Unrelenting rain sent water rushing into streets, homes and businesses as forecasters warned of more wet weather ahead.
Rain overwhelmed storm drains, leaving standing water in streets and slowing the commute.
A crash in the Battery Street tunnel, one of dozens related to the weather Wednesday morning, closed down southbound lanes of Highway 99.
"It's been dry for so long and peoples' confidence builds up because they've been driving in the dry weather," said JD Elliott, a Seattle Police Traffic Officer. "We're in Seattle, so you have to get used to the rainy weather."
The rain -- some of the heaviest the region has seen in nearly a year -- has also triggered flood watches on some rivers in King and Pierce counties, and a flood advisory for most of Western Washington for urban and small stream flooding. The Skokomish River is above flood stage.
"We have to go back to November 22, 2011 to find a day with this much rainfall at Sea-Tac Airport," said KIRO 7 meteorologist Sam Argier in a blog post.
Up to 3 inches of rain fell on the coast and west slopes of the Cascades while the interior lowlands of Western Washington got 1 to 2 inches. The mountains could get another 1 to 2 inches of rain Wednesday while the lowlands get one-half to more than another inch, the Weather Service said.
More than 3 inches of rain have fallen this week in Seattle. A record 1.36 inches fell Tuesday at Sea-Tac Airport, breaking the old mark for the date of .87 set in 1997.
A small mudslide covered railroad tracks three miles south of Mukilteo and delayed three Amtrak trains, but freight trains continued using a parallel set of tracks along Puget Sound, said Burlington Northern Santa Fe spokesman Gus Melonas.
In Mukilteo, a generator pumping water out of a pond failed around 11 p.m. Tuesday. The failure allowed water into several apartments and damaged thousands of dollars' worth of personal property at the Hamptons Apartments on Harbour Pointe Boulevard.
Firefighters at first said the pump may have run out of gas, but on closer inspection, it became clear that someone had cut the spark plug line on the generator. The maintenance manager said he thinks the generator was sabotaged by someone who was fed-up with the loud noise from the machine.
An armed security guard will be on duty Wednesday night to be sure a new generator keeps the pump running.