textproduct: Chicago

KEY MESSAGES

- There is a threat for significant flash flooding tonight primarily near and south of Interstate 80. Rainfall amounts in this area could exceed 3-4 inches in places.

- Additional period of showers and thunderstorms Saturday night into Sunday with a cold front.

- A brief break in the stormy pattern is expected Monday into Tuesday before another storm system moves through towards midweek.

DISCUSSION

Issued at 123 AM CDT Sat Aug 15 2026

A shortwave trough is pivoting across eastern IA and northern MO and will eventually traverse across northern IL and northwest IN later tonight into Saturday morning. Ahead of the wave, isolated to widely scattered showers and sprinkles continue to fester over IL and IN but with the very limited forcing and instability their intensity is meager to say the least. Closer to the shortwave, however, an axis of thunderstorms is developing near the IA-MO border along a remnant outflow boundary from last night's storms. These storms are expected to become more widespread in coverage and spread east across north- central IL and northwest IN through the night and will linger into Saturday morning. Given the near record PWATs (around 2.5 inches) in conjunction with modest instability, a developing low- level jet, and the aforementioned wave, conditions are favorable for torrential rainfall (rain rates of 2+ in/hr) to occur with these storms. While most of northern IL and northwest IN is expected to see showers and storms tonight, a localized axis is expected to see the highest rainfall amounts. At this time, the axis of heaviest rainfall looks to be setting up near and south of a Peru, IL to Rensselaer, IN line but as storm coverage expands cannot rule out this axis shifting a hair northward towards I-80. Regardless, rainfall amounts within the heaviest axis look to be in excess of 3- 4 inches which given the saturated soils and elevated river levels from previous days of rain could lead to significant flash flooding. For this reason a higher-end Flood Watch remains in effect for areas south of I-80 through 10 AM CDT Saturday to highlight this threat.

Outside of this heavier rain axis, heavy downpours are still expected with the showers and storms tonight but with the lower likelihood of training convection the flood threat looks to be more localized to poor drainage and lower-lying areas for those north of I-80. Nevertheless, a Flood Watch remains in effect for our IL counties between I-88 and I-80 through 10 AM CDT. Additionally, resent mesoanalysis does show around 30-35 kts of effective shear overhead which may support a few storms becoming stronger. While the overall threat for widespread severe weather looks low due to the poor lapse rates and modest instability present, locally gusty winds upwards of 40-45 mph cannot be ruled out with the stronger storms tonight.

As we head into this afternoon, the aforementioned shortwave will exit the area and allow any lingering showers and thunderstorms to diminish in coverage and push east of the area. Overall it looks as if most of the afternoon hours Saturday should be dry for most, but forecast soundings do show some lingering moisture in the presence of weakly unstable air which may support some isolated showers festering across parts of the area through sunset. With some broad synoptic ascent forecast to be overhead Saturday, it is hard to pin point any specific areas showers will be favored so have opted to just keep all of northern IL and northwest IN under some 15% POPs for now. Otherwise, expect mostly cloudy skies to prevail through the day though temperatures should still be able to warm into the lower to mid-80s with continued warm advection.

Another shortwave trough is expected to pivot across the Mississippi River Valley tonight with a cold front in tow. These features look to lead to the development of additional showers and thunderstorms across northern IL and northwest IN late this evening into the overnight hours. Initially the coverage of shower/storms along the front may be more muted as our area resides in the subsidence behind this afternoon's wave and the new one tonight, but coverage should increase overnight as the front progresses southward. With the humid air mass still in place in combination with the modest instability and forcing, these showers/storms will once again be capable of locally heavy to torrential downpours. However, since the front should be rather progressive it seems the threat for training storms and flooding should be lower. Finally, there will also be another threat for locally gusty winds with the storms tonight though the overall severe threat does look low.

The cold front is expected to just about exit our area by Sunday morning which will allow drier air to filter in and finally conclude this active pattern. However, there is a chance that some isolated showers or storms may attempt to develop near the front Sunday afternoon south and east of I-57, but with the drier air and capping confidence is low on showers/storms actually materializing. Therefore, expect most areas to be dry for Sunday afternoon with temperatures still seasonable in the lower to mid-80s. Winds will be out of the northwest with gusts peaking around 20 mph during the afternoon.

Dry weather will persist through at least Tuesday afternoon as a surface high builds into the Great Lakes on Monday. So expect temperatures to remain seasonable with highs in the 80s but with lower humidity and cooler overnight lows in the 60s. Unfortunately the dry weather does not look to last long as an amplified trough is forecast to dive into the Great Lakes towards the middle of the week and bring with it more chances for showers and thunderstorms. As things sit right now, the higher coverage of showers/storms looks to be late Tuesday night into Wednesday as guidance shows a complex moving through the area but exact timing is still uncertain. That said, will need to monitor this period for the potential for a threat of stronger storms and/or heavy rainfall as well.

On the bright side, long range ensemble guidance is hinting that high pressure may move in behind Wednesday's storm system which may keep the bulk of any additional storms south and west of us for the later half of the week. Obviously this is lower confidence right now but at least something to possibly look forward to.

Yack

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/

Issued at 1258 AM CDT Sat Aug 15 2026

Key aviation forecast messages:

- SHRA likely this morning. Lower confidence in northward extent of TS coverage.

- Several hour period of IFR CIGs likely this morning through midday, MVFR then persists through the afternoon.

- Another round of SHRA/TS may arrive late tonight into early Sunday AM (30% chance).

The corridor of heavy rain and thunderstorms remains well southwest of the terminals at this hour. Confidence in the northward extent of the greater shower and TS coverage remains on the lower side but in general continues to favor areas south of I-80. There continues to be some guidance as far north as the I-88 corridor, however, and have accordingly maintained prevailing SHRA and a VCTS mention with this update this morning. Did move up the time by an hour to account for the potential for slightly earlier development ahead of the main system. Showers could then linger through mid to late morning before the system fully exits to the east.

Ceilings will steadily decrease with time early this morning with widespread low-end MVFR to IFR CIGs likely by daybreak across the area. Guidance remains consistent in IFR CIGs remaining in place through the morning hours before gradually lifting to MVFR in the afternoon and possibly briefly VFR in the evening before lowering again with our next disturbance late tonight into Sunday AM.

Winds will gradually trend east and then southeast through the overnight hours ahead of the increasing coverage of SHRA and potential TS. In the wake of any convection winds will trend southwest in the afternoon through the remainder of the period.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch until 10 AM CDT this morning for ILZ008-ILZ010- ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032- ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Flood Watch until 10 AM CDT /11 AM EDT/ this morning for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.