textproduct: Chicago

KEY MESSAGES

- Scattered thunderstorms expected this afternoon and evening, a few may be severe with large hail and damaging winds.

- Another chance of showers and possibly a few thunderstorms during the day Friday, with another chance of showers and thunderstorms Saturday night perhaps into early Sunday morning.

- First heat wave of the summer expected early next week with the potential for highs in the 90s and peak afternoon heat indices over 100 degrees.

DISCUSSION

Issued at 146 PM CDT Wed Jun 24 2026

Shortwave trough over the western Great Lakes along with associated weak sfc front from northwest Wisconsin south into eastern Iowa should encourage scattered thunderstorm development this afternoon. These storms should move southeastward across our CWA late this afternoon into this evening. Jet streak is evident on water vapor imagery nosing into SE MN/SW WI early this afternoon. Guidance depicts a 40-50kt 500mb speed max spreading across southern WI and northern IL this afternoon and evening. This will result in a increase in deep layer shear into the 40kt range.

Rain associated with lead shortwave should continue to exit the area early this afternoon with at least partial clear already spreading into our far western CWA. This should allow for moderate instability to develop across the area with MLCAPES of 1000-1500 J/kg. The moderate instability coupled with fairly strong deep layer shear should support supercells with a potential for large hail and perhaps damaging winds. Weak mid level lapse rates may limit the magnitude of the hail/wind threat, but the long straight hodographs would support splitting supercells and a hail threat.

This front should clear the CWA late this evening bringing an end to the showers/storms by late evening. In the wake of the front, looks for dry weather Thursday and most of Thursday night. There is a signal in most guidance for another low amplitude shortwave to potentially becoming convectively enhanced before moving toward the mid Mississippi Valley Friday. Instability should be pushed well south of our CWA, so if this MCV takes a more northerly track it would likely be in a weakening state as it encounters more stable and drier low level air mass over our area Friday. Conversely, it is possible that the MCV may tend to propagate more southward toward the instability gradient. In collaboration with neighboring offices, lowered NBM pops for Friday some and kept slight thunder chances confined to areas south of the Kankakee/Illinois Rivers. Cloud cover should keep temps cooler Friday, but if it does rain then highs would need to be lowered even more, especially southern CWA where better rain chances should be.

Over the weekend, an upper level ridge is progged to begin developing over the middle and lower Mississippi Valley region. A warm front is progged to lift northward Saturday night into Sunday. Some threat of showers and thunderstorms will exist ahead of this front and ahead of the EML advecting eastward with the developing upper ridge. Medium range guidance is in good agreement in developing a fairly strong (595-596dm) 500mb ridge over the Mississippi Valley early next week.

Sunday, there's likely a small chance of showers/storms in the morning, then probably dry in the afternoon. With the warm front moving through the area, a weak lake breeze is possible along the IL shore perhaps offering a bit cooler conditions right along the lake. Elsewhere, highs should climb into the upper 80s to around 90 with higher dewpoints likely pushing afternoon heat indices well into the 90s to near 100.

Monday through Wednesday, current forecast has highs in the low to mid 90s with dewpoints in the 70s resulting in heat indices solidly over 100 degrees. Based on the current model guidance, these values are reasonable, but with the heart of the upper ridge progged to be centered south toward the lower Ohio and mid-lower Mississippi Valley it is possible that ring of fire convection currently progged to be mostly to our north could sneak farther south or send outflow south into parts of our area offering some reprieve from the heat. Harder to forecast that this far out, but something to watch as a possible spoiler to what otherwise looks like the first formidable heat wave of the summer.

- Izzi

AVIATION /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 635 PM CDT Wed Jun 24 2026

Forecast concerns include...

Scattered thunderstorms this evening. Wind shift/lake breeze to northeast Thursday evening.

A line of strong/severe thunderstorms will move across the Chicago terminals in the next 1-2 hours. Another line of thunderstorms across southwest WI and far northwest IL may move across the terminals during the mid/late evening. Have maintained prob thunder mention for this potential for now.

Prevailing southeast winds are expected to continue this evening and may become more easterly behind the first round of storms. Winds are expected to shift light westerly overnight, then west/ northwest to 10kt Thursday morning. A lake breeze/cold front is expected to shift winds to the northeast late Thursday afternoon into early Thursday evening for the Chicago terminals. cms

LOT WATCHES/WARNINGS/ADVISORIES

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