textproduct: Chicago

KEY MESSAGES

- Warm and humid conditions will continue through Saturday.

- Smoke persists across portions of northeast Illinois and northwest Indiana through this morning. A renewed push of smoke may be in store for late Saturday and Sunday.

- Isolated to scattered showers and thunderstorms are anticipated this afternoon and evening. A couple of showers cannot be ruled out during the morning as well.

- Additional thunderstorms are possible on Saturday with the potential for strong to severe storms, especially near and south of I-80.

DISCUSSION

Issued at 325 AM CDT Fri Jul 17 2026

The occasional shower has been popping up all night and into early this morning around the outskirts of the Chicago metro area. These are forming along a theta-e gradient and subtle convergent axis encircling northeast IL. This feature is, in part, the leftovers of a lake breeze boundary that produced some isolated showers and storms last evening as it pushed inland. Less than 20 kt of cloud layer flow have rendered these showers very slow moving, if not nearly stationary at times. And while these convective showers have been generally low-topped with no lightning since last evening, around 1.75" of PWAT along with the lack of shear have allowed for pockets of moderate to heavy rainfall with most showers that have popped up. One storm late last evening managed to drop over 2.5" in one hour just outside of our SE CWA boundary and prompted a Flood Advisory immediately east of the Porter-White County line. Over the past few hours, the isolated showers stirring over our CWA have been pulsey and progressive enough where there haven't been any concerns outside of perhaps some localized ponding in low spots.

A broad shortwave impulse in the process of ejecting NE out of the lower Missouri Valley early this morning. A slug of low-mid level moisture is being transported northward into our general vicinity on the nose of the decaying wave and will continue to overspread the area as it gets gobbled up by the upper ridge atop the Great Lakes today. The theta-e advection with the added upper forcing from the wave should continue to promote isolated shower development through the rest of the morning. An isolated thunderstorm or two cannot be ruled out either, especially after dawn as conditions will quickly destabilize between the theta-e advection and diurnal heating. PWATs are forecast to climb as high as 2.25" in our south and southeast CWA by the end of the morning as this wave continues to fill in over the area and latest CAM guidance suggests parts of our CWA, mainly outside of the Chicago metro, may be uncapped by late morning. This will set the stage for renewed shower and thunderstorm chances during the day, especially during the afternoon and evening.

The main low level vort lobe tied to the wave will lay out across the area later this afternoon and interact with the largely uncapped near-surface environment. However, subsequent runs of the RAP and HRRR have been on slight downward trend in precip coverage around the area today, likely owing in part to a modest downtick in the expected strength of the wave and resultant forcing. The medium- range camps are a bit more bullish with coverage, but the 06Z suite has yet to populate as of this writing. Given the low level instability at play, wouldn't imagine it'd be difficult to get a fair coverage of storms later today. Regardless, ultimately the highest coverage and QPF is strongly favored across eastern portions of the CWA, primarily around areas near and east of I-55. In the forecast, broad-brushed chances for scattered thunderstorms during the afternoon. Biggest concerns with thunderstorms today would be heavy rainfall and the potential for localized flooding or ponding, especially if more than one of these slow-moving cells stir over the same area. In other news, it will be another warm one today with temperatures progged in the 80s and lower 90s and afternoon heat indices as high as 100F, but storm coverage may have a say on temperature trends today as well.

Regarding the smoke and air quality, today will begin similarly smokey to the latter part of yesterday. Early morning AQI readings from around the area reside well into 'hazardous' territory and an Air Quality Alert is in effect for today across northern and northeastern IL and northwestern IN. Although, notable improvements are anticipated through the day, especially later in the day, as westerly winds build through the column. Those should push the higher smoke concentrations back off to our east and northeast. The reprieve may be short-lived, however, with another plume progged to overspread the region on Saturday behind a southward-moving cold front which may degrade air quality yet again.

That cold front will drop across the WI stateline during the earlier part of Saturday morning and continue working south through our area into the afternoon. Highly unstable conditions are expected to build ahead of the advancing front. However, the environment ahead of the front looks markedly capped through at least mid-morning, by which time the front should be approaching, if not south of, I-80. Capping should wane ahead of the front into the afternoon and a line of convection is favored to develop atop the area. 3,000 to 4,000+ Joules of MLCAPE ahead of the front may support severe convection despite only around 20 kt of effective shear. Biggest severe concern on Saturday will be damaging downburst winds. A small to marginally severe hail threat may also exist as well as a heavy rain and localized flooding threat. With their newest Day 2 outlook, the SPC has most of the CWA in a Marginal (level 1 of 5) severe risk for Saturday with a Slight (2 of 5) risk for areas roughly near and south of I-80 where the environment will be more supportive as the front moves across than areas north.

High pressure building in behind the front will bring mostly sunny skies to Sunday. However, they may be filtered by that second plume of smoke on tap for the region. Additionally, gusty northerly winds in the wake of the front is expected to bring hazardous swimming conditions to area beaches on Sunday. Another synoptic system with additional chances for strong or severe storms around the region is favored for Monday as an upper trough swings into the Great Lakes. SPC issued a proactive 15% risk across much of the upper Midwest, including northern IL, in their most recent Day 4 outlook. Look for more details regarding this Monday potential in future discussions.

Doom

AVIATION /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 106 PM CDT Fri Jul 17 2026

Key Messages:

- Scattered showers and storms to continue this afternoon.

- Another round of storms likely tomorrow afternoon along a cold front.

Scattered showers have developed across northern Illinois and northwest Indiana early this afternoon and will likely persist in some capacity through the early evening. The tallest of these cells have produced and will continue to produce intermittent lightning as associated updrafts pulse towards and above the 30000 ft AGL level. That said, subpar mid-level lapse rates and the pulse-like character of these cells (due to minimal deep- layer shear) should tend to keep storm sizes somewhat small and storm coverage widely scattered today.

Another round of storms is likely tomorrow afternoon as a cold front drops southward through the area. This round of storms won't be as prolonged as today's thunderstorm window as it will likely only last 1-3 hours as the front passes through, but a few stronger storms with erratic downburst winds may occur at or near the Chicago metro terminals. Westerly winds with 20+ kt gusts are expected prior to the arrival of the front and associated storms. After the storms and front have passed, winds should eventually settle into a northeasterly direction.

Lastly, any lingering visibility reductions from wildfire smoke should end by mid-afternoon today. Another wave of wildfire smoke will attempt to make inroads behind tomorrow's front, but it appears that it may not arrive in earnest until after 00Z tomorrow.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...Air Quality Alert until midnight CDT tonight for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Air Quality Alert until 1 AM EDT Saturday for INZ019.

Air Quality Alert until midnight CDT Saturday night for INZ001- INZ002-INZ010-INZ011.

LM...Dense Smoke Advisory until 4 PM CDT this afternoon for the IL and IN nearshore waters.