FEMA drills. I've heard that they're going to focus on the ports and viaduct, so steering clear of those areas for the next week would be good. Traffic might be bad.
http://www.fema.gov/about/training/exercises.shtm#0
The overall exercise scenario unfolds as a Category 4 hurricane threatens the National Capitol Region (NCR) while events in the Pacific Northwest include terrorist attacks involving weapons of mass destruction (WMD) in the state of Washington and an accidental release of a chemical agent at chemical stockpile facility in Umatilla, Oregon. The scenario culminates with the hurricane's imminent landfall combined with a credible terror threat to the NCR which causes the federal government to invoke continuity plans and capabilities.