textproduct: Chicago

KEY MESSAGES

- Early morning fog to dissipate quickly after sunrise.

- Next chance of rain (and maybe a few thunderstorms) comes during the day Friday, with another chance of showers and thunderstorms Saturday night 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 250 AM CDT Thu Jun 25 2026

A surface cold front will settle south of our area today. While primarily dry weather is anticipated across the area in the wake of this boundary, another mid-level impulse across the eastern Dakotas early this morning is progged to slide eastward across southern WI this afternoon. As it does, some guidance is insistent that some isolated shower/storm activity will develop into this afternoon, potentially focusing along the lake breeze boundary. It appears the best potential for this activity will largely be to our north in WI. However, given the close proximately, we could see a few such showers developing as far south as far northeastern IL. Accordingly, we have opted to maintain a low (20%) chance for a couple of isolated showers, mainly across Lake County IL this afternoon. Otherwise, following some early morning fog, expect partly to mostly sunny conditions today with inland temperatures topping out in the upper 70s to near 80. Closer to the lake, an afternoon lake breeze will keep conditions a few degrees cooler.

Late tonight into Friday, forecast attention will be on the next wave of surface low pressure expected to track eastward from Plains out across the Mid-Mississippi Valley region. Model and ensemble guidance continue to depict a decent amount of spread with the strengthen, exact timing, and track of this system. This appears to be tied to the degree of convective enhancement that occurs to the parent mid-level short wave later today into tonight across the Plains. Accordingly, this does lend lower confidence with the extent of rainfall that falls across northern IL on Friday. Nevertheless, we continue to advertise 20-30% chances of rain north of I-80 Friday, with 60-70%+ chances south. Regardless of the extent of rain that falls, the threat for thunderstorms will remain low in our area, owing to the track of the system keeping the unstable airmass well south of our area.

The large scale upper-level pattern will amplify through the weekend as large upper-level trough digs into the western CONUS. As this occurs, heat and humidity will be on the increase across the central CONUS as a large subtropical upper-level ridge amplifies over the mid-Mississppi Valley and lower Great Lakes regions into early next week. As this pattern change unfolds, a surface warm front will shift northeast into the region Saturday night into Sunday. This may foster a period of showers and thunderstorms Saturday night into Sunday morning along the leading edge of the EML advecting into the region under the building upper ridge. With conditions drying out into Sunday afternoon, we should really begin to notice the building heat and humidity, particularly inland from the lake, where a lingering onshore southeast wind looks to keep things cooler through the day. Inland temperatures are currently forecast to top out in the upper 80s, but with dewpoints potentially climbing into the low 70s, heat indices will make it feel like its in the mid and upper 90s. Lakeshore areas in northeast IL should remain cooler (upper 70s/low 80s) through Sunday.

The low-level flow looks to turn offshore by Monday, so hot and humid conditions are expected to impact all areas (including the lakeshore areas). Persistent upper-level ridging across the region is forecast to keep temperatures in the 90s through at least mid-week. Combined with high dewpoints (in the 70s), heat indices are likely to be solidly above the 100 degree mark. 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. It is too soon to tell if and when this may occur, but it something that will need to be monitored.

KJB

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 645 AM CDT Thu Jun 25 2026

Key Messages:

- A lake breeze will result in a northeasterly or easterly wind shift this afternoon or evening.

Any lingering mist and low clouds from last night should dissipate by 13Z or so this morning, allowing for VFR conditions to prevail today. A lake breeze will develop and eventually push inland during the afternoon and evening, first resulting in a northeasterly wind shift at GYY around or a little after midday, followed by northeasterly or easterly wind shifts at the remaining Chicago metro terminals in the hours that follow. Ample low-level moisture in the marine-influenced air mass behind the lake breeze could result in another period of low clouds and/or fog tonight, but confidence in that occurring remains fairly low at this time.

Couldn't rule out a stray shower or two popping along the lake breeze near ORD, MDW, or GYY this afternoon, but the better chances for that should remain north of the Illinois-Wisconsin state line. Additionally, there will be another chance for rain towards the end of the current 30-hour TAF period for ORD and MDW as another weather system approaches the area from the southwest tomorrow morning. However, there is still a fair amount of disagreement across the 00Z/06Z model guidance suites in how far north this rain will get, so opted to keep the ORD and MDW TAFs dry for the time being.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

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