"Dynamic Weather" is a small mod which implements a random chance to change the weather every now and then.

There are nine weather types ranging from nice and friendly to heavy and unfriendly.

There also is a FO3-Version which includes weather themes from Fallout 3 that I personally like.

The official DLC "Honest Hearts" is required.

http://www.nexusmods.com/newvegas/mods/61600.