Demand to/from SEA is very seasonal, however this might also be a result of the fruits of the UA codeshare, as SEA is UA's 6th biggest domestic station IIRC (after the 5 hubs) with plenty of frequent flyers. Also better to use resources to serve SEA where US has a nonstop monopoly. Maybe US should consider adding BOS-SEA (move one of the PIT flights), though this would compete with AS and AA.
Seasonal is an understatement! In January and February, I've often seen A319s with maybe 30 passengers. It's always interesting when an airline needs to move passengers around on a mainline aircraft to balance weight for takeoff.
