2 Dual Band AEBS, Roaming On, but no Auto Switch from MB Pro?

I'm curious to see if anyone can help. I have two Dual Band Airport Express routers, one set up as a router and another on bridge mode. They have the same wireless name/password, wireless security setting, but different channels. My understanding was that any computer that connects would have the strongest signal strength available, meaning, my laptop would automatically connect to whichever router was closest. My macbook pro, unfortunately, is not doing that. If I connect to a router upstairs, and then move downstairs, even directly next to the downstairs router, my laptop will still be connected to the wireless router upstairs instead of automatically switching.

Is there some easy setting I'm missing on my MBP or airport base stations to make this transition smoother? Or is it this way simply because the wireless coverage overlaps? If I turn off my airport on my MBP and then turn it on, then will find the closest router and connect to that, but it will not automatically switch.

Any ideas?

Message was edited by: Yaser Herrera