few thunderstorms) near and north of I-80 has mostly washed out, taking away the enhanced lower level convergence. As such, given capping, and mid-level dry air and subsidence in place for most of the area, the threat for additional convection will be minimal through sunset. A minor exception will be into portions of central Illinois, as widely scattered showers and storms developed near and south of a PIA to DNV line. RAP analysis and near term forecasts indicate a pretty sharp mid-level moisture gradient into our central IL counties, so any thunderstorms that do manage to reach into far southern areas may tend to dissipate with farther northeastward extent.

Tonight-Early Friday:

Strong to severe thunderstorms ongoing as of this writing across the Upper MS Valley are tied to a low amplitude short-wave emanating from the Central Plains, interacting with a much less capped, more strongly unstable environment in that area. While the strongest large scale forcing will generally remain north of our area tonight, expectation is for a combination of erosion of capping and possibly congealing outflows from storms just to our to north and northwest to yield increasing thunderstorm coverage with time.

The exact extent of convective coverage is still a bit unclear, so opted for scattered wording in the grids near/north of I-80. The threat for storms should start earliest near the IL/WI state line (mainly after 8 or 9pm) and then build south later in the evening and early overnight. With a large instability reservoir, high PWATs (conducive to precip loading and wet microbursts), DCAPE still upwards of 1k J/kg, and modest deep layer shear, the main thunderstorm threat will be damaging wind gusts. It's certainly plausible that some of our far northern IL counties will need to be included in a convective watch this evening.

If more organized clusters/lines of storms are able to develop and send a coherent outflow boundary farther south prior to exhaust of available MUCAPE, we'll also need to watch for corridors of training convection and an associated flash flooding threat, most favored across far northern Illinois.

Shower and thunderstorm coverage is then expected to wane after 4 AM or so and translate to a primarily dry start to Friday. However, we did hold onto slight chance PoPs from 7-10 AM to account for rogue lingering convection.

Heat and Humidity on Friday:

Similar to today, lingering outflow boundaries, any associated wind shifts, and outflow modified profiles are quite plausible. Sufficiently high thunderstorm coverage tonight and sagging outflows tonight could somewhat delay air mass recovery with northward extent (and vice versa). All in all, the lower thunderstorm coverage expected tonight south of I-80 should enable an earlier jump on diurnal warming, plus higher dew points (solidly mid to locally upper 70s) are forecast. Therefore, confidence is highest in peak heat indices approaching or locally exceeding 105F for locations near and south of the Kankakee River. With Friday being day 5 of the current heat wave, felt the path of least regret was to extend the Extreme Heat Warning until 7 PM CDT Friday for the aforementioned locales. Depending on convective evolution this evening into early Friday, we may need to consider changes to heat headlines (the warning for the rest of the area expires at midnight CDT tonight).

Castro

Friday Afternoon-Evening Convection:

A more potent wave currently crossing northern Arizona will become another focus for convection over our area on Friday. This wave will cross northern IL and southern Wisconsin during the afternoon, setting the stage for a potential line or clusters of thunderstorms in the area Friday afternoon into the early evening traversing northern Illinois into northwest Indiana. Inevitably, effects of prior convection (or lack thereof in spots) can be expected to augment the location and timing of convective initiation, lowering confidence in specifics. With this afternoon's forecast issuance, we're messaging a favored timing of about 1 PM to 7 PM CDT for highest thunderstorm coverage on Friday (earliest west, latest east). Modest deep layer shear and an inherent high DCAPE reservoir with the warm/humid airmass support the risk for damaging winds with any linear segment or multicell clusters.

Conceptually, subsidence in the wake of the short-wave and associated convection should result in a steady downward trend in thunderstorm coverage the remainder of Friday evening, which *may* bode well for late evening community fireworks displays. It's certainly too early for an all clear area-wide in that period (mid-late Friday evening), though there has been a somewhat consistent signal (including in products such as the HREF calibrated thunder probabilities) in lower convective coverage to be able to note this in our graphical messaging.

Castro/Kluber

Friday Night Onward:

A mid-level impulse exiting the northern Rockies late on Friday will provide renewed convective episodes in the region as the mid-level pattern becomes zonal. The track of this convection will likely focus along an effective cold front produced by the combined effects of convection over the next 48 hours. So while convection seems likely in the region on Saturday/Independence Day, the location of convection could range from southern Wisconsin to southern Illinois. Furthermore, with forecast guidance consistently depicting a slowing or stalling mid-level wave over the western Great Lakes late Saturday into early next week, timing out precip chances becomes even more difficult if not impossible.

As the aforementioned wave eventually drifts eastward, drier and somewhat cooler conditions are expected by the middle of next week.

Kluber

AVIATION /00Z TAFS THROUGH 06Z SATURDAY/

Issued at 630 PM CDT Thu Jul 2 2026

Key Aviation Messages for the current TAF period:

- Scattered showers and thunderstorms this evening and possibly into the overnight.

- Additional thunderstorms during the day on Friday, most likely during the afternoon.

Southwest wind gusts around 20 knots and VFR conditions prevail in the immediate short term. However, outflow from storms in southwest Wisconsin is surging southward into northern Illinois. Winds with the outflow could turn to the northwest, with scattered showers and storms moving in behind it. Increased confidence in arriving around KRFD prompted not just an earlier time of arrival, but converting the PROB30 to a TEMPO.

Models are suggesting that this outflow will continue to move southeastward with a broken line of showers and storms moving along with it. But as the outflows moves away from the storms, they should be arriving in a decaying state. This adds some uncertainty in timing and location of storms specifically, keeping the chances as a PROB30, for now, though the arrival time was bumped up based on current observations. There is lower confidence in showers/storms remaining behind this round. As a low level jet increases in the pre-dawn hours, there is a chance that elevated convection could tap into a region of instability resulting in the need for VCSH/VCTS through 12Z. For now, it was decided to see what occurs with the first round and then potentially adding additional precipitation mention at the 06Z TAFs.

Winds will return to the west southwest tomorrow. Other than some lingering showers in the morning, a drier period in the morning is expected for most terminals. The next impulse is expected to move over northern Illinois from the west in the late morning to early afternoon prompting PROB30 mentions for TSRA at all the TAF sites.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...Extreme Heat Warning until midnight CDT tonight for ILZ003- ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013- ILZ019-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107- ILZ108.

Extreme Heat Warning until 7 PM CDT Friday for ILZ023-ILZ032- ILZ033-ILZ039.

IN...Extreme Heat Warning until midnight CDT tonight for INZ001- INZ002.

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

LM...None.