Sunny, cloudy, windy and stormy all refer to weather concepts, rain and snow are physical weather elements. So you can't say :

 - If it suns
 - If it clouds
 - If it winds
 - If it storms

whereas:

 - If it rains
 - If it snows (hails/sleets)

and you are also allowed:

 - If it is rainy
 - If it is snowy

but I'm not sure if they sound quite right, so instead:

 - If it is raining
 - If it is snowing