Raid on the Savage Oasis

From the introduction:

Supernatural storms are common in the Western Wastes and most of them have dangerous side effects. One type of storm, at least, results in a somewhat positive outcome. At random intervals—typically no more than twice annually, but often not reoccurring for several years—massive storms brew in the wastes, dangerous thunderstorms that unleash torrents of rain from clouds that flicker with multi-colored lightning.