The tornado at 7:20 a.m. Monday hit the industrial area of Frederickson, tearing a hole in the roof of the Northwest Door factory, blowing out car windows at a nearby Boeing factory, and damaging a building where sections of a downtown Seattle tunnel project were being assembled.
A team from the Weather Service office in Seattle went to the scene and confirmed the tornado from eyewitness accounts, meteorologist Johnny Burg said.
There were no injuries.
The damage, including a jagged 40-by-40-foot hole in the roof at Northwest Door, stopped work at the factory that makes garage doors. About 100 workers evacuated.
"It looked from the inside like a wave going along. You could actually see the roof flexing," Northwest Door President Jeff Hohman said.
Work at the Boeing plant resumed while repairs were underway. There was no damage to parts or equipment, Boeing spokesman Doug Alder said.
The tornado blew out the windows of about two dozen cars in the Boeing parking lot. Several thousand employees work at the Frederickson site, which makes parts and sections for just about every Boeing airplane, including the vertical tails for the 777 and 787.
The tornado also ripped off one-third of the roof and destroyed a metal garage door at a tent-like structure in Frederickson where a company called EnCon is welding rebar cages for use in the tunnel project under downtown Seattle. Project manager Kasandra Paholsky said the damage forced work to halt but ultimately will not affect the schedule for digging the Highway 99 tunnel.
Washington may get a tornado or two every year, but they are usually small. One of the largest was an F3 in 1972 in Vancouver that killed six people.
Parts of the Northwest got more rain in a day or two over the weekend than typically falls in the entire month.
"We basically had conditions well off shore that were very reminiscent of late fall-early winter," said Dana Felton, meteorologist at the National Weather Service office in Seattle.
With Mondays' precipitation still to be added, it's been the wettest September on record in Olympia and the second-wettest in Seattle.
Nearly 8 inches fell in Olympia, topping a 1978 record and swamping the usual 1.7 inches that fall in that time, the National Weather Service said. Sea-Tac Airport's September total of 5.6 inches came second to a 1978 record, while downtown Portland saw 6.2 inches - the most since record-keeping began in 1872.
Puget Sound Energy had about 12,000 customers out of service late Sunday, the Bellevue-based utility reported. Seattle City Light reported it had about 3,200 customers out of service overnight. Portland General Electric had more than 90,000 customers out of power since the storm began.
The storm brought the first significant snow of the season to the mountains. Forecasters expected 6-to-12 inches by Tuesday morning in the Olympics and 10-to-20 inches in the Cascades.
Gene Johnson, Tim Fought in Portland contributed to this report.