Air Force jet fighters are to swoop across Washington skies early Wednesday as part of a periodic training exercise, authorities said.

The flights are scheduled to take place between midnight and 2 a.m. A main purpose is to hone the abilities of the North American Aerospace Defense Command (NORAD) to intercept aircraft intruding in Washington air space, NORAD said.

The exercise, which has been conducted periodically in this area for years, generally involves two or three F-16 fighteers and a Coast Guard helicopter. The hour is chosen to avoid interfering with civilian aircraft operations.