textproduct: Chicago

KEY MESSAGES

- A Flood Watch for flash flooding is in effect for areas roughly along and north of the Illinois and Kankakee rivers through early Sunday morning. - Heavy rain-producing showers and thunderstorms are expected to redevelop late tonight. Flash flooding will be possible where the heaviest rains occur over the same areas.

- Showers/drizzle and perhaps an isolated thunderstorm likely to continue for most of the area through the day on Sunday. Periods of showers to persist off-and-on through Tuesday.

UPDATE

Issued at 943 PM CDT Sat Sep 19 2026

An area of showers with isolated embedded thunderstorms blossomed over northern Illinois during the mid-late afternoon and is still ongoing over a good chunk of our forecast area at press time. Rainfall rates with this activity have been kept in check for the most part with the greater instability still displaced to the south and southwest of our forecast area, but the footprint of this shower activity ended up being a bit more expansive than previously expected. This has prevented the frontal zone referenced in this afternoon's forecast discussion below from advancing as far north as many high-resolution forecast models had earlier depicted that it would have by this time.

Robust isentropic upglide, paired with upper-level jet divergence and the approach of a shortwave trough coming out of the central Plains, is still expected to yield expansive convective development across the sloped frontal zone that is draped over the region during the latter half of the evening. Have started to see some new convection bubble recently over south-central Iowa, and the footprint of this convection should continue to expand and progress eastward in tandem with the stronger isentropic and DCVA-induced forcing, reaching our forecast area around midnight and continuing through the overnight hours. With the frontal zone likely to remain a bit farther south than previously expected, though, ended up nudging our highest PoPs and QPF a bit farther south to be maximized across and near the I-88 and I-80 corridors. These grid updates look to be well-aligned with most of the 00Z CAM guidance that's rolling in, which continues to support peak rainfall rates of 2+"/hour occurring with the most robust convective elements in our area overnight. Also expanded the going Flood Watch to include LaSalle, Grundy, southern and eastern Will, and Kankakee counties based on these forecast trends and the rainfall that has occurred in these counties in the past few hours.

Thus, there is still concern for flash flooding occurring in our area overnight, and our level of concern has grown a bit from earlier given that the preceding round of rainfall this afternoon and evening dumped a swath of 0.5-2" of rainfall over some of the areas that are likely to see heavier rainfall overnight tonight. The one potential saving grace may be that there may be less instability for the overnight convection to work with as a result of the preceding round of convection, but even 0.5-1"/hour rainfall rates will likely cause some flood impacts in areas where soils are already saturated and in our typical flood-prone urbanized areas. That said, if the heavier rainfall rates do materialize as anticipated and convection takes on an optimal west-east-orientation for training like many models have been showing, then significant flash flooding will be a distinct possibility in our CWA overnight, so we will be monitoring radar and rainfall trends very closely through tomorrow morning.

Ogorek

DISCUSSION

Issued at 1242 PM CDT Sat Sep 19 2026

A broad surface trough extends from northeast Iowa, southwest across Kansas and into the Texas and Oklahoma Panhandles. A convectively- reinforced outflow/gravity wave associated with widespread morning convection had pushed well south through central Illinois but should start to slowly return northward as the aforementioned trough begins advancing towards the Mississippi river through the rest of the afternoon. This boundary may make it back as far north as about a Cedar Rapids to Ottawa to Rensselaer line through late this afternoon with temperatures recovering back into the lower 80s and dewpoints into the lower 70s.

To the north of this returning (and eventually stalling) boundary, isentropic upglide will increase significantly late this afternoon and evening as strengthening 925-700 mb southwest to westerly flow rides up and over the sloping frontal zone as a positively-tilted shortwave shifts out of central Iowa. As this occurs, a remnant dry layer from 925-700 mb--leftover from morning convection--will saturate, leading to the redevelopment of showers and thunderstorms.

Trends in high-resolution guidance have generally been towards a slower progression to things, with redevelopment not occurring meaningfully until after about 7-8 PM or so. That said, given the magnitude of forecast isentropic upglide developing late this afternoon, would not be surprised if more vigorous convective development still occurs during the late afternoon and early evening. Regardless, the environment in place will be supportive of torrential rain-producing thunderstorms tonight, with PWATs well into record territory, quasi-persistent upper jet divergence, and a modest (yet sufficient) 25-30 kt southwesterly low-level jet impinging on the aforementioned stationary frontal zone. Generally easterly/east-southeasterly cloud bearing flow, aligned more-or-less parallel to the initiating boundaries, will result in a potential for training convective elements. Latest guidance suggests the main potential heavy rain corridor may essentially overlap with the location of this morning's widespread steady rainfall which has started to saturate soils. Have extended the previous Flood Watch through early Sunday morning. Did contemplate adding a few zones on the south end of the watch, but at this time it looks like activity that does push farther southward may end up being somewhat more progressive, limiting overall residence time. Can't rule out the need for an expansion at some point, but the widespread heavy rain threat appears too low to warrant that at this time.

The main synoptic cold front will push south across the region on Sunday morning. Fairly widespread showers and embedded storms may be ongoing after daybreak, but expecting the bulk of activity to gradually shift southward through the morning and afternoon. A trailing shortwave may result in a bit of a flare up of shower activity towards midday/early afternoon, but the main MUCAPE axis will steadily be shifting southward during this time. Lingering saturation up through about 10 kft and modest ascent may result in areas of showers or even steady drizzle continuing north of the frontal zone into the afternoon (north of I-80), but we should eventually start to scour out the low cloud deck sufficiently to bring most of the lingering precip chances to an end late in the day.

An additional series of disturbances are slated to rotate atop the frontal zone Sunday night through Tuesday as broad/persistent cyclonic flow develops across the region. Forecast soundings suggest we may be dealing with at least periods of showers and drizzle from Sunday night through the better part of Tuesday evening before a drier low-level airmass pushes in from the east. Persistent cloud cover and east to northeasterly winds will keep temperatures seasonably cool in the 60s.

High pressure is forecast to build more meaningfully across the region on Wednesday and Thursday, but another disturbance may clip our area towards the end of the week.

Carlaw

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 716 PM CDT Sat Sep 19 2026

Key Messages:

- Showers this evening with thunderstorms likely overnight.

- Sub-VFR ceilings likely tonight through most of tomorrow.

- Gusty northeasterly winds will develop tomorrow.

A wave of showers with embedded thunderstorms is working its way through northern Illinois and northwestern Indiana early this evening. It appears that most of the lightning with this activity should remain to the west and south of the terminals, but still couldn't rule out a few isolated lightning strikes occurring near the terminals over the next few hours. The steadiest rainfall with this wave of showers has knocked down visibilities to as low as IFR levels, and would expect that to remain the case through the mid-late evening.

Another wave of showers and more expansive thunderstorm coverage will develop later this evening to our west and spread into the Rockford and Chicago metro areas around/after midnight. Pushed back the timing of the inherited VCTS and TSRA mentions a few hours based on the latest observational and model forecast trends. There is a chance that the strongest storms and heaviest rainfall with this round could focus just south of the terminals, but even if that were to occur, would still expect lightning and sub-VFR visibilities to be issues within the C90 airspace. This activity should largely begin to taper off by around sunrise, but some showers may linger in the area for a while longer into tomorrow.

IFR ceilings are also likely to develop or drift into the area later tonight, and LIFR ceilings also appear to be a possibility at times -- particularly at RFD and wherever the more persistent heavier rainfall occurs overnight. Gradual improvement to MVFR conditions is expected during the daytime tomorrow, though how quickly this will occur remains a point of uncertainty.

Lastly, winds will turn northeasterly behind a surface boundary on the backside of a surface low tomorrow. Sustained winds will likely approach 15 kts tomorrow afternoon with regular gusts in the 20-25 kt range.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch until 7 AM CDT Sunday for ILZ003-ILZ004-ILZ005- ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020- ILZ021-ILZ023-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

Beach Hazards Statement from 7 AM CDT Sunday through Tuesday evening for ILZ006-ILZ103-ILZ104.

IN...Flood Watch until 7 AM CDT Sunday for INZ001-INZ002.

Beach Hazards Statement from Sunday morning through Tuesday evening for INZ001.

Beach Hazards Statement from Sunday morning through Monday morning for INZ002.

LM...Small Craft Advisory from 7 AM Sunday to 7 PM CDT Tuesday for the IL nearshore waters.

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

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