textproduct: Chicago

KEY MESSAGES

- Warm and humid conditions will continue into Saturday with peak heat indices around 100 degrees at times.

- A period of reduced visibility due to smoke is expected today and tonight, generally along and northeast of Interstate 90.

- A few storms are possible this afternoon, with better chances Friday and then again late Saturday afternoon into Saturday evening.

DISCUSSION

Issued at 213 AM CDT Thu Jul 16 2026

The main focus this morning is on the incoming plume of wildfire smoke, which has crept in along a lake-enhanced front. This boundary continues to slowly shift southwestward, but has become increasingly diffuse/washed out in the vicinity of a Freeport to Joliet to Valparaiso line. In the wake of this boundary, smoke is gradually working its way westward and towards the surface. Backward model trajectories continue to indicate the primary source region for near-surface parcels later this morning originating primarily from western lower Michigan where visibilities continue to slowly fall towards 2-3 miles. Recent webcam imagery from Chicago does reveal a slowly- worsening pall drifting off the lake though, and have been watching visibilities slowly work their way towards 4-6 SM near the lake.

Observations from Wednesday afternoon across central Wisconsin showed widespread BKN-OVC ceilings with cloud layers around 3000 feet. HRRR/RRFS smoke output suggests slightly lower total smoke concentrations in our area, but given the footprint of the observed ceilings, pushed sky cover grids towards the partly sunny range across northeast Illinois and northwest Indiana through the day. It's unclear to what extent (if any) this will impact surface insolation and subsequent high temperatures. With the notably thicker plume across central lower Michigan on Wednesday, observed high temperatures were some 4 to 9 degrees cooler than guidance. At this time, don't suspect things will be thick enough to result in that drastic of a temperature difference, but do think the NBM delivered highs in the mid 90s were a touch overdone. Additionally, onshore flow will prevail to start the day, which is a big change from the past few mornings. This will allow for a notably more expansive lake cooling footprint today, which will be reinforced by an additional lake breeze push this afternoon. Lakeside highs are expected to be in the lower 80s today. Peak heat indices will still likely flirt with the century mark, but mainly south and west of a Rockford to Aurora to Kouts, IN line.

Boundary layer moisture will pool along the aforementioned quasistationary boundary this afternoon. Assuming smoke isn't so dense that it stunts insolation, we should see an increase in cumulus with heating. Forecast soundings do show the presence of a bit of a "CAPE robber" upstairs today (the previous subsidence inversion which is gradually being lifted and cooled) which will reduce overall instability a bit and may tend to keep some degree of inhibition present. That being the case, convergence on this axis looks respectable, and modest upper- level divergence is forecast to develop on the southern flanks of a jet streak situated to our north. With otherwise negligible large scale forcing but a moist and generally uncapped environment, isolated airmass-type convection will be possible this afternoon in the vicinity of a Rockford to Joliet to DeMotte, IN line. Skinny CAPE profiles and the previously mentioned mid-level warming may work a bit against lightning production, but guidance is nearly unanimous in developing at least isolated activity during peak heating. Little/no shear will result in any updrafts quickly raining themselves out, perhaps with some locally gusty winds. Finally, given the forecast steep low-level lapse rates, light mid-level flow, and environmental vorticity along the surface boundary, can't rule out the potential for a few funnel clouds today.

Lingering activity should tend to diminish this evening but may continue to fester in the moist environment. Smoke output depicts a potential secondary surge across northeast Illinois and northwest Indiana this evening and overnight, and have continued the smoke mention in the gridded forecast through Friday morning. By Friday afternoon, boundary layer flow will turn southwesterly, and should gradually push lingering near- surface smoke back towards the lake.

Tropospheric moisture is forecast to increase further on Friday, with PWATs over 2 inches across a broad swath of the CWA. While most CAM guidance remains decidedly unimpressive regarding convective coverage, do continue to wonder if coverage will end up a bit higher given the eastward advancement of a low- amplitude shortwave into this richly moist environment. At least scattered showers and storms are expected to develop midday into the afternoon. Slow storm motions, deep warm cloud layers, etc. will lead to a threat for heavy rainfall and localized flooding.

A cold front will then move through the region sometime Saturday afternoon and evening. Ahead of the front, afternoon heat indices may once again push into the 100 to locally 105 degree range as dewpoints rise into the mid to possibly upper 70s. While the bulk of the large scale ascent will slide into Michigan, height falls/DCVA with the frontal passage may be sufficient to ignite isolated to perhaps widely scattered convection during the afternoon. Instability is forecast to be fairly robust, with MLCAPE values around 3,000 J/kg (or more) along with a hint of additional mid/upper level flow. Could envision a multicell cluster or two with a potential localized wind threat as a result, but still quite a ways out with a good deal of uncertainty, and ultimately expect the greatest coverage to our east.

Northeasterly winds will increase Saturday evening which will lead to at least a moderate swim risk. Whether waves build further remains a bit unclear due to a significant spread in the low-level wind field.

Sunday looks like a nice day with seasonable temperatures in the mid 80s (a bit cooler near the lake) and more comfortable humidity levels. A stronger synoptic-scale trough will then move across the region Monday afternoon and evening, sending another cold front through the area. Saturday's front will only send the low-level moist axis into Iowa, so this may not have much of a problem surging back east across the forecast area on Monday. Dewpoints are progged to rise back into the mid to upper 70s ahead of the front while a plume of steeper mid-level lapse rates build in aloft. While capping could pose a challenge to convective development, significant flow through the column, coupled with building instability suggests this will be a period to monitor closely for more of a strong to severe weather threat in the region. Depending on timing, the threat for storms could continue into Tuesday.

Carlaw

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 609 AM CDT Thu Jul 16 2026

Key Aviation Messages for the current TAF period:

- Smoke from wildfires is reducing visibilities across the region

- Isolated shower/storm chances this afternoon, but chances at any specific terminal remains less than 30 percent and thus kept out of the TAF

Vis sensors continue to drop this morning with webcams showing the smoke spreading over northern Illinois. Most obs north of a KRFD to KGYY line are 3 to 5 SM; however, localized obs at KMKE and KRAC are down to 1SM. There is a chance that diurnal mixing helps improve things slightly during the day, but the TAFs were written more conservative and kept MVFR vis (localized IFR through at least 15Z) from smoke through the day. Gradual improvement is expected through the evening, but with winds becoming light, there is lower confidence in exact improvement timing. Better chances for a return to P6SM tomorrow by mid morning as winds turn to the southwest.

Winds will remain light and northeasterly through the current TAF window, up to 10 knots in the afternoon. Light southwesterly winds are expected tomorrow morning.

There remains a non-zero chance for daytime showers and storms this afternoon along a moisture axis to the west. Any storm that develop would have a very slow storm motion to the east. Coverage should remain isolated, so the chance for a shower or storm at any particular terminal is too low for formal TAF mention. Radar trends in the afternoon will need to be monitored for quick AMDs if needed.

DK

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 midnight CDT tonight for INZ001-INZ002- INZ010-INZ011.

LM...None.