State troopers responded to the fatal crash, on I-5 near the Highway 530 interchange, at about 9:50 a.m.
The motorcyclist, a 24-year-old Bellingham woman, was pronounced dead at the scene, said Trooper Mark Francis.
He said a car had spun out earlier, hitting the left guardrail, causing a traffic slowdown and backup to form on the southbound lanes of the freeway.
The motorcyclist, on a Honda touring cycle, was traveling at a speed too fast for conditions as she approached the backup in the southbound lanes of I-5. She slammed into the back of a VW Touareg that had either stopped or slowed for the backup, said Trooper Cliff Pratt.
The motorcycle burst into flames and hit the pavement after the crash.
The driver of the VW, a 48-year-old Issaquah woman, was taken to a local hospital to be checked out for minor injuries.
Two lanes were blocked at the scene during the investigation of the crash. All lanes have now reopened.