The randomness is not just a stream of totally random numbers, it's produced by a coherent noise generator, meaning that next value is highly probable to be near the current one. Currently it simply drives the cloud density, and other values are derived from it. For example, wind strength is computed from absolute cloud density change. That gives a cyclic behavior, though it could be better adjustable if there were additional controls, like for the ratio of good/bad weather.
Anyway, there will be an interface for weather plugins that will allow controlling it in different ways.