It's pretty in depth, and takes a little to tweak, but when it's done, it gives some damn good detail, and also remains consistent to the weather from the previous days and the season... it's shareware though, and they ask you to purchase it after a trial of 30 days... worth giving a try though.
I have an excellent excel weather generator I downloaded from www.3ednd.com, although I can't seem to find it on that site anymore. It's written for Greyhawk but can be adapted fairly easily. If you're interested, I'll email it to you.