I believe the OP noted she would be traveling Monday afternoon between 4 and 5 pm. I drive often and have been sitting in traffic at precisely 4 pm on weekdays on southbound 405, trying to get to SEA.

I do agree with prismwiz about the other timings. Assuming you have checked-in online, working backward: clear security by 5:45 pm, enter security by 5:00 pm, arrive at airport parking garage by 4:45 pm. Leave Bellevue by 4:00/4:15. If you are confident you can leave by this time, you have a 99.9% chance of making your 6:20 pm flight.

Alternatively, you can book the later flight. AS is very good about placing you on standby for earlier flights. I have flown SFO/OAK/SJC to/from SEA on AS, and have had very good luck snagging a seat on an earlier departing flight. (It is always better than missing your flight due to traffic.) I know the later flight makes for a late arrival home. If you have a changeable ticket, you can always call AS from Seattle... even from the road on your way to the airport... to make changes to your itinerary (before your scheduled flight departs.)