pierce, California during Dec is winter time.. ave temperature is around 20 deg afternoon, 10 deg at night and can be cold, especially if u go inland. got several good places to recommend depending on how many days u intend to tour. think 12 days is enough to see half of Ca (starting south Ca, LA or mid Ca, San Fran) can still go on to Las \/egas, Nv and up to Grand Canyon, Ar, before heading back to where u started from.