I've seen a couple different posts about random weather tables, including a thread on Dragonsfoot. My own contribution there was pretty simple: make a reaction roll for the weather. If the weather is hostile, it gets worse, whatever "worse" means for that region and season; Hostile weather in spring or fall means rain, usually, and Very Hostile weather is a thunderstorm. If the weather is Friendly, it eases up; Very Friendly weather will be unseasonably mild and pleasant.
Durations for weather is usually longer for the higher rolls. Hostile weather rolls every four hours, Neutral weather every eight hours, Friendly weather every day. Either Very Hostile or Very Friendly weather rolls every hour. Repeated results generally increase the degree of the weather as well as extend the effect; Neutral reactions keep the weather the same.
I'm kind of wondering, though, if I should add a dicemap for even more options. Perhaps that's overkill. I do kind of like the idea of Very Hostile weather having a 1 in 6 chance of turning into something weird, like a sudden bank of smog that chokes and stings, or actual icicles stabbing down from the clouds. Maybe even some weirder results?