Incoming stormy weather and operational problems caused by a fire last week at a Chicago-area air traffic control facility forced the cancellation of more than 800 flights on Thursday at Chicago airports.

More than 525 flights at O'Hare International Airport, one of the world's busiest, have been canceled and delays are averaging 45 minutes, according to the city's department of aviation.

Nearly 300 flights were canceled at Chicago Midway International Airport and some flight delays were averaging 40 minutes or more. Southwest Airlines, the largest airline serving Midway, canceled all flights after noon.

The National Weather Service said much of the Chicago area would get about an inch of rain from severe thunderstorms on Thursday.

A fire last Friday forced the evacuation of the Federal Aviation Administration control center in Aurora, Illinois, a Chicago suburb, and severely affected air traffic, with thousands of flights canceled in the first two days.

Brian Howard, 36, a field technician, was charged with a federal felony count of destruction of aircraft or aircraft facilities for setting the fire, prosecutors said. He is being held without bond.

The Federal Aviation Administration reported on Wednesday that arrivals and departures at the two airports were running above 85 percent of the average Wednesday air traffic over the past two months at O'Hare and above 80 percent at Midway.

The FAA said technicians were continuing around-the-clock work to restore telecommunications service at the Aurora facility.