All of northern Illinois is at significant risk for thunderstorms Thursday night that could drop as much as 2 inches of rain per hour, overwhelming the dry soil and causing flash floods, as hail, 60 mph winds, lightning and possibly tornadoes also hit the area, forecasters said.
The storm system will move in from the west and eventually cover all of the state north of Interstate 80 and east into Indiana, beginning as early as 7 p.m. Thursday, according to Brian Leatherwood, a meteorologist with the National Weather Service. Areas to the northwest should expect the thunderstorm to strike earliest, with the Chicago area being hit closer to midnight, he said.
“It looks like conditions are fairly ripe for our first severe weather event in some time developing tonight,” Leatherwood said. “It’s a rather wide area of precipitation we’re looking for, it’s easily going to cover most of our watch area.”
Leatherwood said silver dollar-sized hail is possible, and the weather service’s website categorized it as the possibility of pingpong ball-sized hail. Along with that, there will be “frequent lightning,” winds as strong as 60 mph, an elevated “fire weather risk,” and Leatherwood said forecasters “cannot rule out” the potential development of a tornado or two.
But a chief concern is flash flooding, Leatherwood said.
“Since we’re in a drought, the ground is solid and dry. It would be nice if we could get some light showers first, to moisten up some of the soil, but it doesn’t look like that’s going to be the case. We’ve got our fingers crossed, but it looks like it’s going to come down fairly hard and therefore the water’s going to run off fairly quickly because of the dry, hard ground,” Leatherwood said.
Forecasters expect the rain to fall at a rate of 1.5 to 2 inches per hour in the hardest-hit areas.
“If you happen to get underneath one of these core storms, we think the rain could fall as fast as 2 inches an hour. For total precipitation for the event, some places might get under these cores and easily see 2 to 3 inches for their heaviest totals,” he said.
The system should be out of the Chicago area by 5 or 6 a.m., forecasters said. But the moisture and humidity won’t necessarily relent. The city could reach a heat index of 100 degrees Friday.
“All those things combined are just setting us up for a heat index approaching 100,” Leatherwood said. “It will probably still be in the mid-90s, but approaching 100, definitely. Even with all the cloud cover, it’s not going to be comfortable. It’s going to be just very muggy out there.”
Meteorologists said the area may get a reprieve Saturday when the heat index will drop and the air will be a bit drier. The high temperature should be in the mid-80s.
But by Sunday, forecasters expect another round of thunderstorms.
For more information, visit the Tribune’s weather page and check back for updates.
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