I'm going to have to face this issue in about a month or so. Up here on the eastern slopes of the Cascades (PNW), it doesn't rain in the summer. OK, we had ONE rain storm during the summer this year, but essentially, it's totally dry.
The end of September brings a whole different scenario; rain. Frequent rain. For weeks at a time. And it gets chilly.
I've not been as active in cycling in the last 30 years or so as I am now, so I've got some interesting times ahead. Usually, I just stop riding and hit the rollers once in a while, but that hasn't been as frequent as it should have been.
I'm considering setting up my ancient mountain bike with slicks so I can still get some miles in and not trash out my nice road bike on the wet roads. Later in the fall/winter they salt the roads which will ruin a bicycle in short order. During those times, it's rollers or nothing.
So, I've got some decisions to make for sure!