From staff reports:

The Washington State Department of Transportation will close the two right lanes of westbound Highway 18 between Weyerhaeuser Way and I-5 from 10 p.m. to 5 a.m. Monday and Tuesday (Jan. 11-12), reopening Wednesday morning (Jan. 13). The ramp from westbound Highway 18 to northbound I-5 will be closed at the same time. A signed detour will direct drivers to the southbound I-5 exit, then to South 348th Street and eastbound Highway 18, then back to northbound I-5.

The closure is due to construction involving the Olympic pipeline, according to WSDOT.