No one was hurt.
The twister struck just after 7:20 a.m. near the Boeing Frederickson plant and caused roof damage at the EnCon and Northwest Door near 190th Street and East Canyon Road.
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.
Also in the tornado's wake, a dozen trees, a Rohn tower and several parking lot lamp posts had toppled, the National Weather Service said. Several empty rail cars were also knocked over in the gusts.
"I could see it, I could hear it," said Mark McNeil who was in a large vehicle outside the EnCon building when the tornado struck. "I looked over and I could see it coming from my right front as I looked through my windshield -- and it was a little one and I could see some debris flying in it, and it came over and took off part of the building to my left."
He said he could feel the strong gusts while inside his vehicle that weighed over 78,000 pounds.
"The wind rocked me around pretty good," McNeil said.
The National Weather Service said the tornado rated an EF-1 on the 6-rung Enhanced Fujita Scale with estimated wind speeds of 110 mph. It was on the ground less than 5 minutes and carved a path that was a mile long and 75 yards wide, the Weather Service said. Damage was estimated at $25,000.
Tornadoes aren't unheard of in Western Washington -- there have been 16 in King, Pierce and Snohomish County since the 1950s, almost all very weak, although this one that struck Kent in 1969 sure turned heads.
Statewide, Washington averages about two tornadoes a year, most in the weak EF-0 to EF-1 category. That average recently was declared up from one a year -- most likely helped by our 1997 season that had a record 14 tornadoes.
See list of all tornadoes to strike Washington
Only four tornadoes ever officially injured anyone (including the aforementioned Kent one in 1969). There has only been one deadly tornado in recorded history in Washington -- an F3 tornado that touched down in Vancouver on April 5, 1972. Six people were killed and 300 were injured in that tornado.
The Associated Press contributed to this report