In a post at Gaming Ballistic, +Douglas Cole mentions the use of real-world weather records for your in-game weather, which is what I do. The game is set in Lodea, which is more or less like Italy in climate. I set game year 850 to be equivalent to real-world year 1990, and use Rome for the center of the country, Palermo for when the party is in the south, and Turin when they're in the mountainous north. (Scroll down to find the "History & Almanac" box on the left where you can look up weather records going back decades.)
You can check the temperature, wind, precipitation, and so on, and also the time the sun and moon rise and set, which has been important on several occasions in my game. And if anyone ever casts Predict Weather, I'll have answers ready at hand.
My game's world, Mond, just happens to have a year that is exactly as long as Earth's, so this works out just fine for me. If your world's year is a different length, you can cut out some day to make it shorter or repeat some days to make it longer. On Earth, about two-thirds of the time, tomorrow's weather is pretty much the same as today's, so this is reasonably realistic.
Today was a game day. Hopefully I can get a write-up of the session done tomorrow.