textproduct: Chicago

KEY MESSAGES

- A Flood Watch for flash flooding is in effect for much of northern Illinois and far northwest Indiana through early this afternoon.

- Periods of heavy rain and scattered thunderstorms through mid/late this morning and then a second wave of heavy rain and scattered thunderstorms tonight into Sunday. Localized flash flooding will be possible where the heaviest rains occur over the same areas.

- Below normal temps early/mid next week with low chances for showers.

DISCUSSION

Issued at 148 AM CDT Sat Sep 19 2026

Primary forecast focus and challenge remains heavy rain timing and location through Sunday.

A large area of convection currently extends from far northeast IA across most of southern WI and is slowly drifting south into far northern IL. This activity will continue to slowly move into northern IL during the predawn hours and into the I-80 corridor by mid/late morning. There remains some uncertainty for where the heaviest rains will setup and how long they'll persist in any given area. The heaviest rains this morning may end up falling along and northeast of a Rockford to Chicago line. Scattered showers and possibly a thunderstorm may continue this afternoon. Current flood watch area has this threat handled well both for area and timing, ending at 18z today and no changes are planned.

Another period of thunderstorms with heavy rain is expected later this evening, through the overnight hours and into Sunday morning, likely slowly sagging south across the cwa during this time period, which may bring the heaviest and most widespread rains to at least parts of the area. Where these may overlap with heavy rains from this morning is still uncertain. Another flood watch is likely from late this evening into Sunday morning, for the same areas as the current watch and possibly extending another row of counties south to the Illinois and Kankakee River Valleys. Given the current watch end time and a large time gap until the next wave tonight, will hold off on a new flood watch for tonight.

The new SPC Day1 and Day2 outlooks have trimmed the level 1 severe risk to the far southern portions of the local area. There will still be the potential for a few stronger storms and perhaps an isolated severe storm or two, mainly across the southern cwa.

A few of the models, NAM/HRRR, suggest another period of heavy rain during the day on Sunday. Confidence is low during this period, but if these trends were to continue, a second possible flood watch may need to be extended out into this time period as well.

High waves and dangerous swimming conditions will continue into this afternoon at IL Lake Michigan beaches as easterly winds turn more southeasterly. Northeast winds are expected to quickly increase early Sunday morning and then remain fairly strong through the middle of next week, resulting in several days of high waves and dangerous swimming conditions at IL and IN Lake Michigan beaches. An additional beach hazard statement will be needed for this long time period.

The models are now trending toward an upper level low across the region through midweek. While below normal temperatures still look on track, there will likely be at least low chances for showers, especially during the afternoon/evening as various weak waves rotate around the upper low. cms

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/

Issued at 1234 AM CDT Sat Sep 19 2026

Key aviation forecast messages:

- SHRA/TSRA develop into far northern IL 07-10Z this morning with TSRA and near-LIFR visibility in heavy rain through much of the morning.

- SHRA and MVFR ceilings persist at times this afternoon into tonight, with another round of TSRA possible this evening into the overnight hours.

SHRA/TSRA continue to develop across southern WI in response to low-level jet forcing atop an elevated warm frontal zone. Guidance remains in good agreement that this convection will continue to develop east-southeastard over the next few hours and impact the terminals for a prolonged period into mid-late morning. While TS coverage should diminish by midday, scattered SHRA may persist as weak, moist ascent persists within the warm frontal zone. Another mid-level wave currently emerging from the High Plains is progged to approach the forecast area from the west this evening, with another increase in SHRA and TSRA coverage likely tonight.

MVFR ceilings (already in place at RFD at issuance) are expected to become widespread this morning and persist through the period, with IFR CIGs possible at times. IFR to near-LIFR vsbys are possible in heavier SHRA/TSRA this morning and again tonight.

Surface winds are expected to remain generally east-southeast to southeast across the terminals, though may become more southerly at RFD (and possibly DPA) later today depending on how far north a surface low pressure wave and surface warm front lift into the region. TSRA may also temporarily produce gusty, more northerly surface winds.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch through this afternoon for ILZ003-ILZ004-ILZ005- ILZ008.

Beach Hazards Statement through this evening for ILZ006.

Flood Watch from 4 AM CDT early this morning through this afternoon for ILZ006-ILZ010-ILZ011-ILZ012-ILZ013-ILZ020- ILZ103-ILZ104-ILZ105-ILZ106.

Beach Hazards Statement through this afternoon for ILZ103- ILZ104.

IN...Flood Watch from 4 AM CDT early this morning through this afternoon for INZ001-INZ002.

LM...Small Craft Advisory until 3 PM CDT this afternoon for Winthrop Harbor to Northerly Is. IL.