Prevent from switching Wi-Fi networks

I have two wifi access points at home: "downstairs" and "upstairs". They're both WPA2.

If I'm upstairs and connected to "upstairs" and my laptop goes to sleep, when I reactivate it, it connects to "downstairs" as it is higher in the list, but the signal is too low and I have to switch it manually to "upstairs". How can I prevent that?

Remark: I don't want "upstairs" or "downstairs" to have priority: it depends on where I stand in the house. I just OSX to stick with the network I have selected, whichever it is. I don't want to re-type the lengthy password all the time neither ("forget network" is probably not the way to go).

Script or third-party program welcome...

^{OS X 10.10}