The threat of storms was enough for airlines to cancel nearly 800 flights at Chicago's airports Thursday, as last week's fire at the Aurora radar center continues to disrupt air traffic across the country.



As of 3:30 p.m., more than 490 flights were canceled at O'Hare International Airport and nearly 300 flights at Midway Airport, according to the Chicago Aviation Department. Delays were averaging 20 minutes at both airports.



Southwest Airlines announced early in the morning that it was canceling all of its flights scheduled after noon at Midway.



American Airlines began canceling flights Wednesday night and, by noon, had scrubbed 172 flights at O'Hare. "We are trying to be proactive ... so that we can get customers notified and rerouted/rebooked before they arrive at the airports for their flights," American spokeswoman Leslie Scott said.



United nixed 124 departures out of O'Hare today because of "significant weather in Chicago and continued FAA airspace restrictions," airline spokeswoman Mary Ryan said.



This is the first bad weather to roll across the Midwest since the fire last Friday grounded thousands of flights in Chicago and across the country. Repairs to the center won't be completed until the middle of the month, and the air traffic control system is still not at full capacity.



Thunderstorms are expected to hit the Chicago area this evening, according to the National Weather Service. The storms could produce winds in excess of 55 mph and up to an inch of rain, but the worst is expected to hit south of Chicago.



Clusters of storms began moving through the Plains on Thursday morning, mostly in western Illinois and near St. Louis, according to the weather service. The Chicago weather should be mostly dry until around 7 p.m. when storms are expected to move in, the weather service said.



The most severe weather will likely hit south of Interstate 88 and along the Kankakee River, according to weather service meteorologist Rick Castro. The Chicago area is under a "slight risk" for severe storms.



The extremely conservative approach by the airlines has everything to do with the loss of the radar facililty in Aurora, called the Chicago Center, according to the carriers.



Up through Thursday and continuing until about Oct. 13 when Chicago Center is expected to resume operations, the FAA is routing a lot of high-altitude traffic around the airspace normally run by controllers in Aurora. That alone, even in good weather, reduces the number of air corridors available and results in increased congestion along those highly used corridors.



The severe thunderstorms and high winds that are expected to reach the Midwest today will further reduce the options for air traffic controllers to direct planes along less-turbulent paths in and out of airports.



The outlying FAA facilities that are picking up the slack from Chicago Center are not accustomed to working traffic bound for O'Hare and Midway. They are following basic air traffic procedures -- ''nothing fancy,'' said one aviation source.



So some of the special air traffic programs that controllers at Chicago Center and air traffic facilities in Elgin normally employ in bad weather to squeeze extra flights in and out each hour "are not in the current toolbox,'' the source said.



Even during the relatively good weather that coincided with the arson at the Aurora facility and in the five days following, the airlines had canceled thousands of flights because management of Midwest and Chicago-area traffic had to be handed off to FAA radar facilities in Indianapolis, Cleveland, Minneapolis and Kansas City.



Some 85 percent of flights at O'Hare and Midway Airport were canceled or severely delayed last Friday and through much of last weekend. Most O'Hare flights -- 65 percent -- were still being canceled or were hit with lengthy delays as recently as Monday.



The trend continued on a reduced level Tuesday and Wednesday with the cancellation of about 200 flights each day at O'Hare.



It has become the norm in recent years for airlines to cancel flights well in advance of approaching thunderstorms and snowstorms. There are several good reasons to do so.



It allows passengers more time to reschedule trips and avoid the hassle of being stranded for hours at airports where no flights are moving.



It also greatly reduces diversions to other airports of flights already in the air and once the bad weather clears the airlines are usually able to resume normal service quicker because planes, pilots and flight attendants are where they need to be to get back up and running instead of being scattered haphazardly across the system.

