I believe all the weather is defined in atmosphere settings, which is global. There might be some NIS (animation) stuff that you could do, but I'm no expert, and I think that is for cinematic..etc.
You could apply the same sort of detailing, as in the frozen rivers on the stock maps. There is quite a bit of SPLT's and splines, in those. You could always add some rocky island in the middle of the lake, even if it cannot be accessed, it would look natural.
Thank You! I appreciate it! That really helps!