At that time of year, the only places that will not be too cold in the USA would be in the Southwest USA (lower elevations). For mountain biking I would consider Phoenix, Arizona and Las Vegas Nevada. These places sometimes get cold storms but they normally do not last more than a few days.
Southern California could be beautiful, but it could also be wet and rainy, its more unpredictable because we are closer to the coast. If you do decide on Southern California, I would consider the Coachella Valley (Palm Springs, CA) its probably the warmest place with mountain biking in Southern California.
Ciao!