Should be okay, but it can get very cold at night. I've ridden both the Alps and Pyrenees in mid September and got lucky with the weather both times. Dry and warm by day, but once the sun goes down the temperature plummets. Made an early start in the Croix de Fer, Galibier route a bit challenging.
Thinking about doing the Pyrenees and northern spain in late september. Will the weather be good?
Would be keen to base myself at a campsite for one of two weeks somewhere in Pyrenees, suggestions on where?