Sunny, cloudy, windy and stormy all refer to weather concepts, rain and snow are actual 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

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