textproduct: Chicago

KEY MESSAGES

- Showers and possibly a few isolated thunderstorms this morning, shower chances may linger near/south of the Highway 24 corridor this afternoon into tonight

- Dry weather with seasonable temperatures expected to end the work week.

DISCUSSION

Issued at 533 AM CDT Wed Sep 9 2026

Vigorous upper level trough noted on water vapor imagery over the northern Plains early morning with an attendant upper level jet extending from WY to Lake Superior. Evening ROABs sampled a nearly 100kt jet in this area. Convection continues to develop over IA early this morning, in the broad, divergent right entrance region of this jet. Objective analysis data depicts a reservoir of 1000+ J/kg of MUCAPE across IA with a 40+ kt low level jet aimed right into IA. As this convection develops and pushes eastward into northern IL cloud tops have been rapidly warming and convection diminishing as it moves east of the nose of the LLJ and encounters a substantially more stable air mass.

A pair of small MCVs are evident on regional WSR-88d mosaic, one moving into NW IL just after 07z and a second one over central IA. The area of rain associated with that lead MCV is likely to continue weakening and probably gradually dry up as it reaches lake Michigan. However, this lead MCV will likely increase moisture in the low-mid levels of the atmosphere and should allow for the second MCV to maintain itself better as it moves into and across northern IL later this morning. Still anticipate minimal instability, but a second wave of rain and perhaps a couple of isolated thunderstorms (especially western CWA) should move across northern IL and extreme northwest IN in the 11-15z time frame.

While the second MCV and its associated rain shield should tend to weaken later this morning, remnant cloud cover and footprint of rain cooled air will likely result in the effective frontal boundary being pushed into our southern CWA by this afternoon. Cannot rule out some showers continuing into our southern CWA later this morning into the early afternoon, but coverage is expected to be on the decrease. Renewed convective development this afternoon appears unlikely across IL given the lack of forcing and weak instability.

Another shortwave over the northern Rockies is expected to shear rapidly eastward into the lower Missouri River Valley later tonight, likely resulting in an uptick in coverage of showers and thunderstorms over Missouri late tonight. As this activity pushes east into Illinois, it is possible that the northern flanks of the showers could scrape across our far southern CWA late tonight into Thursday morning, but the vast majority of this rain and all of the associated thunderstorm activity should stay south of our area.

Temperatures today will be heavily dictated by cloud cover with highs potentially topping out in the lower 80s far southern CWA prior to thicker cloud cover and potential showers. Also, far northwest CWA could see highs in the lower 80s assuming some partial clearing this afternoon. Over the majority of the CWA, highs will probably hold in the 70s given the cloud cover.

Other than the small chance of showers near/south of Highway 24 in the morning, Thursday looks to be a pretty nice day weather- wise across the area. Skies are expected to clear out, lower dewpoints will advect in, and highs will top out in the 70s. By Thursday night, the center of the high pressure ridge should set up over the area. This should set the stage for nearly ideal radiational cooling with clear skies and calm winds. Have nudge low temps downward Thursday night, especially in the typical cool spots where lows could dip into the 40s. The urban heat island should keep the heat of the city of Chicago in the lower 60s for low temps. Friday looks like another spectacular early autumn day with highs around 80, low humidity, and sunny skies.

The weekend forecast overall looks ok, though forecast confidence is a bit lower with some chance it could rain a bit at some point. Another trough is expected to pass well north of the area over the weekend with a weakening cold front reaching the area Saturday or Saturday night before probably washing out altogether. Cannot totally rule out a few showers with that front, but given weak frontal convergence, antecedent dry air mass, and minimal upper forcing, the more likely scenario looks to be dry weather for most. Southerly winds in advance of this front should allow temps to warm more solidly into the 80s on Saturday. Another trough is expected to move into the region early next week bringing what looks to be a somewhat better chance of showers and thunderstorms in the Monday/Tuesday time frame. There is still model variability in the timing and track of that trough, so forecast confidence is still on the lower end of the spectrum.

-Izzi

AVIATION /12Z TAFS THROUGH 18Z THURSDAY/

Issued at 645 AM CDT Wed Sep 9 2026

Key messages for the 12Z TAF period:

* Intermittent showers through the morning

* MVFR cigs expected mid to late morning and lasting into the afternoon

A system of showers will continue to work across northern IL through the morning hours. Pockets of moderate rainfall with vsby reductions will be possible mid-late morning. An embedded thunderstorm cannot be ruled out either, particularly at RFD. Rain should move away toward the end of the morning.

MVFR cigs are favored to settle overhead with the rain later this morning and stick around into mid-afternoon. A period of IFR may be possible, especially at RFD.

Meanwhile, SW winds early this morning will veer to W for the daytime and get up to around 10 kt for the afternoon. Expect light NNW winds late tonight flipping to NNE Thursday morning

Doom

LOT WATCHES/WARNINGS/ADVISORIES

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