Showers and thunderstorms -- and possibly hail -- are expected to continue overnight, with some areas seeing as many as 4 inches of rain, according to the National Weather Service.
"We're already seeing the leading edge move in," meteorologist Ed Fenelon said Wednesday morning. "There's a good flow of gulf moisture heading northward, which is causing the trend."
Reports of pea-sized hail across the area came in earlier Wednesday, Fenelon said.
The city of Chicago's Department of Aviation said many flights at O'Hare were delayed by an hour or more, and more than 300 flights had been canceled. At Midway International Airport, a few flights were canceled earlier Thursday and there were some delays of 30 minutes or more.
As of Tuesday, Chicago's official recording station at O'Hare International Airport measured 2.28 inches of rain so far in April. The average through April 16 is 1.75 inches, Fenelon said.
Average rainfall for the entire month of April is 3.38 inches, an amount that could very well be surpassed by the end of Thursday.
The water equivalent in 2013, which considers the depth of water that would result from melting a snow sample, is 10.87 inches, or well above the average of 7.77 inches.
"We're seeing some river flooding in the area, and these storms will further exacerbate it," Fenelon said. "Lake County is particularly vulnerable."
According to National Weather Service data, the Des Plaines River near Lincolnshire is near flood stage, while minor flooding is taking place further north near Gurnee.
In Gurnee, eastbound Route 132 between O'Plaine Road and First Street is closed and westbound traffic is limited to one lane due to standing and rising water on the pavement.
A report from the National Weather Service Wednesday morning stated that the Des Plaines River near Des Plaines was at a flood stage of close to 4 feet, and it is expected to crest at more than 6 feet around 1 a.m. Friday. The flood action stage is at 5 feet.
The Fox River near the McHenry Dam also is experiencing minor flooding, while the river near Algonquin and Montgomery is near flood stage. As of 1:30 p.m., there were reports that the Fox River was at a 10.21 feet.
Residents in the floodplain are encouraged to set up sandbags, which are available at the Algonquin Wastewater Treatment Facility located at 125 Wilbrandt St.
The Kane County Office of Emergency Management sent out a news release reminding residents to not walk through moving water or drive over flooded roads.
The DuPage County Stormwater Management anticipated it will begin operation of its flood control facilities between 9 p.m. Wednesday and midnight.
Water elevations in Salt Creek and the West Branch DuPage River may rise enough to warrant operation of the county's mechanically operated flood control facilities, including the Fawell Dam in Naperville, the Spring Creek Reservoir in Bloomingdale and the Wood Dale-Itasca Reservoir in Wood Dale.
"We encourage homeowners to keep a close monitor of the situation," Fenelon said. "Rivers may rise quickly."
• Daily Herald staff writer Jessica Cilella contributed to this story.