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 /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 635 AM CDT Sat Sep 19 2026

Key aviation forecast messages:

- SHRA/TSRA early this morning, tapering off to scattered showers by mid-late morning. RFD TS threat likely leans toward the latter timing. Brief IFR with heavier precip this morning.

- Another wave of SHRA/TSRA develops toward/during this evening and persists into the overnight hours.

- Surface winds temporarily northeast this morning, then mainly southeast to south-southeast into tonight. Winds shifts north- northeast and become gusty early Sunday morning. IFR CIGs possible with/post wind shift.

Area of SHRA/TSRA occurring across the terminals at issuance time this morning. Thankfully, southeastward moving outflow boundary has resulted in the deepest convective cores being a little more progressive and these should slide southeast of the Chicago terminals over the next 1-2 hours. Farther west, RFD is closer to regenerating TS on the nose of the low-level jet and may see a little longer duration of at least embedded TS into mid-morning. Guidance continues to suggest a decrease in SHRA coverage (and TS) mid-late morning into this afternoon, though with persistent forcing within the moist frontal zone there will probably be continued scattered SHRA through the day.

Several mid-level waves are expected to ripple along the baroclinic zone into tonight, with somewhat lower confidence in timing of TS, though guidance continues to generally support an uptick in TSRA potential again this evening. As there remains some uncertainty with timing (and to allow a for a likely longer period of TS than a TEMPO allows) have maintained PROB30s for now, though those will likely transition to more targeted prevailing or tempo TS mentions with later updates.

Surface winds have been disturbed by convective outflow early this morning, but northeast winds should become east-southeast again shortly. The warm front across central IL is expected to lift north toward the terminals later today and depending on how far it lifts, winds may become more south-southeasterly for a time before turning back southeast this evening. Eventually, the front will push back south as a cold front early Sunday morning, with a shift to north-northeast winds which may become gusty in the 20-25 kt range after sunrise.

MVFR ceilings are likely to prevail for much of the period, with brief IFR CIG/VIS in heavier convection early this morning and again tonight. Guidance is in pretty good agreement in IFR CIGs prevailing in the northerly winds behind the cold front Sunday morning.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch until 1 PM CDT this afternoon for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ020- ILZ103-ILZ104-ILZ105-ILZ106.

Beach Hazards Statement through this evening for ILZ006.

Beach Hazards Statement until 3 PM CDT this afternoon for ILZ103-ILZ104.

IN...Flood Watch until 1 PM CDT this afternoon for INZ001-INZ002.

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