textproduct: Chicago

KEY MESSAGES

- Showers and storms will move across the region this morning. Localized flooding may occur in typical prone locations.

- A brief break in the pattern is expected tomorrow afternoon through Friday before another period of wet weather materializes this weekend.

DISCUSSION

Issued at 310 AM CDT Thu Sep 17 2026

Isentropic upglide associated with an intensifying southwesterly low-level jet overrunning a quasi-stationary surface boundary over central Illinois continues to facilitate blossoming convection from eastern Iowa eastward into portions of northern Illinois early this morning. While storm coverage south of I-80 remains scattered as of this writing, we continue to monitor expanding showers and thunderstorms over eastern Iowa. This activity is expected to spread eastward into northern Illinois through the predawn hours as the low-level jet veers.

The environment in which these storms are developing is anomalously moist (precipitable water value near ~2" per the 00z DVN RAOB) with very high freezing levels (just under 15,000 ft) supportive of deep warm cloud depths. Accordingly, very heavy rain rates (1+" per hour) will likely accompany these showers and thunderstorms this morning, potentially leading to a few bands of 2"+ amounts (locally higher), particularly where multiple storms train over the same area in northern IL. While rainfall of this magnitude could result in localized flash flooding, recent dry weather (30-day precipitation deficits of 10-20% of normal) and low soil moisture may limit this threat to some degree. Nevertheless, we will continue to monitor radar trends for potential hydro issues this morning, particularly in the more urban areas around Rockford and Chicago. Otherwise, the shower and thunderstorm threat is expected to shift eastward out of the area by late morning.

Aside from a low (~20-30%) chance of a few afternoon and evening storms near the surface frontal boundary across central Illinois and Indiana, a period of rain-free weather is expected this afternoon through at least the first half of Friday. We should see another fairly large variation in temperatures across the area, particularly this afternoon. Low clouds are likely to persist across northern areas (especially along and north of I-80), holding temperatures steady in the lower to mid-70s through the afternoon. Conversely, areas farther south into central portions of Illinois and Indiana will experience another very warm and humid afternoon, with temperatures and heat indices topping out in the mid-to-upper 80s (low 90s).

Attention then turns to another wet period across the region this weekend into early next week. Ensemble guidance remains in agreement that the upper-level wave over the western CONUS will eject a couple of mid/upper-level shortwaves northeastward into the Midwest Friday into the weekend. Increasing upper-level jet diffluence and low-level convergence along the nose of an intensifying low-level jet should lead to widespread showers and storms in the Upper Mississippi River Valley Friday into Saturday morning. As the parent Pacific wave drifts eastward and a surface high-pressure system moves into southern Canada this weekend, a low-level frontal zone will shift southward across the Lower Great Lakes and serve as a focus for several rounds of showers and storms Saturday afternoon through the middle of next week. We will maintain broad-brushed PoPs from Saturday afternoon onward, along with a temperature gradient (coolest north, warmest south) similar to current conditions.

KJB

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 652 AM CDT Thu Sep 17 2026

Main Concerns:

- SHRA/TS this morning

- Low flight category trends: Temporarily down to LIFR CIGs and IFR VSBY in heavier rain and BR this morning, improving some by early afternoon (except at GYY). IFR CIGs possible tonight.

Heaviest SHRA/TS have generally sagged south of the terminals except for GYY, which will remain in the persistent ~1SM +TSRA for longer. Still expect occasional IFR VSBY in heavier rain rates north of the current most intense axis. Widely scattered SHRA (no TS anticipated) may then persist until midday/early afternoon, followed by primarily dry conditions through Friday morning.

CIGs will bottom out at occasionally LIFR this morning, though be primarily IFR to MVFR with some variance (including brief scattering of low CIGs). MVFR CIGs this afternoon should linger into tonight, with uncertainty on improvement timing (or if they build back down to IFR overnight). Confidence is on the lower side with specific details in the TAFs. There may also be MVFR VSBY outside of Chicago tonight.

Light and VRB winds this morning will become steady northeasterly this afternoon through tonight, and then east- northeast 10-15 kt Friday morning.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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