press release:
Seattle Department of Transportation’s (SDOT) contractor for widening the Spokane Street Viaduct will close both westbound lanes of the structure between I-5 and SR 99 each weeknight next week. Closures will begin at 10 p.m. and wrap up by 5 a.m. the next morning in each instance.

During the overnight closures, southbound I-5 traffic will be directed to the S Forest Street exit to access surface streets leading to the lower Spokane Street Swing Bridge. Northbound I-5 traffic and westbound traffic from Columbian Way will be detoured to surface streets at Sixth Avenue South to the detour route leading to the swing bridge. Signage will be posted to direct drivers along these routes.

Motorists will also be able to use southbound SR-99 to access the high-level West Seattle Bridge.

See http://www.seattle.gov/spokane for more details on this project.