textproduct: Chicago

KEY MESSAGES

- Cooler daytime temperatures, afternoon showers, and chillier nights through the end of the week

- Patchy frost possible tonight and Friday morning, with the potential for widespread frost Saturday morning.

- Warmer and wetter conditions potentially late in the weekend.

DISCUSSION

Issued at 149 PM CDT Wed Apr 29 2026

A 500 mb long wave trough over the upper Midwest will become a closed low overnight over southern Ontario and gradually move east through Saturday. Embedded shortwaves within the upper level troughing will provide occasional opportunities for showers this afternoon through Friday. Cloud cover this afternoon has limited daytime heating, but with an area of convergence near the lake due to the onshore winds, spotty showers are possible through the early evening, but will diminish quickly after sunset. Temperatures will fall into the 30s overnight. If winds decouple enough along with the decreased cloud cover tonight, patchy frost is possible tomorrow morning.

The wave is expected descend southward within the upper level pattern on Thursday afternoon. While the entire forecast area has a slight chance of a shower tomorrow, hi-res models are suggesting a slightly more southerly track to the wave leaning higher chances south of I- 80 tomorrow. Persistent northerly winds will continue to filter in cooler air to the region. As skies clear out again and winds diminish, there is another chance for patchy front Friday morning.

Guidance suggests yet another wave to descend down through the pattern on Friday providing another chance for showers Friday afternoon. The coldest air is expected to arrive overnight for probably the best chance for widespread frost to develop Saturday morning.

Weak ridging moving in from the west should finally break the string of shower chances and allow for a drier weekend. As winds turn to the southwest, it should filter in a (relatively) warmer air mass. It will not be a heat wave, but should help bring temperatures back up a little and approach the low 60s by Sunday. However, the next trough is expected to descend out of Canada Sunday providing the next chance for rain. There is still model disagreement on timing and location. With better forcing and instability, there could be some thunder embedded with these showers, but most guidance has the better instability to the south. Warmer temperatures will stick around early next week with another storm system potentially midweek.

DK

AVIATION /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 701 PM CDT Wed Apr 29 2026

Aviation forecast concerns for the 00Z TAFs:

- Few lingering spotty SHRA early this evening, then another chance of SHRA Thursday afternoon/evening. Patchy MVFR cigs possible in precip.

- NNE winds at ORD/MDW/GYY become NW/WNW overnight into Thursday, then E-ENE again Thursday afternoon.

Shower coverage has decreased markedly since mid-afternoon, though a few spotty showers will remain possible across the terminals early this evening as a mid-level trough moves across the area. Have maintained a VCSH mention for this, though coverage may be on the light side. Patchy MVFR cigs were noted over/near Lake Michigan and into southeast WI at issuance time, though are mainly affecting GYY with ORD/MDW just west/south of more solid MVFR coverage. ORD/MDW are expected to remain primarily VFR with a gradual decrease in overall cloud cover overnight.

Surface winds will shift northwest at ORD/MDW/GYY later this evening and remain so through Thursday morning. Another lake- assisted wind shift to east-northeast is expected early-mid Thursday afternoon however. This will about coincide with increasing rain shower chances which will develop from west-east across the terminals in association with another approaching mid-level disturbance. Highest coverage of showers looks to be from mid-afternoon into early Thursday evening, then decreasing beyond that time. Patchy MVFR ceilings will be possible mainly in precipitation during that time.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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