Flood warning for Fox River issued from Johnsburg to St. Charles; winter weather advisory for McHenry County

The view from Bob Letmonkski's deck on Mineral Springs Drive, looking across the Fox River on Sunday, Oct. 1, 2023. He and other family members watched Saturday at a boat hit the shore next to the blue house and flipped, killing the two people aboard.

A flood warning has been issued along the Fox River from Johnsburg to Red Gate Road in St. Charles starting Tuesday night and will continue “until further notice.”

At the same time, a springtime winter weather advisory has also been issued for parts of northern Illinois through noon Wednesday, bringing with it a potential for two to four inches of snow.

The National Weather Service issued the advisory effective 4 p.m. Tuesday through noon Wednesday for McHenry, Ogle, Lee, DeKalb, Winnebago and Boone counties, including the towns of Crystal Lake, Algonquin, McHenry and Woodstock.

The Fox River is forecast to get above flood stage early Wednesday morning and continue rising through early Sunday afternoon to 10.5 feet at Algonquin, according to the National Weather Service, with potentially affected areas in McHenry, Lake and Kane counties. At that level, low-lying sections of Cornish Park in Algonquin will be inundated, the weather service says. As of Monday evening, the stage was 8.7 feet. Flood stage is 9.5 feet.

A flood warning “means water levels above flood stage are imminent or may already be occurring. Persons along rivers and streams in the warned area should take immediate precautions to protect life and property,” according to the National Weather Service.

“Moderate” flooding is forecast. Additional rises in the river stages are possible after Sunday afternoon, depending on updates in the forecast. That forecast includes steady rain Tuesday that could change entirely to snow Tuesday evening along a line from LaSalle to McHenry counties.

The Illinois Department of Natural Resources reported that Fox Lake gauge is predicted to crest at almost 5 feet on April 8, while the Lower River was forecast to peak at about 4.5 feet at the McHenry Tailwater gauge at the same time.

“Every attempt will be made to prevent impacting structures on the Lower River; this may result in lake levels above 5.0′ at Fox Lake. As forecasts change, all attempts will be made to keep water levels as low as possible and prevent damage of structures through the entire system,” IDNR reported Tuesday.

“Adjustments will be made in an attempt to maintain the Lower River” below 4 feet on the Stratton Lock and Dam near McHenry,” the agency said, noting the seven-day forecast shows 2 inches of rain for the Chain O’ Lakes.