It didn't calm down. I worsened the weather, and it went from cloudy, to rainy, to snowy. I had to wait for it to calm down to lightning before I could use the spell. But I recall I have used thunderstorm/call lightning in snow before. I think. Goddam my memory...
Oh, yeah, and contrary to it's name, the weather isn't always snowing in Winter. Well, it may be in the intro to winter bit, where the Drynoks and Razor Clawed Beasts, if anything (not sure on that, I'm going to look when I logon next)...but not anywhere else. 
Dey