textproduct: Chicago

KEY MESSAGES

- Showers and some thunderstorms are expected later tonight into Wednesday. - Temperatures will remain near to above average through the upcoming weekend, warmest today and then again over the weekend.

DISCUSSION

Issued at 220 AM CDT Tue Sep 8 2026

The upper ridge that has been responsible for our near record warm first week of September is poised to be suppressed southward a bit mid week as a pair of shortwave troughs pass by to our north. These two shortwaves show up nicely on water vapor imagery early this morning as a pair of closed circulations embedded with a broader trough extending from southern Saskatchewan into the Pacific Northwest. Associated sfc low will track east into Manitoba today with a cold front trailing south into the lower Missouri Valley by later this afternoon.

Out ahead of this system, southern winds will become established across our CWA today leading to another unseasonably warm day. Despite cirrus cloudiness obscuring the sun a bit, still anticipate high temps to climb into the mid to upper 80s today. With deeper low level moisture pooled ahead of the boundary to our west, we could see dewpoints mix out some this afternoon, especially eastern CWA, which should keep heat indices near or a hair below ambient temperatures.

Convection is expected to develop over IA this afternoon ahead of the cold front in an area of maximized instability and moderate shear. This convection is expected to move eastward into northern Illinois tonight, but as it does so, it will be moving into a more stable environment with less moisture. In addition, the lead shortwave will be lifting farther northeast, becoming even more removed from what will likely be a decaying convective complex moving into northern IL overnight. The low level jet is expected to be focused to our north and also not be of much assistance in maintaining the intensity of this MCS as it moves into our area. With MUCAPE expected to be 500 J/kg or less, there may not be much thunder left with this activity as it moves into our area.

The remnants of that activity could linger into Wednesday morning over our area, mainly bringing a threat of showers without much threat for thunder. Wednesday's set-up looks potentially pretty messy, with the potential for outflow from this prior convection resulting in an effective frontal boundary setting up over central IL into north central Indiana Wednesday afternoon, well ahead of the synoptic cold front which is progged to move into northern IL during the afternoon. Mid and upper level height falls in advance of trailing shortwave trough is likely to support renewed convective development Wednesday afternoon into Wednesday evening, but it isn't clear if the more widespread and stronger convection will be over central IL with the lead outflow boundary or if the air mass in the wake of that boundary will recover sufficiently allowing the true synoptic cold front to be the primary focus for late day into evening storms.

Mid-level lapse rates should remain pretty weak Wednesday, limiting instability despite 70F+ dewpoints likely to be pooling near and ahead of the boundary/boundaries. That degree of low level moisture is still likely to support an axis of 1000-1500 J/kg of MLCAPE (given sufficient heating/breaks in the cloudiness). The moderate instability paired with respectable deep layer shear around 40kt, could lead to a localized threat for severe storms Wednesday afternoon, particularly over our southern CWA as it looks now. The showers and thunderstorms should gradually settle south of our CWA Wednesday night as the synoptic front pushes south of the area.

In the wake of the front, look for a brief cool down and return to temperatures closer to average Thursday. Warm air advection is already progged to start ramping up Friday in advance of the next shortwave, allowing temps to warm a bit Friday and more so into Saturday. The next cold front should move across the region later Saturday or Saturday night bringing with it the next chance of showers/storms followed by another return to near average temps late in the weekend into early next week.

- Izzi

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 633 AM CDT Tue Sep 8 2026

Key messages for the 12Z TAF period:

* Showers and perhaps a couple of thunderstorms beginning late Tuesday night and Wednesday morning

Opening up the day with light SSE'erlies. A lake breeze will protrude inland during the afternoon but is favored to hold just short of the Chicago sites and GYY. If it does make it across the airfields, a period of SSE will result during the late afternoon. Winds will be moving today at near and largely below 10 kt.

A system of showers will approach RFD late Tuesday night and move into Chicagoland early Wednesday morning. Most likely window for rain in Chicago is roughly 12 to 16Z. A couple of embedded thunderstorms will be possible, as will a period or two of MVFR. Isolated showers may linger into the early afternoon.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

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