textproduct: Chicago

KEY MESSAGES

- Hot and humid conditions will continue through the holiday weekend with daily peak heat index values near 100 degrees.

- Waves of thunderstorms are expected Thursday evening through the weekend. With that said, there will be many dry hours.

DISCUSSION

Issued at 138 PM CDT Wed Jul 1 2026

Tonight and Thursday:

Regional GOES-19 water vapor imagery augmented by RAP analysis data imagery depicts a quintessential summertime pattern characterized by a large upper-level high pressure system (around 596dm) centered over the southeastern United States. Such a "heat dome" continues to provide hot and humid conditions in the general region with heat indices around 100 degrees in the local area at press time. Meanwhile, a recent hand surface analysis revealed an outflow-reinforced frontal boundary stretching from central Lower Michigan through central Wisconsin and through northwestern Iowa. The frontal boundary continues to serve as a highway for episodic clusters of showers and storms in the central and upper Great Lakes region, including a supercell that recently produced a 3 inch hailstone near Fond du Lac, Wisconsin. This is all to say, the weather outside is definitely living up to the standards of July.

Have been watching an area of clumping cumulus clouds across far northeastern Iowa early this afternoon for signs of initiation into thunderstorms as they approach the Illinois/Wisconsin state line. With an absence of large-scale forcing as well as around 100 J/kg of MLCIN noted on the 18Z DVN RAOB, do not currently expect thunderstorms to emerge out of the clumping cumulus clouds. However, if the off chance they do take off, they would be prone to impacting portions of northern Winnebago, Boone, and perhaps McHenry counties later this afternoon/evening with a localized threat for damaging winds, damaging hail, and flash flooding.

With that said, another round of thunderstorms is expected along the stalled frontal zone across northern Iowa and southern Minnesota along the nose of a modest southwesterly 850mb low- level jet. A somewhat uneven overlap of moisture-laden instability but marginal deep-layer shear will encourage thunderstorms to evolve into loosely organized outflow-driven clusters that tend to track into central Wisconsin through the night. Our area will hence be dry, warm, and muggy tonight with overnight lows generally in the mid to locally upper 70s.

If convective coverage is relatively high this evening and overnight across Wisconsin, amalgamation of repeated cold pools should lead to a southward-drifting region of outflow that would be prone to spilling into far northern Illinois or perhaps southwestward off Lake Michigan toward daybreak. Such an evolution is depicted by both the NAM3/HRDPS, which typically perform well in these sorts of regimes. Even if outflow does reach our area tomorrow morning, current thinking is that it would have minimal impact on hot and humid conditions with highs expected to soar back into the upper 80s to lower 90s with peak heat indices of 100 to 105F. Most daylight hours look dry.

Thursday evening into early Friday morning:

By tomorrow evening, the upper-level heat dome will begin to "flatten" as the frontal boundary stalls from central Iowa toward the Illinois and Wisconsin border. CAM-based guidance is fairly consistent in showing an upper-level shortwave riding the top of the heat dome into the Lower Great Lakes during the evening hours leading to the development of storms along the frontal boundary. With forecast soundings depicting minimal capping by early afternoon, suppose a thunderstorm or two may develop as early as 4 PM. However, coverage should increase markedly with the shortwave itself favoring after 7 or 8 PM. With the trajectory of the shortwave favoring southern Wisconsin, storms should remain largely north of Interstate 88. However, if there is a high coverage of storms, can certainly envision the southward spill of outflow-driven storms toward or even further south of I-80 by daybreak Friday. Any storm may produce locally damaging winds or flash flooding tomorrow evening and night, again favoring areas north of I-88.

Friday through the weekend:

Ensemble model guidance remains in agreement that the upper- level heat dome will break down Friday and transition toward somewhat weak but quasi-zonal upper-level flow through the weekend. With 850mb temperatures remaining somewhat stagnant and near +20C, will go with a persistence forecast for both heat and humidity levels with daily peak heat index values near 100 degrees. Subtle upper-level shortwaves embedded within the quasi-zonal flow (if not just afternoon heating of the boundary layer) should support episodes of thunderstorms in the area. In this kind of pattern, any storm can be prone to producing damaging winds and flash flooding, though moreso on a localized basis than widespread. With all of that said, many hours should be dry.

We are very cognizant of the large number of outdoor events and activities scheduled for the upcoming holiday weekend with heightened attention on the forecast for the timing and coverage of thunderstorms. This type of pattern is one where confidence in timing and location of storms comes much more into focus about 12 to 24 hours beforehand. So, stay tuned to the forecast.

A note about heat headlines:

Yesterday was definitely the most oppressive day of the ongoing heat wave. While forecasted heat index and HeatRisk values this afternoon through the upcoming holiday weekend outside of Cook County are forecasted to fall well short of local Extreme Heat Warning criteria (four or more consecutive days with peak heat index >105F and/or widespread HeatRisk level of 4 out of 4), will maintain the product across the entire area for the sake of a consistent message with neighboring forecast offices.

In terms of any extension in heat headlines beyond Thursday, am not sure we will meet local criteria even for a Heat Advisory given forecast heat indices are expected to remain below 105F. With that said, there is something to be said about the cumulative effect of the heat (as Friday would be day 5 of heat index values near or above 100F), so future shifts may need to consider a Heat Advisory anyway for Friday or Saturday regardless barring, of course, the cooling effects of thunderstorms.

Borchardt

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1232 PM CDT Wed Jul 1 2026

Main Concern:

- Potential for weakening SHRA/TS nearby to drive an outflow associated wind shift across the area early Thursday morning

VFR conditions are expected through the period, with the small possible exception of any precip related VSBY effects. Southwesterly wind gusts are expected to increase into the 25-30 kt range (at least occasionally) late this afternoon into early this evening, then likely ease with or just after sunset. There may be sporadic gusts near 20 kt, especially at ORD and MDW.

The area to watch late tonight is from far NE Iowa across to central and southern Wisconsin. Exact coverage of TS in that area will likely contribute to potential for an outflow boundary to push across the area and result in a temporary wind shift (NW to NE to SE before returning to SW). Also can't rule out SHRA and a few isolated TS making it into northern IL (~20% chance). Due to uncertainty, confidence was not high enough for any formal mention of a wind shift and precip in the TAFs. The next decent chance for TS at the TAF sites is looking to be Thursday evening, beyond the current ORD/MDW 30-hour TAF period.

Castro

CLIMATE

Issued at 1246 AM CDT Wed Jul 1 2026

.Daily Records for the Upcoming Week........

Chicago (KORD) Record High Record Warm Minimum July 1 103 (1956) 78 (1931) July 2 99 (1970) 79 (1911) July 3 102 (1949) 78 (1911) July 4 102 (2012) 80 (1921)

Rockford (KRFD) Record High Record Warm Minimum July 1 101 (1931) 76 (1970) July 2 99 (1910) 74 (1970) July 3 99 (1925) 75 (2012) July 4 102 (2012) 77 (1977)

NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES

IL...Extreme Heat Warning until midnight CDT Thursday night for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Extreme Heat Warning until midnight CDT /1 AM EDT/ Thursday night for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.