CHICAGO – A winter storm dropping rain, ice and then snow as it lumbered across much of Illinois on Wednesday snarled traffic, closed a shopping mall early, grounded more than 1,000 flights, kept thousands of kids home from school and caused rivers and creeks to overflow.

In northern Illinois, the storm that was expected to arrive overnight didn’t hit in earnest until the morning commute hours. But the National Weather Service was calling for perhaps a foot of snow in parts of Lake and McHenry counties and other parts of northern Illinois and as much as nine inches in Chicago.

And with temperatures dropping, and the rain turning to ice and snow, officials were warning commuters to brace for an extremely slow and treacherous drive home on Wednesday night.

“We’ve already got water and ice building up and snow on top of the ice makes for hazardous conditions,” said Illinois State Police Master Sgt. Luis Gutierrez.

Gutierrez said there had already been several reports of minor weather-related accidents by midmorning and the Chicago police said the nasty weather may have helped cause a truck loaded with chocolate to overturn on the Chicago Skyway on Wednesday morning, prompting the closure of two entrance ramps.

The deteriorating conditions came at a time when some communities, including Gurnee in northern Illinois and Peoria in the central part of the state, were reporting that they were running low on road salt.

At Chicago’s O’Hare International Airport, approximately 1,000 flights were canceled as of 4 p.m., and there were delays ranging from 45 minutes to more than two hours for its remaining flights, said Gregg Cunningham, spokesman for the city’s aviation department. Across the city at Midway Airport, there were 100 cancellations and delays on remaining flights of about two hours, Cunningham said.

“Airlines proactively canceled a lot of flights to try to minimize the impact on passengers,” she said.

Meanwhile, many schools, particularly in the northern suburbs, were closed Wednesday and others were closing earlier than normal. Northwestern University canceled evening classes at both its Chicago and Evanston campuses.

Even a shopping center, in the suburb of Vernon Hills, decided to close its doors at 2 p.m. because of the weather.

In central Illinois, the storm hit overnight, with just over two inches of rain falling in Mattoon by midnight Tuesday, flooding streets and basements, as well as cornfields on the outskirts of town. As much as a foot of water stood in fields as rain fell late Tuesday.

By Wednesday morning, the water had receded, leaving the city to clean up.

“We went out and had to clear off some corn stalks because the water carried some corn stalks out onto roads,” city public works director David Wortman said.

Meanwhile, the weather service said there were reports of several roads closed in Iroquois and Benton counties due to flooding caused by heavy rains.

And the weather service on Wednesday issued a flood warning for Champaign and Vermilion counties east central part of the state, noting, that streams and rivers continued to overflow.

The weather service said, for example, that one road near the Salt Fork River was covered with as much as three feet of water.

The story was different in southern Illinois, where things were a bit calmer Wednesday than the day before, when much of the region got drenched by pounding rains.

Cairo, along the Ohio River in far southern Illinois, got an inch of accumulation, Pinckneyville nearly two and a half inches and Gallatin County’s Ridgway Community about 3.7 inches as wind-packing thunderstorms buffetted the area, the National Weather Service said.

Damage was minimal; a camper trailer was reportedly blown across a road into a utility pole near Du Quoin, and there were numerous reports of snapped utility poles and tree limbs.

Associated Press writers Jim Suhr in East St. Louis and David Mercer in Champaign, Ill., contributed to this report.