It's always partly cloudy around Fort Dawnguard! Always! The weather never changes...
I've always wanted to look at the castle in different kinds of weather and light, but it's always partly cloudy. No rain, no nothing. Well, actually, once I exited the castle and it was raining, but after 3 seconds turned back to clouds.
It's always like http://cdnstatic.gamesas.com/akqacms/files/tes/screenshots/FortDawnguard.jpg. And no, Clear Skies is only effective with a 100% cloudy sky.