While weather was implemented in both vanilla WoW and The Burning Crusade, most of the Wrath of the Lich King zones and all of the new Cataclysm zones did not have weather changes. Having rain in the MoP areas will be a nice addition to an already visually gorgeous expansion.
A few forum posters have reported seeing weather in some zones, but as it is up to the RNG, your mileage may vary.
Have you seen any precipitation or other meteorological events in the latest beta patch?
It's open warfare between Alliance and Horde in Mists of Pandaria, World of Warcraft's next expansion. Jump into five new levels with new talents and class mechanics, try the new monk class, and create a pandaren character to ally with either Horde or Alliance. Look for expansion basics in our Mists FAQ, or dig into our spring press event coverage for more details!