textproduct: Chicago

KEY MESSAGES

- A cold front will move across the area tomorrow accompanied by isolated to scattered showers (20-40% chance at any given spot). A lightning strike cannot be ruled out (10-15% chance).

- Periods of showers and a few storms are favored Friday night through early Saturday afternoon (>60% chance areawide). Coverage of showers may taper in time for outdoor Halloween activities.

- Generally mild conditions will continue into the first week of November before a pattern change leads to more showery and cooler conditions by the first weekend of the month.

DISCUSSION

Issued at 133 PM CDT Wed Oct 28 2026

Early afternoon water vapor imagery reveals a vigorous upper- level short-wavelength trough propagating northeastward across Minnesota toward Lake Superior. A parent surface low is located in western Ontario with secondary surface low development underway near the Twin Cities along a trailing cold front. A plume of modestly steep mid-level lapse rates (sampled near 7.5 K/km per the 18Z DVN RAOB) extends ahead of the front across the middle Mississippi River Valley. Meanwhile, closer to home, it is another beautiful late October day with temperatures in the mid 70s, mostly sunny skies, and a southwesterly breeze.

This afternoon and evening, coverage of showers and thunderstorms will increase from central Iowa into central Minnesota as the secondary low deepens and cold front shifts southeastward. Isentropic ascent ahead of the front will encourage a few showers or storms to develop as far southeast as northern Illinois after sunset and through the overnight hours, though a resident dry low-level airmass should render coverage isolated at best (20 to 30% chance, especially north of I-80). Overnight lows will be unusually mild for the time of year and in the mid to upper 50s.

The cold front is expected to cross southward into Illinois after sunrise accompanied by a band of low to mid-level clouds. Suppose a few showers may accompany the front north of I-88 during the morning hours though minimal instability suggests otherwise. With that said, can easily envision renewed shower development (diurnally-driven) along the front by mid-afternoon somewhere near the I-88 or I-80 corridors (30 to 50% chance). Forecast soundings do not look particularly impressive from a thermodynamic perspective, but will tuck in an isolated thunder chance (10-15%) just to be safe as the front progresses southeastward through the afternoon. Temperatures tomorrow will be somewhat regulated by the position of the frontal boundary with highs in the lower 70s near US-24 to generally the low to mid 60s elsewhere.

A surface high pressure system will quickly build into the region Thursday evening setting the stage for ideal conditions for radiational cooling (slackening winds, clearing skies, dry airmass). Will undercut NBM guidance and feature overnight lows generally in the upper 30s to lower 40s, but will forgo mention of frost for now (would be near the Wisconsin state line should it occur). A relatively cool start to the day, increasing mid- level cloud cover, and easterly flow off Lake Michigan will hold temperatures on Friday to the lower 60s north of I-80 to around 70 near US-24.

Attention then turns toward a conglomerated upper-level wave emanating from northern Mexico due to arrive to the Lower Great Lakes this weekend. Am noting a trend toward stronger isentropic ascent ahead of the wave Friday night which would support a healthy coverage of showers and storms from eastern Iowa into northern Illinois, though will note quite a bit of guidance that isn't as excited. Nevertheless, do anticipate at least scattered activity moving through the area through Saturday morning with a signal for gradually waning coverage through the afternoon hours (hopefully in time for outdoor Halloween activities). Temperatures should cool toward the lower 50s after sunset.

Looking toward next week, ensemble model guidance depicts a storm system or two moving through the central CONUS followed by a pattern change toward more showery and cooler conditions.

Borchardt

AVIATION /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 640 PM CDT Wed Oct 28 2026

Forecast concerns include...

Chance of showers tonight through midday Thursday. Mvfr cigs Thursday morning. Low level wind shear overnight. Wind shift to northwest with a cold front Thursday morning.

Some sprinkles and isolated showers will be possible across northwest IL this evening but there will be a better chance of showers overnight through midday Thursday as a cold front moves across the area. Current prob mention has this potential handled well. There may be a gap between a period of showers across northern IL in the morning and then new development of showers mainly south of the terminals midday into Thursday afternoon. There is also the potential for a few isolated thunderstorms, but confidence for thunder is too low for any mention with this forecast.

South/southwest winds around 10kt will continue this evening, with a few higher gusts at times. Directions will turn more southwest toward daybreak, then shift to the northwest as the cold front moves across the area Thursday. Low level winds will steadily increase this evening with low level wind shear expected overnight and through daybreak Thursday morning.

A period of mvfr cigs is expected mainly from daybreak through late morning associated with the cold front. The mvfr cigs may become prevailing for at least a few hours before scattering out by early/mid Thursday afternoon. cms

LOT WATCHES/WARNINGS/ADVISORIES

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