In my limited experience, (3 passing through trips in July / August) it's always rainy and cloudy in the Black Forest, especially in the late afternoon and evening. And it always rains in the Vosges, regardless of time of year or day. Doesn't stop me going back there year after year as I suspect it'll be stunning when I finally get a dry trip there.
Hah! Unlucky.
I was in the Vosges last July. The roads were very slippery at times. Not from rain but it was so hot that the over-banding used to seal cracks was molten. Proper shits you up it does when the front slides half way through a hair-pin. Apart from that it was great as most roads were empty of trafffic, apart from a few cyclists on the Ballon d'Alsace

