textproduct: Chicago

KEY MESSAGES

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

DISCUSSION

Issued at 252 AM CDT Sat Jun 27 2026

Spotty light showers persist across northern Illinois early this morning with a lingering low to mid-level deformation axis wringing out a remaining layer of modest moisture. Expectations are for remaining showers to erode by mid morning as dry air advection increases over the area and the deformation axis exits to the east. For areas south of the Kankakee/Illinois River, the northern extent of a very moist airmass (PWATS 2+ or higher) combined with limited low-level capping and diurnal influences should yield isolated shallow showers with efficient warm rain processes to develop this afternoon. Expect highs today to range from the upper 70s inland to the upper 60s along the immediate shore.

Steepening mid-level lapse rates above an incoming EML on Sunday will provide at least a small chance for a NW to SE oriented axis of convection to develop toward the Mississippi River late tonight. However, limited moisture near the base of the incoming EML and the lack of a decent LLJ directed toward the area imply that any upstream convection will struggle to continue with northeast extent over the area by Sunday morning. Otherwise, will also need to monitor the track for a potential MCS developing over South Dakota late this evening as the notable instability gradient (albeit driven by the incoming EML) will be focused across northern Illinois Sunday afternoon.

Heat and humidity will begin to build with heat index values rising into the lower 90s inland during the afternoon, with persistent onshore flow maintaining much cooler conditions along the shore.

Focus remains on the potential for an extended period of dangerous heat through most or all of next week as an anomalously strong mid-level ridge (heights very near 600dam) drifting along the Ohio River Valley. H850/H700 temps of over 25C/12C Monday through Wednesday will yield unseasonably warm conditions with daytime highs well into the 90s while building an incredibly strong cap over the local area. This should deflect convection well to the northwest (Minnesota through Upper Michigan) through at least Wednesday. Combined with persistent southwest low-level flow, there looks to be no relief from lake breezes or upstream convective outflow boundaries during this time. Beyond that point, cooler temps aloft may allow more robust convective complexes to near from the north and provide some relief for northern portions of the area. If this does not occur, similar heat will likely continue heading into the Independence Day weekend.

For Cook County, current forecast conditions support the eventual need for an Extreme Heat Watch/Warning for a longer duration event with afternoon heat index values over 100 degrees for three days or more in conjunction with nighttime lows likely struggling to fall much below 80 in the core of the metro.

For areas outside of Cook County, it appears likely that at least a Heat Advisory (Heat Index of 105+) will be required for some days or every day from Monday through Thursday. Whether conditions meet Extreme Heat Warning criteria (110+) are less clear and will hinge heavily on whether dew points rise into the upper 70s versus low to mid 70s.

Kluber

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 626 AM CDT Sat Jun 27 2026

Aviation forecast concerns for the 12Z TAFs:

- Some potential for MVFR or low-end VFR ceilings across the terminals today, with the greatest likelihood at KGYY.

A weak surface low pressure was noted near KSTL this morning, and will track east along a nearly stationary frontal boundary stretching west to east across southern IL/IN. Relatively moist low-levels north of the front was supporting a variety of cloud layers from VLIFR across central IL to 3500-6000 foot VFR bases farther north across our terminals. Can't rule out some patchy MVFR ceilings developing this morning with diurnal warming, with the greatest potential mainly south and southwest of the Chicago area. KGYY is likely most susceptible to MVFR development, with decreasing probabilities for terminals to the north. Onshore flow off of Lake Michigan may result in scattering of lower clouds this afternoon, though various guidance remains in good agreement in VFR ceilings in the 4000-5000 foot range redeveloping tonight before eventually scattering out more completely Sunday morning.

Surface winds will remain east-northeast through most of the forecast period with speeds around 10 kt today. Winds will eventually turn more east-southeast Sunday morning.

Ratzer

CLIMATE

Issued at 252 AM CDT Sat Jun 27 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...None. IN...None. LM...None.