I don't remember the thread or who it was, but there's a poster who has a great set of climate and weather files, if you want to go the complex route.
As a meteorologist, I have a suggestion on how to generate weather. Simply put, pick a real-world location like the campaign area, and use data from that location. For US locations, you can get historic data on National Weather Service websites; left-hand column select "Climate/Local" and then on the tabs that appear, select the "Observed Weather" tab and then "Preliminary...(CF6)" To the right of that, select "Archived data" and a month you think is appropriate.
Why do this instead of random generation? If you're concerned about any sort of realism, the weather needs to have some continuity. What happens tomorrow isn't entirely independent of what happens today. Random weather generation can produce really wacky combinations of lows, highs, winds, and precip on any one day, and the change to the next day can be even more jolting. Most players probably won't care or notice, but it's easier to make up weather than run 8 pages of charts. If you want the realism, just pull the CF6s. (If players in the UK or other countries are interested in that approach, post here and I'll take the time to dig up the equivalent websites.)