I've been to the Sierra Nevada. You can expect to see snow there for all of February and March. There is a good chance that you will see snow there in most of January and most of April also.
If you are renting a car ensure that the rental company provides chains for the wheels. If you go up the Sierra Nevada without wheel-chains the local poilice will allow you complete only a fraction of the journey.
Also, I would recommend a visit to Granada and before you go do your homework on the place, otherwise you could miss out. Try and visit Granada while its universities are open as the street cafés are all the more colourful and vibrant then. Don't forget your camera.
