textproduct: Chicago

KEY MESSAGES

- Showers and a chance for thunderstorms this afternoon/early evening

- Dangerous heat is expected across the area this week with afternoon heat index values forecasted over 100 degrees. Little to no relief is expected at the lakeshore or at night.

DISCUSSION

Issued at 125 PM CDT Sun Jun 28 2026

A complex of showers and thunderstorms is progressing eastward through northeast Iowa at the time the discussion was published. The current expectation is for it to follow a northwest to southeast instability gradient across northern Illinois. But as the upper level ridge pattern is amplifying and advecting in warmer temperatures in the mid levels, there is lower confidence in thunderstorms being able to maintain their strength. Perhaps there is lower confidence in the eastward extent of the lightning activity, but currently projecting the system to decay as it approaches the Fox Valley with only isolated embedded thunderstorms. Localized small hail is still possible, mainly west of the Fox Valley, due to over 40 knots of effective shear available. Otherwise, gusty winds are possible with any shower or thunderstorm. However, as this complex of showers passes east, an area of localized low pressure is expected to develop in its wake. Southeast wind gusts unrelated to showers and thunderstorms have the potential to increase late this afternoon/early evening for an hour or two up to 40 mph.

As an anomalously deep low continues to sit over the Intermountain West, another embedded impulse is expected to move northeastward into Wisconsin. This may trigger some scattered light showers or sprinkles around daybreak, mainly for areas closer to the Wisconsin stateline and the Lake Michigan shoreline. But any moisture is going to have to contend with the growing dry air mass, preventing any formal mention of precip in the PoPs forecast. More intriguing is that high-res guidance is now suggesting an MCS could develop tomorrow off this impulse in northern Wisconsin and head toward Michigan. If an MCS were to occur, there is a chance that some outflow could race southwestward toward are area, but the current guidance is that the MCS tack would be far enough east to not impact our area.

Otherwise, the main story remains the heat wave. An amplified upper level ridge over the area will set up prevailing southwest winds, a drier air mass, and higher surface pressure. The southwest flow will increase 850 mb temperatures north of 20C and pull 70+ dewpoints across the forecast area. Hot and humid conditions are forecast starting tomorrow through at least Wednesday. Based on current guidance, heat indices are expected up to around 105F. There's fairly good confidence that 110F (the criteria for an Extreme Heat Warning) will not be reached. Therefore, no changes were presently made to the Heat Advisory, though cannot rule out upgrading it to a Warning IF the heat is expected to continue through Thursday (more on that in a minute). Otherwise, the Extreme Heat Watch for Cook County (with its lower heat criteria) was converted to an Extreme Heat Warning given the confidence in the heat for Monday through Wednesday.

All the uncertainty on the duration of the heat wave focuses on the strength and location of the ridge over the eastern CONUS. Models have been bouncing around a lot over the last several runs. The stronger the ridge late in the week, the better chances for higher heat indices to remain in place and the potential for that upgrade from the Advisory to a Warning. However, a weaker ridge and/or a drift farther east would lead open the door for convective systems to pass closer to the area which could help give "some" heat relief. Given the uncertainty and model disagreement, felt no need to change any of the chance PoPs in the extended forecast though it is accompanied with lower confidence. One last note, despite the model disagreement on the ridge placement both the GEFS and Euro ensembles are keeping 850 mb around 20C through the end of the week and weekend. So even if heat index values were to fall at the end of the week below an Advisory or Warning criteria, they could still remain well into the 90s through the holiday weekend. Welcome to summer, everyone!

DK

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 1256 PM CDT Sun Jun 28 2026

Key messages for the 18Z TAF period:

* A decaying system of showers and thunderstorms moving across northern IL this afternoon. Greatest thunder chances at RFD.

* A period of gusty SE winds expected behind the showers late this afternoon.

* MVFR cigs into this afternoon, possibly staying just outside of ORD. Low confidence on return to VFR.

A decaying system of showers and thunderstorms is working into far northwest IL early this afternoon. It will continue weaken on its way into Chicagoland, but could bring some scattered showers for the mid-late afternoon period. Embedded thunderstorms are more likely over RFD, but a storm cannot be ruled out into Chicagoland. A brief period of gusty SE winds can be expected behind these showers as a result of a wake low.

MVFR cigs over the terminals have managed to stay just outside of ORD so far and appear to be lifting toward and above 3kft as of late. MVFR may persist as late as mid-afternoon, although exact timing on a return to VFR is uncertain.

Meanwhile, ESE winds will periodically gust to around 20 kt this afternoon outside of the showers. They'll stay up near 10 kt overnight and veer to SSW by early Monday before increasing to over 20 kt for the late morning and afternoon.

Doom

CLIMATE

Issued at 252 AM CDT Sun Jun 28 2026

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

Chicago (KORD) Record High Record Warm Minimum June 29 97 (1954) 80 (1931) 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 29 103 (1931) 75 (1931) 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...Heat Advisory from noon Monday to 10 PM CDT Wednesday for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ106-ILZ107-ILZ108.

Extreme Heat Warning from noon Monday to 10 PM CDT Wednesday for ILZ103-ILZ104-ILZ105.

IN...Heat Advisory from noon CDT /1 PM EDT/ Monday to 10 PM CDT /11 PM EDT/ Wednesday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.