Hello
I know it may be a bit late, but i'm a local there and i can tell you to check out meteo.hr for croatian forecasts. I prefer to use the weatherpro app on my phone, they are really really good, and yr.no has good forecasts for our area too.
It will really depend on where you're going actually. the islands tend to have shorter showers and more sunshine than the mainland. So, for instance there may be a lot of rain in Dubrovnik but the nearby Peljesac peninsula will be dry and sunny.