textproduct: Chicago

KEY MESSAGES

- Warm and humid conditions will continue through Saturday.

- Smoke persists across portions of northeast Illinois and northwest Indiana through Friday morning.

- A few showers and storms are possible this afternoon, with scattered coverage expected on Friday. Additional storms may develop along a cold front Saturday, some of which may be capable of strong to locally damaging wind gusts.

DISCUSSION

Issued at 251 PM CDT Thu Jul 16 2026

Through Saturday:

Wildfire smoke from fires in Ontario and northern Minnesota descended into portions of northern Illinois and northwest Indiana early this morning resulting in unhealthy to hazardous air quality conditions. Visibility reductions within this smoke layer have been as low as 1/2 to 2 miles, lowest across far northeastern Illinois and along the lakeshore. There has been some slight improvement on the southwest edge of the smoke layer in part due to diurnal mixing this afternoon. Hi-res near- surface smoke models suggest the smoke layer may slosh back farther inland into this evening though with a reinforcing lake breeze leading to continue hazardous air quality through the overnight hours. Low-level flow will turn southwesterly during the day on Friday which may help lift the smoke layer out of the area and improve air quality gradually through the day.

Diurnal cumulus has developed this afternoon over much of the area south of the area of greatest concentrations of smoke, currently south of a Harvard to Chicago to Valparaiso line. Can't rule out a stray shower or storm within this region of cumulus between now and sunset, though a locally higher potential exists within a low-level convergence axis along a Dixon to Morris to Fowler line. In addition to a few lightning strikes and localized downpours, brief funnel cloud also can't be fully ruled out. Severe weather is not expected and the majority of the area is expected to remain dry.

Looking ahead to Friday, temperatures are forecast to warm into the mid 80s to around 90, warmest in the Chicago metro. Increasing cloud cover and shower/storm coverage, particularly south of I-80 initially, may lead to slightly lower temperatures in those areas. If coverage ends up lower, temperatures and heat indices may end up higher than currently forecast. Hi-res guidance continues to be rather sparse on coverage compared to some of the global guidance. However, forecast soundings are largely uncapped during the afternoon coinciding with a weak mid-level vort pivoting into the area. Have accordingly included "scattered" coverage wording in the latest forecast. Primary hazards with any storms include lightning, gusty winds, and torrential rainfall rates (tropical PWATs to 2"+) which could lead to localized flooding.

Hot and humid conditions continue on Saturday, potentially slightly warmer than Friday, with highs in upper 80s to lower 90s and peak heat indices in the 95 to 105 range. A mid-upper shortwave and associated surface cold front is expected to sweep across the area in the afternoon-early evening timeframe. Ample instability in place ahead of the front (MLCAPE around 3000 J/kg) will be supportive of scattered thunderstorm development along the front as it moves across the area. Given increasing mid-upper flow with the approaching wave, the strongest storms may be capable of gusty to locally damaging wind gusts (severe threat level 1 of 5).

We will also have to monitor smoke levels in the wake of the front into Saturday evening, as it may bring additional smoke back to the surface.

Petr

Sunday through Thursday:

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 /00Z TAFS THROUGH 06Z SATURDAY/

Issued at 634 PM CDT Thu Jul 16 2026

Forecast concerns include...

Low visibility from wildfire smoke. Isolated showers/thunderstorms this evening. Isolated thunderstorms Friday afternoon.

A lake breeze boundary is moving inland early this evening and is allowing a few showers and isolated thunderstorms to develop. These will dissipate with sunset this evening and will remain west and south of the Chicago terminals.

The lake breeze is also pushing the smoke and low visibilities further inland. Prevailing visibilities this evening are expected to be around or just under 1sm and will likely continue in this range overnight. Winds will turn southwesterly by mid/late Friday morning and this should allow for improvement to the visibilities as the smoke shifts back to the east, though only medium confidence and some additional mvfr vis may be needed into early Friday afternoon.

Isolated to scattered thunderstorms are possible Friday afternoon though confidence is low for coverage. As storms form, they will likely create outflow boundaries that will then allow additional storms for develop. Current prob mention handles this potential for now, but as trends emerge, timing refinement can be expected with later forecasts. cms

LOT WATCHES/WARNINGS/ADVISORIES

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

Air Quality Alert from midnight tonight to 1 AM EDT Saturday for INZ019.

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