textproduct: Chicago

KEY MESSAGES

- Hot conditions are expected for much of the week; though somewhat cooler near Lake Michigan.

DISCUSSION

Issued at 135 AM CDT Tue Jul 14 2026

The core of a sprawling mid-level ridge will expand east and build farther across the region today. Thicknesses will increase, supporting a several degree jump in high temperatures compared to yesterday, with upper 90 degree readings in play in and around the more deeply-mixed urban Chicago corridor (including Midway and O'Hare). Based on observed dewpoint trends on Monday, guidance with the strongest PBL mixing schemes (RAP and HRRR) where too deeply mixed, with notable dry biases during the afternoon, while the NAM/NAMNest/NSSL WRF were all a bit too moist. The ECMWF seems to have handled trends the best across the deterministic guidance suite, and have commensurately weighted the latest dewpoint forecast towards the EC solution. This results in dewpoints generally in the low 70s today away from the Chicago core, and this tracks with the idea of somewhat increased evapotranspiration (ET) effects as the surface high builds a bit farther eastward. Peak heat indices will likely be near 100 degrees (locally higher or lower due to proximity to agricultural areas and enhanced ET).

Air temperatures will likely be similar on Tuesday. Dewpoints may tick up a few degrees as the low-level moisture profile is forecast to deepen ever so slightly. It still looks like the NAM remains a bit too moist/undermixed with dewpoints in the mid to upper 70s, with low to locally mid 70s values looking more appropriate given the lack of moist Gulf trajectories. Heat indices are forecast to once again largely top out around 100 degrees, but with the slight increase in dewpoints, could end up seeing more in the way of a smattering of 105 degree readings. If this outcome becomes more apparent and widespread, a Heat Advisory could be needed. At this time, coverage and confidence in 105+ values remains too low/limited to justify an advisory.

By Thursday, the upper ridge will begin to break down and weaken as synoptic troughing amplifies across eastern Canada. As a result, highs will likely come down a few degrees, but low- level moisture will remain in place. Guidance which had previously advertised a lake-enhanced backdoor front rolling through northeast Illinois and northwest Indiana have all backed off on this, with a trend towards a stronger high/ridge axis overhead. It still looks like we'll get a stronger lake breeze/northeast push off the lake, but this may not be until late in the day after peak heating.

This trend brings the potential for one of the three Extreme Heat Warning criteria for Cook County/Chicago (three days of 100-105 heat indices) more into play. Am confident in peak values around 100 today and Wednesday away from the immediate lake influence (more than about a mile to two inland), but whether we see a full third day of solidly 100+ values does still remain unclear, particularly given the potential for temperatures to come down a degree or three. Additionally, this heat episode just doesn't seem to be to the same caliber as the event leading up to Independence Day. I ultimately would like to see a more significant spatiotemporal extent of solidly 100-105 readings to justify hoisting the highest level heat product we have. Given this, and overnight temperatures forecast to dip into the mid 70s, decided to hold off on issuing an Extreme Heat Warning for Chicago/Cook county on this shift, but did give this consideration. If confidence in 100+ heat indices increases for Thursday and peak values increase further for Wednesday, can't discount the need for a Warning.

Regarding lake cooling: based on how temperatures have behaved the last few days, any cooler temperatures will be confined to the immediate lakefront, with conditions warming quickly more than one mile inland. The lake breeze pushes will be directed mostly into Lake County IL, and as such, the lake cooling presence will be even further muted across Lake and Porter counties in northwest Indiana.

Additionally for Thursday, can't entirely rule out some isolated afternoon thunderstorms, particularly in the vicinity of the inland-advancing lake breeze. Forecast sounding still generally looked capped, although not as capped as today and Wednesday. Capped any PoP mentions to slight chance, and suspect things will largely remain dry, but the presence of somewhat deeper moisture does support the potential for some airmass convection in the region.

Finally: numerous wildfires are evident across southwestern Ontario. Associated smoke is largely forecast to remain north of us and aloft through tonight. However, as low to mid-level flow turns more north to northwesterly on Wednesday, there is some potential smoke builds southward towards our region, particularly beyond Wednesday night (per the latest HRRR SMOKE output). No mention of smoke or manually-increased cloud cover yet, but something we'll need to keep an eye on over the coming forecast cycles.

Boundary layer moisture will increase further on Friday and coupled with a weak mid-level wave to our west, could result in scattered afternoon showers and thunderstorms. Very little flow in the column will support slow-moving storms capable of brief locally heavy rainfall and perhaps a gusty downburst wind threat. A series of more pronounced waves will scoot to our north and northeast Saturday into Sunday which will keep at least some low-end storm chances running through the weekend and into early next week.

Carlaw

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 626 AM CDT Tue Jul 14 2026

VFR conditions are expected through the TAF window. Winds are expected to be light west-northwest today. There remains a signal for a weak lake breeze to develop once again midday. However, being so weak, it is possible that it washes out prior to arriving at KORD (similar to yesterday) keeping winds out of the west through the overnight.

DK

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 tonight for INZ001-INZ002- INZ010-INZ011.

LM...None.