From SDOT, which says this might be a two-night closure:

The contractor working for the Seattle Department of Transportation (SDOT) to widen the Spokane Street Viaduct will close the westbound lanes of the structure tonight and tomorrow night. Crews plan to work from 10 p.m. tonight until 5 a.m. tomorrow, November 30 and if necessary also work from 10 p.m. tomorrow, November 30 to 5 a.m. Wednesday, December 1, to completely remove temporary supports under the structure. During this work, southbound I-5 traffic will be directed to the Forest Street exit to access surface streets. Northbound I-5 traffic and westbound traffic from Columbian Way will be detoured to surface streets at Sixth Avenue South.

The Spokane Street Viaduct is the section of the east end of the West Seattle Bridge between I-5 and Highway 99.