Several hours of severe thunderstorms with heavy rains are likely across northern Illinois Thursday evening and overnight into early Friday morning.
All types of severe weather are possible with the storms. Strong winds over 60 mph are the primary threat, but large hail up to half-dollar size and an isolated tornado cannot be ruled out, according to the National Weather Service.
The storms are likely to move into the area from the west and northwest after dark Thursday, but some storms could develop after 7 p.m.
Much of the Chicago area is in the slight risk category, according to the Storm Prediction Center. Areas including McHenry and DeKalb counties and points to the north and west are included in the enhanced risk category. The enhanced risk is the third of five levels of risk for severe thunderstorms.
“These are a little bit more serious if they develop,” said Brian Leatherwood, a meteorologist with the National Weather Service in Romeoville. “There is enough energy in the atmosphere that if a storm develops, it could be pretty severe in nature.”
Temperatures across northern Illinois were in the mid-80s Thursday afternoon with some areas approaching 90 degrees. The amount of moisture is expected to increase in the atmosphere throughout the day, fueling storm development and increasing the potential for heavy rains possibly leading to flash flooding.
“There’s going to be lot of moisture and the ground is very dry, so when rain hits it might take a little while to absorb. [That could lead] to run off,” Leatherwood said.
Much of northern Illinois suffering from a severe drought and has seen little rain over the past several months.
Leatherwood said 2 to 3 inches of rain is possible across the area, but exact amounts will depend on exactly where storms track.
“There could be areas with some storms that could easily drop 1 to 2 inches per hour,” he said.
The National Weather Service advises people, and especially motorists, to stay away from flooding and to not enter flood waters, even if it appears passable.
Leatherwood said meteorologists are expecting a larger complex of thunderstorms and not a derecho – an arched thunderstorm complex with high winds that can cause significant damage like the one that passed through Iowa and northern Illinois last August. He said forecasters will be watching the storms closely as they develop for a possible increased high wind risk, however.
Some storms could linger into Friday morning, especially south of Interstate 80 in Will and Kankakee counties. The potential for more severe storm development will carry over into the day Friday for areas south of I-80 as well.
Temperatures Friday are expected to top 90 degrees across the area with higher humidity levels. More comfortable temperatures will return for the weekend with a new chance of showers and thunderstorms on Sunday.