textproduct: Chicago

KEY MESSAGES

- Hot conditions will continue through the week with high temperatures generally in the upper 80s to 90s and overnight lows in the 70s.

- A period of reduced visibility due to smoke may occur on Thursday generally along and northeast of Interstate 90.

- A few storms appear possible on Thursday, with increasing storm chances Friday and then again late Saturday afternoon into Saturday evening.

DISCUSSION

Issued at 214 AM CDT Wed Jul 15 2026

Another hot day is on tap as an elongated, nearly 600 dam 500 mb high remains directly overhead. Little change to column thicknesses today should result in similar high temperatures to Tuesday. The HRRR/RAP/RRFS continue to be greatly overmixed and too dry during the afternoon, while the NAM/NAMNest/NSSL WRF remain too moist. A combination (HREF) performed respectably well on Tuesday, and have largely followed this output for today, which results in dewpoints at or just a smidge higher than Tuesday. One slight wildcard today is the presence of some upper-level smoke (which has taken a circuitous route from southern Canada, across New England, and then back across the Midwest). Progged upper-level smoke concentrations are a bit higher than they were on Tuesday, but at this point, don't think this will be enough to result in meaningful insolation reductions today. The net result is peak heat indices of 100 to locally 105 degrees today away from the lake. Similar to the past few days, the lake cooling footprint will be minimal and tied to the immediate lakeside areas.

Tonight into Thursday morning, low-level wildfire smoke is ubiquitously progged to surge southward out of Wisconsin and into northeast Illinois and parts of northwest Indiana. HRRR and RRFS smoke output suggest the highest near-surface concentrations will be tied to areas near and behind the previous evening's lake breeze, where winds will be light east to southeasterly and a significant subsidence inversion will allow smoke to rapidly build to the surface. Forecast concentrations resemble the June 27, 2023 event, so if guidance is correct, there would likely be notable visibility reductions before deepening PBL circulations help churn up and disperse smoke (to some degree) during the afternoon. At this time, the greatest concentrations are forecast roughly north and east of a Rochelle to Valparaiso line, but will note a lot of this hinges on the lake breeze passage this afternoon and evening and the general smoke evolution upstream today.

Thursday afternoon's forecast gets complicated a bit by the prospect for surface smoke and reduced insolation. Additionally, seeing a decent signal in the guidance for winds to remain light easterly/northeasterly through the morning, coupled with a more robust lake breeze surge midday into the afternoon. All of these could help notably hold temperatures down across NE Illinois and near the lake. Since this is not a high confidence scenario at this time, have only slightly nudged highs down roughly northeast of I-90 into the mid 80s. Less smoke or winds turning offshore during the morning could easily push temperatures back into the low 90s.

Given all of this, and with peak heat indices only touching 100 degrees around Chicago on Monday, am less concerned about flirting with the 3-day Extreme Heat Warning threshold for Cook County. The continued general heat messaging continues to appear appropriate for this stretch.

Low to mid-level flow will begin to turn out of the south during the afternoon as a broad cyclonic gyre that's been present across the ArkLaMiss gradually advances northward through Thursday afternoon and evening. This will push increasing boundary layer moisture northward, and some very modest upper- level jet divergence is forecast to develop as an embedded vort max meanders across northern Missouri. Forcing obviously won't be robust, and soundings still look pretty marginal from a thermodynamic perspective, but this may be enough to facilitate some isolated shower and thunderstorm development during the afternoon, particularly in the vicinity of the lake breeze. Cell motions would be slow, but the lack of stronger shear suggests any storms will remain pulse-like, reducing the residence time of any heavier rainfall rates.

For Thursday night into Friday morning, Environment Canada smoke output suggests another surge of smoke could occur in the region before deeper southwesterly flow gets established over the region on Friday. Have not continued a smoke mention during this time frame at this juncture, but may need to consider this in future forecast packages.

On Friday afternoon, a series of upper-level perturbations are forecast to drift across the region. This should foster higher coverage of diurnal showers and thunderstorms. Cell motions will once again likely be quite slow, and given higher coverage and forcing mechanisms, may result in somewhat more of a locally heavy rainfall threat.

On Saturday, the guidance trend has been towards a slower progression of a robust shortwave embedded within the broad cyclonic flow to our north. While this feature will eventually shove a cold front through the region, it's possible this doesn't occur until very late in the afternoon or into the evening. Ahead of the front, heat indices could push back into the 100 to 105 degree range. Additional thunderstorm chances will accompany the front late Saturday afternoon and evening. With increased deep-layer flow, will need to keep an eye on the potential for some increased storm organization but at this time, the severe threat still appears generally low across the area.

Sunday now looks like it may remain dry as high pressure builds back across the region, with temperatures falling back into the 80s. Another fast-moving disturbance will bring the next chance for thunderstorms to the region during the Monday/Monday night timeframe. This wave is forecast to be accompanied by significantly more flow throughout the column, so one to keep an eye on for a possible stronger storm threat, although uncertainties at this range remain substantial.

Carlaw

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1255 PM CDT Wed Jul 15 2026

Key aviation forecast messages:

- Potential lake breeze this afternoon at ORD/MDW/GYY

- Low confidence in wind trends on Thursday which may impact timing of any VSBY reductions in smoke

West winds around 5-10 kt are ongoing early this afternoon across the area. We are monitoring the lake breeze yet again today with continued lower confidence on its inland extent through early evening. Have maintained the 20Z MDW and 21Z ORD timing for an east wind shift with this update but it remains possible that it stalls just east of the terminals again. Winds then ease with sunset, becoming light and variable through early Thursday morning. Confidence in prevailing wind directions during the day on Thursday remains low, however, with guidance varying from either west southwest to east northeast This would also have implications regarding the timing of any potential VSBY reductions associated with the wildfire smoke to our north which may drift into portions of the area at times on Thursday, though any potential MVFR/near-IFR VSBYs may not arrive until late Thursday afternoon/early evening in the wake of a reinforcing lake breeze.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

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

Air Quality Alert until 1 AM EDT Thursday for INZ019.

LM...None.