Chance For Heat-Related Illnesses Increasing

Chicago is bracing for a dangerous mix of hot temperatures and high humidity through the middle portion of this week as an expansive heat dome builds across the Midwest.

Timing The Heat Dome

Wednesday and Thursday will feature the most sweltering conditions from this heat wave. A heat advisory has been issued by the National Weather Service across Chicagoland for both of those days.

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Highs will rise into the mid-90s, or about 10 degrees above average for late-July, but still short of threatening record highs at O'Hare Airport.

What's adding to the dangerous heat is a high level of moisture in the atmosphere, which is causing the heat index, or how it feels with humidity factored in, to spike into the triple digits across all of northern Illinois and northwest Indiana on Wednesday and Thursday afternoons. In some parts of the Chicago metro area, the heat index could top out between 105 to 110 degrees.

The Cubs will battle the Kansas City Royals at Wrigley Field at 1:20 p.m. CDT on Wednesday, so those in attendance will need to take precautions through the heart of the afternoon. Heat exhaustion has already impacted players at Wrigley Field this season.

Low temperatures won't offer much relief at night and could hold in the upper 70s to near 80 degrees Wednesday night into Thursday morning. The heat index likely won't fall much below 90 degrees.

A strong bubble of high pressure aloft known as a heat dome is the cause of this burst of hot, humid weather.

Sinking air near the center of this heat dome suppresses clouds, allowing the intense sunshine this time of year to heat the ground and air above it most efficiently.

Storms, Temporary Heat Relief To Follow

A cold front will approach northern Illinois by late Thursday, packing both a chance of severe weather and some temporary heat relief.

The greatest chance of storms will be late Thursday afternoon and especially Thursday night. If any storms turn severe they could bring strong, damaging wind gusts. Additional bouts of storms will be possible Friday into the weekend.

Although it will remain muggy, temperatures will drop back into the 80s Friday and Saturday, providing some relief.

July's reputation for heat and humidity won't go completely away as we look ahead. Temperatures will rise back up into the lower 90s Sunday and Monday, with the heat index climbing toward the middle to upper 90s.

Chris Dolce has been a senior digital meteorologist with weather.com for 15 years after beginning his career with The Weather Channel in the early 2000s.