textproduct: Chicago

KEY MESSAGES

- Fog possible tonight into early Thursday morning, with the next chance of showers and storms late Thursday morning into the afternoon.

- Our active, stormy weather pattern is expected to persist into the weekend with periodic chances of severe weather and heavy rainfall which could lead to flooding.

- A period of dry weather may finally arrive early next week.

DISCUSSION

Issued at 311 PM CDT Wed Aug 12 2026

A much quieter afternoon and evening is anticipated across the area following the stabilizing influences of this morning's round of showers and thunderstorms. Accordingly, we have continued the trend of lowering POPs through tonight, with dry conditions expected across most of the area. Our main concern for tonight is the threat for fog development, as a very moist low-level airmass nocturnally cools amidst light winds. Also cannot rule out the possibility for some of the fog to become locally dense into early Thursday morning, particularly in areas that become mostly clear.

On Thursday, forecast attention will be focused on the evolution of the next MCS expected to track eastward from the Plains out across the Corn Belt tonight. Ultimately, this MCS is expected to begin to cross the Mississippi River around, or a bit after daybreak on Thursday, though likely in a weakening state as the primary instability gradient over IL looks to remain south of I-80 Thursday morning. This may thus act to steer the stronger storm potential with the MCS farther south across central parts of IL later in the morning. In spite of this, high precipitable water values (~1.9") and deep warm cloud depths will support locally heavy rainfall with this remnant MCS.

Later Thursday, and especially Thursday night, a south- southwesterly low-level jet is expected to ramp up across the mid- Mississippi Valley and nose into IA and partitions of northern IL. Interestingly, this could result in regenerating showers and thunderstorms along a west-northwest to east- southeast corridor in the wake of Thursday MCS. Where this occurs, a flash flood threat would be a big concern Thursday night, especially given the storm motion to favor training cells over the same areas. At this time, there are signs that the best potential for several hours of training storms may remain southwest of the Chicago metro area, and potentially even southwest of our area altogether, but this will need to be monitored.

Unfortunately, medium-range guidance continues to depict our active pattern remaining in place and supporting continued waves of showers and thunderstorms across our region into the weekend. Fortunately, there does appear to be some decent agreement in the global models/ensembles that high pressure eventually settles across the western Lakes region by Monday-Tuesday of next week. Once this occurs, it will push our stalled frontal zone to the south and provide us with a much needed break in this active weather pattern.

KJB

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1252 PM CDT Wed Aug 12 2026

Key Messages:

- Threat of low CIGs and Fog late tonight into early Thursday morning.

- Next chance of showers and storms comes Thursday afternoon into Thursday night.

Aside from a small (<20% chance) for a few additional showers this afternoon, it appears we are done with precipitation through at least Thursday morning. The main aviation weather focus thus shifts to the threat for low CIGs and fog tonight into early Thursday morning. Overall, we appear to be in good setup for fog development through the evening, and especially overnight, particularly at RFD, DPA and GYY. The main change for the 18Z TAFs was to hit the VSBYs and CIGs a bit harder at these sites for a few hours into early Thursday morning. We have also added in a mention for lower CIGs and VSBYs at ORD and MDW late tonight. We certainly cannot rule out the possibility for lower conditions (IFR) developing over the main Chicago terminals overnight, though confidence with this is too low at this time.

The low CIGs and VSBYs will improve by mid morning on Thursday. Thereafter, we will be monitoring the evolution of the next wave of showers and thunderstorms expected to shift eastward across IA late tonight into early Thursday morning. The remnants of some of this activity could attempt to shift across the area terminals Thursday afternoon. This may thus result in our next period of showers and possible storms. I opted to omit the threat for TS from the TAF at this time, as the better threat for actual storms may remain south of the terminals. Otherwise, expect easterly surface winds through the day.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

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