Heat advisory as searing heat settles in

The sun beats down on pedestrians on the 11th Street bridge in Grant Park today. (E. Jason Wambsgans / Chicago Tribune / July 17, 2011)

The Chicago area baked in slowly rising temperatures today, as the heat index topped 100 degrees at Northerly Island and outlying weather stations this afternoon.

The heat dome that has settled over the Midwest should bring a hot and sultry week, although passing thunderstorms today and Tuesday could briefly temper the sweltering temperatures.

Those thunderstorms have the potential to be severe based on their history of high winds and hail as they passed through western Wisconsin and Minnesota. The National Weather Service predicts a 40 percent chance of thunderstorms late this afternoon and evening.

A heat advisory is in effect from 11 a.m. to 8 p.m., with temperatures expected to rise into the lower and mid 90s. But the heat index -- how it feels -- could reach 105 degrees today and again on Wednesday and Thursday, the National Weather Service warned.

At 4 p.m., Northerly Island--the former site of Meigs Field--and Chicago Executive Airport in Wheeling both recorded a temperature of 93 degrees, with a heat index of 101. Waukegan recorded 94 degrees, with a 103-degree heat index.

Since 1990, it has been "quite rare" for the heat index to reach the 105 degree mark, the weather service said.

"There have only been 31 days where the heat index officially reached 105 degrees at O'Hare International Airport," the weather service said in a statement. "To find the last hourly observation with a heat index of at least 105 in Chicago, one would have to look back to 6 p.m. on Aug. 2, 2006."

Area residents are advised to drink plenty of fluids, stay in air-conditioned rooms, stay out of the sun and check up on relatives and neighbors. Children and pets should never be left in unattended vehicles under any circumstances, because car interiors can reach lethal temperatures in a matter of minutes.

The hottest weather is expected to arrive Wednesday and Thursday, with the mercury likely to reach 100 degrees in some parts of the metropolitan area.

Severe thunderstorms packing large hail and damaging winds could hit northern Illinois and the Chicago area later today. The chance of thunderstorms and showers is 40 percent, according to the weather service.

Tonight will be partly cloudy with a 30 percent chance of showers and thunderstorms, hazy and very warm and humid with lows in the mid 70s except downtown where it will be around 80.

On Sunday, hot and humid weather surged into the area, sending temperatures to the middle 90s, their highest level in more than a month. It was the city’s hottest day since June 7, when both readings of 96 were recorded at both O’Hare International and Midway airports. 

Much of middle America baked in a heat wave Sunday, and the National Weather Service declared excessive heat warnings in at least 14 states, most of those in the upper Midwest.

Historically, July is the hottest month in the area.

About 37 percent of the nearly 3,000 days in which temperatures reached 90 degrees in Chicago from 1928 through last year were in July, more than any other month, according to data gathered at Midway and analyzed by WGN-TV meteorologists.

And the heat wave by which all other hot spells are measured remains the deadly July 1995 stretch that killed more than 700 area residents.

Forecasters said temperatures this week are unlikely to match those conditions—and no deaths were reported in the current heat wave as of Sunday. 

For continual updates, check the Chicago Weather Center.