Area Forecast Discussion

KEY MESSAGES

- A Flood Watch for flash flooding has been extended for areas roughly near and north of I-80 through early Sunday morning.

- Heavy rain-producing showers and thunderstorms are expected to redevelop, as early as late this afternoon, but most likely this evening and overnight. Localized 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.

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 /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1242 PM CDT Sat Sep 19 2026

Forecast concerns for the terminals...

- Isolated to widely scattered showers to linger across northern IL this afternoon. Any showers could result in MVFR to IFR ceilings.

- Widespread showers and embedded thunderstorms arrive late this evening with a cold front and will persist through early Sunday morning resulting in IFR conditions.

- Increasing northeasterly winds behind the front Sunday morning with 20-25 kt gusts expected.

- Ceilings should gradually lift to MVFR as winds increase on Sunday.

A cluster of showers associated with a mesoscale low pressure center continues to fester across northwest IL and has been trying to drift east. However, the combination of some mid-level drying and sinking air in the wake of this morning's showers/storms has been working to erode these showers. While confidence is low as to whether or not these showers will actually make it to the Chicago area terminals, have opted to maintain a VCSH mention to tease the low chances (20-40%) for isolated to widely scattered showers this afternoon. Though, did keep a prevailing SHRA mention at RFD where confidence in showers persisting is higher. Aside from these low-end rain chances this afternoon, expect generally VFR conditions this afternoon with otherwise 8-10 kt east-southeast winds. That said, any showers that do develop this afternoon could result in a period of MVFR to IFR ceilings.

Heading into this evening, the cold front stretching from southern MN to southwest IA will begin to move into northern IL (and eventually northwest IN closer to midnight) and bring with it a more widespread coverage of showers and embedded thunderstorms. Once these showers and thunderstorms arrive they are expected to persist through the night and into the early morning hours on Sunday. Given the humid air mass and strong forcing, these showers/storms will once again be capable of heavy to torrential downpours which will result in IFR ceilings and visibilities. While the bulk of the showers/storms should begins to taper by mid-morning Sunday, forecast soundings do show some stout low-level moisture hanging around which may allow pockets of drizzle to linger into Sunday afternoon. Right now the confidence on drizzle at the terminals is too low for a formal TAF mention, but something we will be watching closely.

Regardless on the drizzle, expect IFR ceilings to persist through at least Sunday morning before attempting to lift to MVFR Sunday afternoon. Lastly, winds behind the cold front will also become northeasterly and increase with 20-25 kt gusts expected late Sunday morning through the end of the TAF period.

Yack

LOT WATCHES / WARNINGS / ADVISORIES

IL...Flood Watch through Sunday morning for ILZ003-ILZ004-ILZ005- ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ020-ILZ103- ILZ104-ILZ105-ILZ106.

Beach Hazards Statement until 7 PM CDT this evening for ILZ006.

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

IN...Flood Watch through Sunday morning for INZ001-INZ002.

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