textproduct: Chicago

KEY MESSAGES

- Dangerous heat wave will likely continue into Thursday for most if not all of the area with daytimes highs in the 90s and peak afternoon heat indices of 100 to 110 degrees. Confidence across far northern IL is lower Thursday, due to potential effect of storms. - Thunderstorm chances will begin to increase late in the week, likely bringing some heat relief for at least some areas.

- While likely not a complete wash out, there will be periodic chances for thunderstorm through the holiday weekend.

DISCUSSION

Issued at 202 AM CDT Tue Jun 30 2026

Have elected to upgrade current advisory to an Extreme Heat Warning, and extended headlines through Thursday based on the potential for a fourth day of hot and humid conditions for much of the area.

An upper level ridge, with H5 heights analyzed ~5960 meters, was centered just south of the Ohio Valley across Kentucky and Tennessee. This ridge is forecast to maintain its strength across the area through at least Wednesday, before gradually elongating west to east into the mid-Atlantic region late in the week. Northwest of the ridge axis, low and mid-level temperatures are progged to warm slightly today, with 925mb temps in the +26/27C range and 700mb temps around +13C. This will support surface temps a bit warmer than on Monday, with mid-90s in many locations. Forecast soundings depict a subsidence inversion in the 800-850mb range, limiting the mixing depth despite otherwise breezy southwest low-level flow. This, likely in combination with increasing crop evapotranspiration, resulted in dew points somewhat higher than expected Monday, especially across the southern half of the forecast area where some upper 70s/80 degree dew points were observed even into the late afternoon hours. Expecting similar trends again today, supporting peak afternoon heat indices in the 105-110 range. Latest guidance continues to indicate that mixing may be deeper on Wednesday, potentially lowering dew points but also allowing slightly warmer air temperatures - which would maintain similar peak heat indices.

Most guidance then begins to shift the center of the upper ridge off to the east across the Appalachians and mid-Atlantic region by Thursday. Slight cooling of the mid-levels especially into northwest and far northern IL and gradual southeastward shift of the upper jet and storm track does suggest an increasing threat of convection slipping into the northern parts of the forecast area, especially by later Thursday afternoon and evening. However, a good portion of the forecast area (or perhaps the entire area if convection doesn't materialize until later or not at all) will likely see a fourth day of highs in the low-mid 90s, mid-70s dew points and peak heat indices around 105. Based on this, and in collaboration with neighboring WFOs will upgrade current advisory to an Excessive Heat Warning and will extend all headlines through Thursday evening.

Global ensemble guidance continues to indicate that the upper ridge will continue to deamplify Friday into the Independence Day holiday weekend. Unfortunately, this results in what looks to be a more active storm pattern for our region, as a zonal jet pattern develops across the northern CONUS with indications of several low-amplitude short waves transiting the flow. While many dry hours are likely, at least periodic thunderstorm potential will exist for the area through the weekend.

Ratzer

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 1230 PM CDT Tue Jun 30 2026

Forecast concerns include...

Gusty southwest winds this afternoon and again Wednesday. Possible mvfr level clouds Wednesday morning.

A high mvfr/low vfr bkn/ovc deck has developed across northern IL late this morning and will continue to slowly lift to vfr shortly, and is then expected to scatter out through the mid/late afternoon. Another period of mvfr level clouds is possible Wednesday morning, similar to the past two mornings. Confidence is low and for now have opted to include scattered mention for this possibility.

Southwest winds will remain gusty this afternoon but the highest gusts appear most likely during the mid/late afternoon and then speeds/gusts will diminish with sunset though periodic gusts will remain possible for the Chicago terminals tonight. Gusts will increase back into the lower/mid 20kt range Wednesday morning. cms

CLIMATE

Issued at 1249 AM CDT Tue Jun 30 2026

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

Chicago (KORD) Record High Record Warm Minimum June 30 99 (1953) 80 (2018) 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 June 30 102 (1931) 76 (2018) 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...Small Craft Advisory until 9 PM CDT this evening for Winthrop Harbor IL to Gary IN.