[rcxdude]

You'll need to define your parameters a bit more. A simple model of each drop which gives a reasonable approximation of a rainstorm? Probably fairly possible, though far from trivial today (a rainstorm may have ~some trillions of raindrops. We can demonstrably simulate systems with an order of magnitude more parameters today, given simple enough rules). A simulation down to the quantum level of each fundamental particle in the rainstorm? In principle possible given enough time, in practice impossible as the computational requirements (absent quantum computers) are too great. And the 'in principle' is assuming that our understanding of physics at that level is accurate, which it may not be.