textproduct: Chicago

KEY MESSAGES

- A few thunderstorms are possible Saturday night into Sunday morning, mainly west of the Fox Valley.

- Dangerous heat will build across the area next week, with afternoon heat index values over 100 degrees each day beginning Monday.

DISCUSSION

Issued at 205 PM CDT Fri Jun 26 2026

Ongoing showers will continue through the evening and overnight. Most of the rain thus far has been fairly minimal. Light rain can be expected as far north as Interstate 90, but the better coverage will remain closer to the higher moisture plume south of Interstate 80 with localized heavier downpours possible along and south of US 24. The better instability remains well to the south and out west near the area of surface low pressure. Thus, pulled the chances for thunder for the rest of today for much of the area, with any lingering (less than 24 percent) chance around US 24.

The surface low, currently in western Missouri, will slowly move northeast and pass just south of the forecast area overnight. The models slowing the progression of the low gave confidence in maintaining chance PoPs for areas southeast of I-57 Saturday morning, though overall drier conditions are expected tomorrow. Winds will remain out of the northeast through the day on Saturday. High temperatures are expected in the 70s to low 80s inland with cooler upper 60s to low 70s along the lakeshore.

While Saturday is looking to shape up as a nice day locally, the overall atmospheric pattern will be changing tomorrow. An upper level low out over the Rocky Mountains will continue to deepen and sink farther south over the western CONUS. Subsequently, an upper level ridge will amplify northward from the Gulf and deepen. This amplified pattern will set up broad southwest flow over the Plains and Mississippi River Valley. A weak wave is expected to eject out of the upper level low and move toward Wisconsin. Decent mid-level lapse rates could support thunder and the presence of a slug of moisture with warm air advection and isentropic upglide could support showers and a few thunderstorms in the overnight hours. However, this will be moving in toward a drier and less favorable air mass, and models are already responding giving confidence to lower PoPs. Perhaps a few showers fester into Sunday morning, but generally drier (and more humid conditions) are expected during the day.

More concerning, starting on Sunday, is the potential for dangerous heat next week. A strong upper level ridge will sit east of the Mississippi River through the week. Southwest flow will continue to advect in warmer air as 850 mb temps increase over the area from 15 to 20C on Sunday to around 25C on Tuesday (the current expected highest temperature forecast of the stretch). Temperatures are expected to climb into the 90s for most of the forecast area, and with overnight lows only in the mid to upper 70s, little relief can be expected at night. Additionally, the southwest winds will bring the afternoon temperatures all the way to the traditionally cooler lakeshore. Dew points climbing into the 70s will make for humid conditions and drive heat index values up to around 105 F.

Setting a record high temperatures is not a guarantee (especially for any records that are 100+, like much of KRFD), but a Climate section has been created below for interested individuals. Two big questions for this heat event will focus on the center of the upper level high embedded in the ridge: 1.) where will its location be? And 2.) how strong will it remain? Monday and Tuesday look to be a good chance for these hot temperatures to set up, but there is greater uncertainty for mid-week and after. If the high were to weaken or even set up along and south of the Ohio River Valley, there could be a convective complex that could ride over the top of the ridge and bring not just showers and storm chances to the area, but also some relief from the temperatures. So no changes were made to the splotchy chance PoPs that the NBM provided for the middle and end of the week. However, recent models have trended towards a slightly stronger high. If that were to materialize, the region could be set up for not just drier conditions, but an extended period of heat indices over 100F. It's still far too early for any headline (e.g. Watch, Warning, Advisory), but there is growing confidence in the first heat wave of the year leading into the holiday weekend.

DK

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 659 PM CDT Fri Jun 26 2026

While remaining mainly non-impactful, light showers/sprinkles continue to redevelop along the I-88 corridor early this evening. Have pushed the end time of the VCSH mention at the Chicago area terminals to 3Z to account for this. The overall trend should still be toward decreasing coverage over the next few hours. Hi-res guidance does hint at the potential for spotty light showers or sprinkles lingering across portions of the area overnight into the day on Saturday, though this is more likely to remain virga and have accordingly left the TAFs dry.

While VFR conditions are currently forecast through the period we will be monitoring a couple areas of LIFR to IFR stratus hugging the lakeshore that may attempt to ooze inland later this evening. The current expectation is that it will struggle to hold together with inland extent and will handle any updates tactically based on observational trends. Any MVFR stratus associated with the system drifting across central IL/IN could also briefly reach the terminals during the day on Saturday though the general trend has been toward it remaining mainly south of I-80.

Winds will remain northeast to east through the period around 5-10 kt.

Petr

CLIMATE

Issued at 143 PM CDT Fri Jun 26 2026

....Daily Record High Temperatures........

Chicago (KORD) Rockford (KRFD) June 28 101F (1951) 103F (1934) June 29 97F (1954) 103F (1931) June 30 99F (1913) 102F (1954) July 1 103F (1956) 101F (1931) July 2 99F (1970) 99F (1910)

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.