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 /06Z TAFS THROUGH 12Z SUNDAY/

Issued at 1233 AM CDT Sat Jun 27 2026

Aviation forecast concerns for the 06Z TAFs:

- Some potential for lowering of VFR ceilings to MVFR Saturday, with the greatest likelihood at KGYY. Otherwise, VFR conditions expected to prevail.

A west-east oriented, nearly stationary frontal boundary remains in place to the south of the terminals from central MO into southern IL/IN early this morning. Aloft, a series of low- amplitude mid-level disturbances continue to ripple eastward along the elevated baroclinic zone north of the surface front. The main effect on weather across the terminals is to maintain persistent mainly VFR cloud layers in the 4-7 kft range, though some lower MVFR/IFR bases have been noted along the Lake Michigan shore and north of KORD where some weak but persistent sprinkles/drizzle has been occurring since last evening. Persistent warm/moist advection aloft is produce a gradual northward shift in the lower VFR deck, resulting in a gradual lowering of VFR cigs across the terminals. Can't rule out patchy MVFR cigs developing as far north as KORD during the day, though the greatest likelihood of MVFR ceilings appears to be at KGYY and locations west and south. Lack of any significant precipitation suggests locations farther north (KRFD/KDPA/KORD and KMDW) are more likely to remain VFR. High-res guidance indicates VFR ceilings will persist Saturday night, though may rise into the ~5 kft range or even scatter out at times.

Surface winds are expected to remain modest (6-11 kts) from the east-northeast to northeast through the period.

Ratzer

...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.