Nearly 900 flights have been canceled across the U.S. and another 500 have been delayed due to the large storm system moving through the middle part of the country.

Flight-tracking service FlightAware showed that more than a third of the cancellations were at Chicago’s two main airports. Another large chunk came from Dallas-Fort Worth International Airport; several tornadoes touched down Saturday in the Dallas suburbs.

Other cities with an unusually large number of cancellations included Houston, Denver, Detroit, Kansas City, Missouri, Oklahoma City, Oklahoma and Lubbock, Texas.

A typical day sees about 150 cancelations and 4,000 delays.

Heavy rain and strong winds, like forecast for parts of Missouri, Illinois and Arkansas on Monday, are often more troubling for airlines than snowfall.

(AP)