I've tried a couple of weather generators in the past and I've always had issues with them being a little too random, and usually fantastical. The possibility of a freak snowstorm in the summer and tornadoes in the winter just don't sit right with me. Sure, you can always just ignore those results, but if you random generator isn't "fire & forget" than you either need to tweak your parameters or get a new generator.
Now I loves me some random table generators, but I don't necessarily like to spend time creating or tweaking what feel, to me, to be tables of secondary (or even tertiary) importance. I'd rather just come up with something that works.
What works for me, and this is the GM Prep tip portion, is to equate fantasy locations to real-world locales and come up with a rough conversion of the fantasy calendar to our real-world calendar. When these rough conversions/translations are done, then I can just look up the real-world data and translate to the fantasy world. Mother nature is a wonderful randomizer and often someone has already taken the time to record her activities.
Weather Underground is a decent enough current weather site, but the gem of this website, which is the Free GM Resource today, is the historical weather section. Let's say my players are travelling in the fantasy analog of Schweinfurt Germany and I'll say that it is the equivalent of the middle of March. I'll just pick an arbitrary year, let's say 1997.....input the info and....well it'll shift to the closest city it has info for:
Now obviously I cleaned up the adds and the historical data will probably only go back so far, but there is more information than you probably need to run your game. There is just so much good info here. If you want, a better look at this particular page is here.
I love the fact that you can look at this data several ways and if you really wanted to go nuts you could probably use this data to build your own weather generator for just this city....now that is way too much work, but you could.....