textproduct: Chicago

KEY MESSAGES

- Periods of heavy rain and scattered thunderstorms late tonight through Sunday night. Localized flash flooding will be possible where the heaviest rains occur over the same areas. A Flood Watch for flash flooding is in effect for much of northern Illinois and far northwest Indiana late tonight through Saturday morning.

- Below normal temps early next week then dry weather expected mid/late next week.

DISCUSSION

Issued at 257 PM CDT Fri Sep 18 2026

An unseasonably strong mid-level ridge with 500 hPa heights up to 600 dam over Texas continues to expand ENE toward the mid- Mississippi River and Ohio River valleys this afternoon. Initial ascent on the periphery of the ridge has maintained a general NNW to SSE axis of high-based showers drifting across the forecast area this afternoon. A diminishing reservoir of MUCAPE with eastward extent across the north half of Illinois has allowed some sparse embedded lightning to approach the far west and southwest CWA. Overall expectations are for isolated to scattered showers to continue tracking across the remainder of the CWA through early evening. Clearing has lead to surface- based destabilization behind an effective surface front over central Illinois. While stronger convection has brushed the extreme southern CWA, remaining activity is expected to remain south this afternoon.

Appreciable deep layer ascent driven by a mid/upper-level jet core passing north of the area tonight and an approaching compact mid- level wave currently over southwest Nebraska will interact with an expansive MUCAPE reservoir advecting northeastward toward the area tonight. Convection will likely quickly develop over far northern Iowa late this evening and expand eastward to southern Wisconsin in response to a strengthening LLJ focused toward far southwest Wisconsin overnight. With the veering LLJ becoming nearly perpendicular to the low-level front, significantly high PWAT values at or above 2 inches (nearly 250 percent of normal for the second half of September), and relatively slow storm motions, there is growing concern that an axis of significant rainfall develops just north of the IL/WI state line overnight. This activity would then slowly translate southward over northern Illinois toward daybreak and through at least mid-morning. Rainfall rates of 2 inches per hour or higher (similar to what was observed early Thursday morning), could lead to flash flooding, especially for urbanized areas around Rockford and the Chicago metro. A Flood Watch for flash flooding has been issued for a vast majority of the Chicago metro and areas around the north of I-88 late tonight through Saturday morning, with the greatest concern in the 5-10am window. Severe thunderstorms are not expected as this activity will remain elevated, but localized sub-severe gusts cannot be ruled out.

Convection may fester over some of the area late morning through much of the afternoon, but coverage is expected to remain low. The low-level environment will quickly destabilize to the southwest Saturday afternoon, with the associated airmass beginning to advect northeastward toward the forecast area. Another mid-level wave currently over southeast Colorado is progged to cross southern Wisconsin very late in the afternoon and evening, aligning with a slow-morning surface low and initiating renewed convection over or just north of the state line by early evening. Organized strong to marginally severe convection cannot be ruled out in a narrow corridor west of the I-39 corridor where low-level destabilization, delayed low-level warming/capping, and modest low-level shear support a very conditional localized damaging wind and brief tornado threat late afternoon into early evening.

The flooding risk will increase again with the evening convection as most ingredients for flooding present in the morning will remain in place during the evening. Convection may then stall somewhere over the central or southern CWA overnight into Sunday morning as the low- level jet strengthens and veers, leading to the potential for a local corridor of significant rainfall totals and additional flooding. A new Flood Watch will likely be required for much of the night for a large portion of the forecast area.

While strong high pressure building across Ontario and Quebec through early next week should provide a feed of fairly dry air toward the area, another compact mid-level low traversing the effective cold front may result in additional heavy rain across central Illinois, including some of the forecast area, on Monday. Dry and cool conditions will then become more favored beginning Tuesday and especially later in the week.

Kluber

AVIATION /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 1248 PM CDT Fri Sep 18 2026

A region of showers will slide over the terminals this afternoon tied to a weakening mid-level wave. With a feed of dry air from the east on the south side of a surface high over the Great Lakes region, suspect the showers will be more of a nuisance than an impact with no cig or vsby restriction locally. Will maintain TEMPO groups generally in the 19-22Z timeframe for plain showers.

Tonight, a low-level jet will develop over the middle Mississippi River Valley affording a continuation of east to southeasterly surface winds. Broken MVFR cigs may develop with time especially toward daybreak as the low-level column moistens. Meanwhile, a large area of thunderstorms is expected to develop in the vicinity of northern Iowa and southern Wisconsin and drop southward across the terminals near or just after daybreak. Will maintain the inherited structure of the TAFs with SHRA starting by 09-10Z and TEMPO groups generally from 12-15Z. Can easily envision transitioning the TEMPO groups to FM groups in a later TAF package. A period of northwesterly winds and IFR cigs may accompany the thunderstorms through the morning.

With continued isentropic ascent in the region, suspect showers will continue on and off unabated through the late morning and early afternoon hours as easterly winds prevail. Do suspect there may be holes in IFR and MVFR cigs through the early afternoon hours before low cigs redevelop and shower and storm coverage increases once again by 00Z.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch from 1 AM CDT Saturday through Saturday afternoon for ILZ003-ILZ004-ILZ005-ILZ008.

Flood Watch from late tonight through Saturday afternoon for ILZ006-ILZ010-ILZ011-ILZ012-ILZ013-ILZ020-ILZ103-ILZ104- ILZ105-ILZ106.

Beach Hazards Statement from 10 PM CDT this evening through Saturday evening for ILZ006.

Beach Hazards Statement from 10 PM CDT this evening through Saturday afternoon for ILZ103-ILZ104.

IN...Flood Watch from late tonight through Saturday afternoon for INZ001-INZ002.

LM...Small Craft Advisory from 10 PM this evening to 3 PM CDT Saturday for Winthrop Harbor to Northerly Is. IL.