Area Forecast Discussion

KEY MESSAGES

- Waves of showers and storms are expected in the general region tonight 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 129 PM CDT Mon Sep 14 2026

A 1024mb surface high pressure system continues to move through the central Great Lakes leading to a generally quiet day. Increasing cloud cover is muting the diurnal temperature rise, though highs should still top off in the mid 70s most places amidst a southeasterly breeze. Meanwhile, an early-autumn storm system is moving along the US/Canadian border driving a large area of warm air- advection driven showers and thunderstorms across the Upper Mississippi River Valley. With a residually dry airmass in place across the Lower Great Lakes on the southwestern side of the departing high pressure system, expectations remain that our area will remain dry through sunset. With that said, cannot rule out a random sprinkle or two this afternoon mainly near/west of I-39 and north of I-88.

Tonight, the northern Plains low will drive a surface warm front northward into the Lower Great Lakes in advance of a cold front sliding southeastward across the Upper Midwest. Strong theta-e advection driven by an intensifying southwesterly low- level jet will usher in a warmer and more humid airmass into our region, pushing atmospheric column moisture (PWATs) near 2 inches. Although large- scale forcing will be modest through the overnight hours, continued isentropic upglide in advance of the cold front will support a blossoming region of showers and thunderstorms later tonight from IA east-northeastward across parts of northwestern IL into southern WI. This activity is expected to shift over areas along and mainly north of I-80 after midnight tonight and will likely linger in the area through Tuesday morning. The primary threat with these showers and storms will be torrential downpours. Fortunately, the steering layer flow tonight will favor faster storm motions (~40 mph), so any potential flash flood threat would likely remain localized and tied to any potential areas of training storms.

Widespread cloud cover and lingering morning showers will limit diurnal warming on Tuesday especially with northward extent leading to the reinforcement of an outflow-driven stationary front across the Lower Great Lakes. Highs should range from the upper 70s near the Wisconsin state line to perhaps the mid 80s near and south of the stationary front south of I-80. By mid- afternoon, decreasing convective inhibition may trigger renewed scattered thunderstorms near the front primarily south of I-80. Access to 30-35kt of mid- level flow may promote localized clustering and associated threats for water-loaded downbursts where convective coverage is highest consistent with the SPC Level 1/5 threat level for severe weather tomorrow.

Tomorrow night through Thursday, the stationary front is expected to stall or modestly retreat back northward as a subtropical ridge strengthens across the southeastern United States. Subtle impulses parading around the perimeter of the ridge along the stationary front will support episodes of showers and thunderstorms in the general region, though forecast confidence on exact timing and locations is lower than average given the overarching pattern. Will have to watch for focused corridors of torrential downpours with an associated threat for localized flooding in this timeframe given PWATs hovering near or just above 2". Also, could envision heat index values poking toward 100 degrees near and south of the stationary front at times. Unfortunately, this is a pattern that'll have to be forecasted day by day as individual waves and the location of the stationary front can be parsed out. Finally, note that the next 2 to 4 days do look wet for many but not everyone in our area. In the absence of persistent large-scale forcing as well as the presence of modest mid-level height rises, downpours may be relegated to scattered coverage rather than enjoyed by all on a widespread basis.

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.

Birk/Borchardt

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.