textproduct: Chicago

KEY MESSAGES

- A couple periods of showers and storms are expected in the general region through tonight. Repeated downpours may lead to localized flash flooding.

- Stormy conditions may return this weekend into early next week.

DISCUSSION

Issued at 248 AM CDT Wed Aug 5 2026

Broad aggregate mid and upper-level troughing over the north central CONUS will steer a few smaller scale impulses across our area, resulting in a few clusters of showers and thunderstorms across the region today into tonight. One such wave is currently driving a line of heavy rain producing showers and storms eastward across northern IL. Fortunately, the eastward movement ~25 mph is limiting the duration of heavy rainfall at any given location, thus limiting the flash flood threat, at least in the near term. Nevertheless, a quick localized inch of rain may fall under the strongest cores.

Following this initial line of showers and storms, we will be monitoring the trends with the ongoing cluster of storms over IA. A good amount of available guidance suggests that this activity will track eastward into northwestern IL around daybreak. Thereafter, it remains a bit unclear as to how long it may hold together as it moves into northeastern IL (especially north of I-80) during the mid-morning hours, owing to the poor diurnal timing and modest instability. Nevertheless, the current thinking is that there will at least be a region of weakening showers moving into the Chicago metro area (mainly in the 8-11 am timeframe). Should this activity hold together into northeastern IL this morning, the presence of deep rich moisture (precipitable water values nearly to 2") will support locally heavy downpours.

After a break in the shower activity this afternoon, another round of developing convection is probable near the surface frontal boundary late this afternoon and evening in response to the approach of the next mid-level impulse. However, with diurnal destabilization expected to remain limited across far northern IL (especially north of I-88 where temperatures will stay in the 70s) as a result of persistent cloud cover through the afternoon, the main focus for stronger convection will likely end up south of I-80. This thinking continues to match the SPC's current Day 1 outlook, which indicates a level 1 of 5 severe threat into west central IL. The main threat with any stronger storms would be locally strong damaging wind gusts and locally heavy rainfall, particularly associated with any strong storm clusters. The overall threat of flash flooding will ultimately be dependent upon if and where multiple storms to move over the same areas (training). While this possibility cannot be ruled out on a localized basis in parts of our area, the better potential for this may end up remaining south- southwest of our area.

The focus for active weather should largely shift south of the area for Thursday and Friday. However, active weather (with shower and thunderstorm chances) is likely to return eighter this weekend, or early next week.

KJB

Weekend onward:

As aggregate upper-level troughing shifts eastward toward the end of the week, a quasi-zonal to northwesterly upper-level flow pattern will materialize across the Upper Great Lakes region supporting additional waves of showers and storms. Some of these may be rambunctious as the eastern edge of an EML plume is repeatedly re-advected eastward atop a stalled frontal boundary draped across the Great Lakes. Naturally, confidence in when/where eventual thunderstorm clusters will roam is low at this range. With that said, the pattern matches conceptual models for threats for severe weather and flash flooding in the general region this weekend into early next week.

Borchardt/KJB

AVIATION /06Z TAFS THROUGH 12Z THURSDAY/

Issued at 1236 AM CDT Wed Aug 5 2026

Forecast concerns include...

Thunderstorm chances through this evening. Wind directions tonight. Mvfr/ifr cigs early Thursday morning.

A broken line of thunderstorms has developed along a SQI/RFD/MKE line and appears to be developing in an instability axis ahead of a weak front. Confidence is low regarding how far east this precipitation and specifically thunder will persist. Given the high uncertainty, opted to include prob thunder mention for ORD/MDW over the next few hours and will need to monitor trends. Its possible this activity will weaken as it moves east and then new thunderstorm activity across parts of IA will spread across northwest IL toward daybreak and then across the Chicago terminals during the mid/late morning. The general trend from guidance is that this activity would weaken before/as it reaches the Chicago area. For now, made no changes to prob thunder for this time period. A lull in the precipitation is still expected from late morning through early/mid afternoon and then there will be another chance of thunderstorms form mid/late Wednesday afternoon through mid evening. Also no changes to current tempo thunder mention for this time period, though only medium confidence for coverage/timing/location. There may still be some showers or isolated thunder late Wednesday night into Thursday morning, but confidence is too low for anything but current vicinity shower mention.

Depending on precipitation coverage overnight into later this morning, prevailing mvfr cigs are possible across northwest IL and at RFD today. There will be a better chance for low mvfr cigs and possibly ifr cigs late tonight into Thursday morning for the entire area. There may also be some fog early Thursday morning.

Southerly winds under 10kts overnight will turn more southwest by daybreak and remain west/southwest Wednesday. A cold front will move across the area Wednesday evening and may stall near the the Chicago terminals. Wind speeds are expected to be light with low confidence on directions, possibly favoring a light northwest Wednesday evening and then a light northeast by daybreak Thursday morning. cms

LOT WATCHES/WARNINGS/ADVISORIES

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