CHICAGO — A powerful storm moving through parts of the Midwest on Wednesday could dump more than a foot of snow in places, leading to the pre-emptive cancellation of hundreds of flights in and out of Chicago and the cancellation of classes by some school districts.

Before the first snowflakes fell in Chicago, airlines had cancelled more than 900 flights in or out of its major airports, O’Hare and Midway. The city had about half of its 300 snowplows on stand-by to aid the evening rush hour. But the heaviest snow was expected to hit northern Indiana, where powerful winds blowing off of Lake Michigan could keep snow piling up past midnight and bury the area in up to 18 inches.