textproduct: Chicago

KEY MESSAGES

- Band of showers and thunderstorms is expected to result in a period of heavy rainfall and flooding overnight. Greatest threats for heavy rainfall and flooding is favored near and south of a Sterling, IL to Rensselaer, IN line.

- A stormy pattern is expected to remain in place through the weekend.

UPDATE

Issued at 823 PM CDT Thu Aug 13 2026

A band of showers continues to drift southward across northern and northeast IL and northwest IN this evening. While the lightning coverage within the line has been low, some scattered thunderstorms have developed along the tail end of the line in eastern IA and as modest instability increases over the next few hours the lightning may become more frequent with eastward extent. In addition to lightning, these showers/storms will also be capable of hefty rain rates (1-2+ in/hr) which will result in reduced visibility and maybe some ponding in poor drainage areas.

Heading into the overnight hours, the band of showers and thunderstorms is expected to stall along an elevated frontal zone that is draped from the Quad Cities southeastward across north-central IL into central IN. As this occurs, a modest low- level jet will develop and help to replenish moisture and instability which should allow showers and thunderstorms to fester along this boundary for much of the overnight hours. While there is still some uncertainty on where exactly this boundary will stall across our area, latest radar trends and forecast guidance is favoring the showers/storms to stall near and south of a Sterling, IL to Rensselaer, IN line. Given the aforementioned forcing and the high PWATs (around 2.0-2.25 inches) these showers and thunderstorms will be capable of heavy rainfall which could result in periods of flooding overnight into early Friday morning. Rainfall totals are currently forecast to be in the 1-3 inch range within the heart of the rain band, but some guidance is showing amounts upwards of 4+ inches. With these forecast rain amounts and the already saturated soils, the Flood Watch has been maintained for our southern counties through 10 AM CDT Friday morning.

In addition to the heavy rainfall, forecast soundings continue to show 35-40 kts of deep layer shear overhead tonight which may support a stronger storm or two. Since most of these storms should be elevated and the lapse rates upstairs are somewhat junky (around 5-6 C/km), it seems the threat for widespread severe weather is low. However, some rain loaded downbursts cannot be ruled out especially if any of the gravity waves seen in IA earlier are able to aid in enhancing the storms' downdrafts. If any gusty winds do occur tonight it seems most of them would be in the 40-50 mph range but locally higher gusts will be possible.

The showers and storms tonight should gradually begin to lose steam Friday morning as the low-level jet weakens. This should in turn give us mostly dry conditions for the afternoon hours on Friday. However, another shortwave disturbance is forecast to pivot through the area Friday night which looks to bring more showers and thunderstorms. These storms could also be heavy rain producers and could worsen any ongoing flooding from tonight and/or cause it to redevelop. The main focus for the evening updates was on the tonight into Friday morning period so the prevailing forecast for Friday night was maintained. For more details on what to expect see the discussion below.

Yack

DISCUSSION

Issued at 230 PM CDT Thu Aug 13 2026

At the time this discussion was published, an MCV was tracking east over US-24. Meanwhile north of the MCV, widespread rain is falling with isolated to widely scattered lightning. This system will continue to track east through the afternoon.

There could be a break in the late afternoon for areas south of Interstate 80, as continued rain is expected north along a projected low level vorticity axis and 500 mb jet axis along the Illinois/Wisconsin state line. With the current MCV moving through the area, it is difficult to pinpoint the exact location of the stationary front that was previously analyzed just south of the forecast area. That becomes important because the right entrance region of the previously mentioned 500 mb jet (and therefore area of focused upper level divergence) will be situated just north of the stationary front. High-res CAMs are projecting weak perturbations within the 500 mb flow this evening and through the overnight. With precipitable water amounts between 1.5 to 2 inches, this set-up is increasing concerns for flooding threat in the area. The one fly in the ointment for this set-up is the absence of a strong low level jet. Had there been a stronger low level jet projected, perhaps there would need to be consideration to expand the previously issued Flood Watch northward. Showers are expected to develop over northern Illinois with the risk of training storms over the same area. It is no surprise that ensembles are showing a broad swath of 1 to 2 inches of rain that could fall overnight, with an axis of locally higher amounts over the southwest portions of the forecast area.

There is lower confidence in the exact timing of when the rain ends on Friday morning. As the upper level jet weakens and moves east, rain should slowly taper off by mid-morning with the potential for a drier period expected in the afternoon. However, as the broad upper level ridge over the southeastern portion of the CONUS remains in place, another short wave will ride over the top of the ridge tomorrow afternoon and evening providing enough forcing for the next system to develop in Iowa and move east. Rain could begin in the early evening, but the better chances for the heavier downpours are expected closer to midnight when the core of the wave moves over. Given what the forecast is for rainfall overnight tonight on already saturated soils, there is growing concerns for flooding threats tomorrow night into Saturday morning and collaborated with the Weather Prediction Center to increase the risk to a level 3 out of 4 for excessive rainfall. The potential for water loaded downbursts keeps much of the forecast area in a marginal (level 1 out 5) risk for severe storms tomorrow night, but weaker shear profiles keep the confidence in severe development low.

Models have shown fairly good agreement that yet another wave is expected to pass over the ridge on Saturday providing another threat for heavy rainfall and flooding threat across portions of the forecast area. Global models are not suggesting a break in the rain until a front drops down out of Wisconsin late on Sunday. Surface high pressure is expected to grow on Monday for the potential for a dry day in the region. However, global models are still suggesting another system potentially bringing a a renewed threat for precipitation mid-week next week.

DK

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 102 AM CDT Fri Aug 14 2026

Key aviation forecast messages:

- Scattered SHRA at times through mid to late morning. -SHRA may linger beyond 18Z but the potential is <30% chance.

- Reduced VSBYs to MVFR in BR outside of Chicago through daybreak.

- Another thunderstorm complex is likely to move into the area late tonight-early Saturday morning.

Scattered showers are expected at times through early this morning. While the focus of greatest +SHRA and TS coverage remains south of I-80, can't rule out a few lightning strikes within the area of showers near RFD (did introduce a brief VCTS there through 7Z). This activity is drifting toward the Chicago metro terminals and may require a brief VCTS mention as well. Will monitor observational trends closely for any short fused amendments. Outside of the showers, high low-level moisture content is leading to VSBY reductions in BR as well, mainly outside of Chicago to around 3-5 SM.

Did introduce a PROB30 for SHRA mid morning through early afternoon to account for what is expected to be a decayed convective complex/MCV emanating from activity developing in the NE/MO/IA border area. While the lightning threat is low it is possible that showers linger as far east as Chicago and may be paired with a period of gusty southeast winds (lower confidence). Can't fully rule out showers at times through the remainder of the day though the potential remains less than 30%.

Model guidance is in fairly good agreement in yet another thunderstorm complex approaching the area late tonight through early Saturday morning. Have accounted for this with prevailing SHRA after 8Z with PROB30 groups for TS after 9Z to account for this potential.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

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

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

LM...None.