I'm thinking about visiting Croatia in September what is the weather like degrees wise?
Well weather is pretty varied and unpredictable everywhere, it seems, but we were there in September and it was very nice. Mid 70s so I wore capris and short sleeve shirts.
What about at night ?
Sweater...I had a polar fleece jacket with me because we were headed north to the Czech Republic. We reached Prague in October and the weather had turned up there - I wore my polar fleece every day up there but didn't use it at all in Croatia. Mid-Sept in Dubrovnik and Zagreb was nice.
If you go to RS tours and click on "Best of Eastern Europe" and then scroll down to the "Weather" listing and click on that. Then scroll down to the Croatia graph, that will give you an idea of the "average" weather every month.
I was on Hvar last September for three weeks. It was c 26/28 degrees most days, with a few rain showers, but the weather broke c 23 September when it dropped to 18/20 degrees, blanket cloud for several days and heavy rain showers. I followed the weather forecast for a week after I returned home and recall commenting that we had left at the right time. Of course, this may not happen every year, but there is a reason why many restaurants hotels etc close at the end of September.
Evenings until the daytime temperatures dropped were plenty warm enough to wear shorts, although there were many mosquitoes around, so we ended up wearing long trousers to avoid being nibbled!