textproduct: Chicago

KEY MESSAGES

- Hot and humid conditions continue today.

- Scattered thunderstorms are expected to develop along a cold front this afternoon and evening, some of which could be strong to severe. Highest thunderstorm chances exist near and south of I-80.

- Wildfire smoke may return to the area in the wake of the cold front late Saturday and continue into Sunday.

DISCUSSION

Issued at 316 AM CDT Sat Jul 18 2026

Early this morning, conditions are quiet locally with mostly clear skies and mild temperatures in the 70s. A few isolated showers had previously been festering around the area, but those cleared out shortly after midnight. Meanwhile, an MCS is tracking south-southeastward across western WI, in the vicinity of La Crosse as of around 3 AM. This formed late last evening along a cold front that will be dropping into our area from the north later this morning. Latest radar and satellite trends would suggest that this feature should largely miss our area to the west this morning with the track of the thunder and heavier shower coverage trending toward the Mississippi Valley and areas immediately west of our CWA. A line of light showers extends east from the core of the system and is more on track to reach our local area, but it's tough to get a handle on exact trends and model guidance isn't offering much help. The most we should expect out of this complex within our area is perhaps a few showers and maybe an embedded thunderstorm across our northern and especially northwestern CWA shortly after dawn, although there's a good chance this leaves us alone altogether.

This complex, if it even makes it into northern IL, will precede the effective cold front by a couple of hours which looks to cross the WI state line closer to mid-morning. This front will be the trigger for potential severe weather later today as it drops south across our CWA through the afternoon. There's excellent agreement on the timing of the frontal passage, but not so much on thunder coverage. As the front moves across the northern CWA mid-late morning, the environment looks to be capped to ML-based convection. A few camps are resolving some isolated elevated convection during the morning across our north, but most guidance keeps this period dry. Should elevated thunderstorms develop during the morning, it's possible that some stronger wind gusts could punch through to the surface.

Thunder chances will build into the afternoon as the environment continues to destabilize ahead of the front. The cap is expected to break sometime around late morning/early afternoon and storms are expected to blossom along the front as it does. There is good agreement on this happening as the front is roughly between the I-90 and I-80 corridors, so unfortunately Chicago and core of the metro area reside in this area of highest uncertainty with the option to stay dry or to see densely scattered, potentially strong thunderstorms build overhead. Confidence in an uncapped environment and, in turn, thunderstorms becomes rather high as the front moves south of I-80. The line will then exit our area to the south near the start of the evening.

Effective shear will be meager this afternoon, on the order of 20 to 25 kt. But over 3,000 Joules of MLCAPE ahead of the front with deep layer moisture and tall ELs will promote deep, robust updrafts capable of producing damaging downburst winds and perhaps small to marginally severe hail. Low level hodographs do not look very supportive of a tornado threat. And while coverage looks pretty tightly confined to the front with the storm threat only lasting as many as a couple of hours over any given area, slower storm motions (toward the south at around 20 to 25 mph) and well over 2" of PWAT will allow for periods of very heavy rainfall and perhaps some localized flooding. Just about all latest CAM guidance depicts pockets of 1 to 2" of QPF over the course of a couple of hours, largely south of I-80, which would likely lead to some ponding or nuisance flooding in spots. In the newest Day 1 Outlook, SPC expanded the wind-driven Slight Risk westward to include all areas roughly along and south of I-80 with a marginal risk up to the WI state line.

The smokey conditions and air quality have greatly improved since 24 hours ago. However, north and northwesterly flow in the wake of the cold front will introduce another plume of smoke to the region late tonight into Sunday. Luckily, the latest HRRR and RRFS smoke forecasts anticipate smoke concentrations far less than what we saw on Thursday and early yesterday. Nonetheless, look for more haze and impacted air quality again on Sunday.

Relative high pressure will slide into the upper Midwest for Sunday and should provide mostly sunny skies to round out the weekend, with the possible exception of a filter of haze. After seeing upper 80s and lower 90s again today with heat indices pushing 100F, more comfortable conditions are expected for Sunday with high temperatures in the lower and middle 80s and lower humidity. Additionally, high waves and dangerous currents are expected to develop on southern Lake Michigan late today and into early Sunday. Accordingly, issued a Beach Hazards Statement around the area for dangerous swimming conditions from this evening into mid-morning Sunday with conditions looking to ease through the rest of the day.

Attention then turns to the Monday and Monday night period with the potential for additional severe convection around the region. A shortwave impulse scooting across the Plains will phase with a strengthening upper trough swinging into the upper Midwest and a surface cold front is expected to pass through the area. A lot of uncertainty revolves around this system with a number of medium range camps even resolving two separate cold frontal passages: one during the daytime and a second late Monday night. For that reason, we could be looking at multiple rounds of thunderstorms, although some models only want to convect along one front and not the other. Despite the traditionally less favorable time of day, the latter potential harbors the seemingly greater severe threat as it will be more strongly forced by features aloft and working with a much better shear profile than during the daytime. But for now, there's too much uncertainty to say much else, so stay tuned as the picture comes into focus over the next couple of days. SPC has introduced a Day 3 Slight Risk to our northwestern CWA and a Marginal Risk elsewhere for Monday.

Doom

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/

Issued at 1211 PM CDT Sat Jul 18 2026

A complex of storms continues to roll through western Wisconsin just east of the Mississippi River well ahead of a cold front stretching from central Lake Superior through southeastern Minnesota. While the leading edge of outflow has surged ahead of the main convective body at press time, gradually increasing 925-850mb east-northeasterly flow acting upon the western side of the cold pool may allow for convection to fester on the western side of the complex near the Mississippi River as far south as the Wisconsin/Illinois/Iowa tri-border region. Should this occur, a northwesterly wind shift at the least could reach RFD in the 10-13Z timeframe. All things considered, did decide to toss in a PROB30 for for thunder and a northwesterly wind shift at RFD later this morning. Elsewhere, quiet conditions are expected at the terminals overnight with clear skies and a southwesterly breeze.

After daybreak, west-southwesterly winds will increase quickly with gusts in the lower 20kt range expected by mid-morning. The aforementioned cold front is expected to sweep southward across the airspace from late morning through early afternoon leading to a northwesterly wind shift at RFD. Winds at DPA/ORD/MDW/GYY are quite a bit trickier as a lake component of the front may surge inland just as the synoptic front arrives. Can easily envision a scenario where there is a northwesterly wind shift quickly followed by a northeasterly wind shift, generally favored in the 18-20Z time window. Thereafter, northeasterly winds should prevail through the remainder of the TAF period.

Convection is expected to develop along the front by early tomorrow afternoon, though exactly where convection initially develops (I-88 or I-80 corridor?) remains an item of lower confidence. With a zone of focused confluence over the Chicago metropolitan area between the synoptic front and lake front, do have concern convection could very well develop directly over DPA/MDW/ORD/GYY. So, will maintain the inherited TEMPO groups in the 18-20Z time window. Meanwhile, at RFD, confidence is increasing in a dry frontal passage. Will maintain the inherited PROB30 for now and will reevaluate whether to go dry in the 09Z AMD or 12Z TAF package.

Finally, another period of wildfire smoke is expected behind the front from tomorrow evening onward. Experimental HRRR-smoke output suggests the smoke will reach ORD/MDW/GYY/DPA in the 00-03Z timeframe and RFD in the 03-06Z timeframe. Will maintain the inherited 5SM visby and let upstream observations inform any (presumable downward) adjustments in later TAF packages.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

IL...Air Quality Alert until midnight CDT tonight for ILZ005-ILZ006- ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106- ILZ107-ILZ108.

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

Air Quality Alert from 1 AM Sunday to 1 AM EDT Tuesday for INZ019.

LM...None.