textproduct: Chicago

KEY MESSAGES

- A Flood Watch for flash flooding has been extended through early this afternoon for areas roughly along and north of the Illinois and Kankakee rivers. - Periods of showers and thunderstorms with torrential rain and localized flash flooding are expected through midday today.

- Below normal temps through the end of the week.

DISCUSSION

Issued at 224 AM CDT Sun Sep 20 2026

The flood watch has been extended until 18z today. Periods of showers and thunderstorms are expected to continue through at least mid morning for much of the watch area. The main surface low won't reach central IN until this afternoon and this may keep persistent moderate/heavy rains across parts of the area through midday. And even from this short(er) distance, there is still some uncertainty where the heaviest rains may fall. The current activity may favor the I-80 corridor in the next few hours and the next wave over eastern IA may favor areas along and north of I-80 with perhaps later waves this morning favoring areas south of I-80. With all this in mind, opted to extend the flood watch through 18z and as trends emerge, the watch can be cancelled as needed later this morning. As for rainfall amounts, there could be one or two swaths of 3-5 inches of rain by the time it ends later today, which could lead to flash flooding.

Showers and scattered thunderstorms may still be moving across the southeast cwa later this afternoon into early this evening before precipitation finally comes to an end. There may be lingering showers tonight and perhaps a few showers on Monday, but only isolated coverage expected.

Fog developed over southern Lake Michigan Saturday evening and briefly moved onshore along the IL shore. Webcams suggest the visibilities have improved right near Lake Michigan and as northerly winds slowly increase through the morning, this fog may lift into a low cloud ceiling. Fog over Lake Michigan is also expected to slowly lift as well, though perhaps not until the winds increase further later this morning.

Strong northeast winds will develop today and remain strong through Wednesday producing high waves and dangerous swimming conditions at Lake Michigan beaches.

Still quite a bit of uncertainty for a possible upper low and shower chances this week. Temperatures look to be well below normal for the end of September with high temps mainly in the 60s through Thursday. cms

AVIATION /12Z TAFS THROUGH 18Z MONDAY/

Issued at 632 AM CDT Sun Sep 20 2026

Key aviation forecast messages:

- TSRA threat decreasing rapidly early this morning, though ra/SHRA will persist through mid-late morning before becoming more scattered SHRA this afternoon.

- IFR CIGs likely through midday, with some patchy LIFR possible early. Gradual improvement to MVFR later this afternoon.

- Northeast winds increase this morning and become blustery this afternoon/tonight with gusts around 25 kts, possibly a bit higher at times.

TSRA have poshed through the terminals just prior to 12Z, with lightning threat now greatly diminished. An area of RA/SHRA extending back into far eastern IA will persist across the area through mid/late morning however, before tapering to lighter scattered showers midday/early afternoon. Can't rule out some spotty light showers at times tonight into early Monday, though coverage ~20% too low for TAF inclusion.

Extensive IFR/LIFR CIGs across the area predawn have scattered out in spots in the wake of the heavier convection, though IFR CIGs likely to redevelop/persist and continue into the afternoon before improving to MVFR late. MVFR CIGs will likely persist overnight and into Monday morning, though may scatter to VFR at times late.

Surface pressure gradient will tighten this morning as high pressure builds across Ontario and the northern Lakes, resulting in increasing northeast winds. Winds will become gusty later this morning with gusts around 25 kts (possibly occasionally a little higher) expected later this afternoon and tonight. Breezy northeast flow will continue into Monday.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch until 1 PM CDT this afternoon for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107- ILZ108.

Beach Hazards Statement through Tuesday evening for ILZ006- ILZ103-ILZ104.

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

Beach Hazards Statement from 10 AM CDT this morning through Tuesday evening for INZ001.

Beach Hazards Statement from 10 AM CDT this morning through Monday morning for INZ002.

LM...Dense Fog Advisory until 9 AM CDT this morning for the IL and IN nearshore waters.

Small Craft Advisory until 7 PM CDT Tuesday for the IL nearshore waters.

Small Craft Advisory from 10 AM this morning to 7 PM CDT Tuesday for Calumet Harbor IL to Burns Harbor IN.

Small Craft Advisory from 1 PM this afternoon to 7 AM CDT Monday for Burns Harbor to Michigan City IN.