Area Forecast Discussion

KEY MESSAGES

- Waves of showers and storms are expected in the general region through much of the week.

- While some areas may not see much rain at all, others may experience repeated downpours with a threat for localized flooding.

- The next few days will also be marked by high humidity.

- A gradual pattern change toward cooler, less humid, and drier conditions is favored by next week.

DISCUSSION

Issued at 307 AM CDT Tue Sep 15 2026

The main focus continues to revolve around the timing and favored placement of each wave of showers and thunderstorms expected over the next few days.

A surface warm front will shift northeastward across the area this morning, ahead of an approaching cold front from the Upper Midwest. Meanwhile, a corridor of strong theta-e advection, driven by a 45+ kt southwesterly low-level jet, is quickly drawing deep atmospheric moisture (PWATs near 2 inches) from the lower Missouri Valley into the western Great Lakes early this morning. Associated strong isentropic upglide within this very moist airmass is supporting a large region of showers and thunderstorms from the Corn Belt into southern Wisconsin. This activity will shift eastward, impacting much of northern Illinois (especially near and north of I-80) through the morning. Continued warm air advection may maintain widely scattered showers through the afternoon across far northern Illinois.

Widespread cloud cover and lingering showers will limit diurnal warming today, especially over northern Illinois, reinforcing an outflow-driven stationary front across the lower Great Lakes. High temperatures should range from the upper 70s near the Wisconsin border to around 90 well south of I-80. By late afternoon, weakening convective inhibition may trigger renewed scattered thunderstorms near the front, primarily south of I-80. Access to ~40 kt of mid-level flow may promote localized storm organization and water-loaded downbursts where convective coverage is highest. For this reason, the Storm Prediction Center maintains a Level 1 of 5 severe weather threat for areas generally south of I-80 later today. Torrential rainfall will also accompany these storms into this evening.

The surface cold front's southward push is expected to slow or stall across the southern half of Illinois tonight as its west- to-east orientation aligns parallel to the mid- and upper-level flow along the northern periphery of a broad subtropical ridge centered over the southeastern U.S. With little pattern change expected through Thursday, the front will remain near the region, serving as the focus for multiple rounds of heavy-rain- producing storms as several convective impulses move around the perimeter of the ridge.

Uncertainty remains regarding the exact coverage of heavy rain tonight into early Wednesday morning. However, storm coverage is likely to increase south of I-80 this evening into early Wednesday as additional convective waves track east- northeastward along and north of the surface boundary. Combined with a modest low-level jet (20-25 kt), this setup may support a corridor of heavy rainfall near and south of I-80, particularly where storms train over the same areas. Late Wednesday into early Thursday, the stationary front is expected to stall or retreat slightly northward as the subtropical ridge strengthens over the Southeast. Additional subtle impulses moving around the ridge along the boundary will likely trigger another round of showers and thunderstorms. This period warrants close monitoring, as a stronger southwesterly low-level jet response could develop Wednesday night, increasing the potential for a more focused corridor of training storms and an associated flash flooding threat in portions of the area.

Extended guidance indicates weak mid-level blocking as a subtropical low over the Gulf slows south of a weakening upper- level ridge. Meanwhile, a upper-level trough moving across the northern Great Plains on Friday may dig far enough south to bring showers and thunderstorms to the area on Saturday.

KJB

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1253 AM CDT Tue Sep 15 2026

Main Concerns:

- Gusty southerly winds (or LLWS) at the start of the period.

- SHRA and embedded TS early this morning. Additional SHRA will be possible at times this afternoon through tonight, though better TS chances should remain south of the terminals.

- Potential for MVFR CIGs at times later this morning into the afternoon and again tonight.

SHRA and TS off to the northwest (closest to RFD) will gradually overspread the terminals early this morning, and then coverage should wane by the mid to late morning. Maintained VCTS and TEMPO TS mention, though adjusted the timing a bit later based on radar trends. Additional adjustments may be needed with time. Winds will veer to south-southwest from current gusty south- southeast with the arrival of the rain, and can't rule out some outflow effects on the winds. There's a signal (strongest at RFD) for MVFR CIGs over northern IL as the morning convection winds down.

Late this afternoon into this evening, winds will shift to northwest as a cold front pushes across the area. Expect further veering to north-northeast tonight. The northern extent of additional convection this evening and overnight is uncertain. Higher chances/coverage should favor near and south of I-80, and as such included VCSH mention at GYY and MDW.

Castro

LOT WATCHES / WARNINGS / ADVISORIES

IL...Beach Hazards Statement until 7 AM CDT this morning for ILZ006- ILZ103-ILZ104.

IN...None. LM...Small Craft Advisory until 9 AM CDT this morning for the IL nearshore waters.

Small Craft Advisory until 7 AM CDT this morning for the IN nearshore waters.