http://www.wsdot.wa.gov/Northwest/King/ExpressLanes/Weekend.htmThis is study that WSDOT did several years ago into Saturday congestion on I5, and the effect of a later change of the express lane direction. Lots of graphs on traffic volumes through out the day.
Apart from the fact that afternoon traffic is heavier than morning, I wonder if there was some event that affected your north bound trip, such as construction or ball game. The fact that there were backups as far south as Seatac and north as Lynnwood suggest that more than the express lane direction was involved.
Look around the WSDOT site. There may be other information on historic travel times. Google Maps also presents travel time estimates, though I don't know if it makes a distinction between weekday and weekend.