September to mid-November will be sunny and mostly warm, like 17-22 degrees (Celsius) each day. Not much rain.

Toward the end of November to February it will be cooler - think 12-15 degrees for the high temperature each day. It will rain more often but in the three years in which I lived in Lisbon I never wore rain boots. (In other words, it's not going to be flooded.)

I mostly wore light jackets when outerwear was required.