This just in from the folks at NORAD:

North American Aerospace Defense Command will conduct exercise flights today as they practice intercept and identification procedures. The exercise will occur over northwestern Washington. Although they are scheduled for the morning, the flights could be delayed due to weather concerns.

The flights are scheduled to take place over northern Washington State and into the Seattle/Tacoma region. People in Seattle and Tacoma may hear and/or see low-flying NORAD-controlled fighter jets in close proximity to a military or military contracted aircraft, which will be taking on the role of aircraft of interest.

In order to test responses, systems and equipment, NORAD continuously conducts exercises with a variety of scenarios, including airspace restriction violations, hijackings and responding to unknown aircraft. All NORAD exercises are carefully planned and closely controlled.