Boy, its a risk. You could get lucky and have a beautiful ride. Or you could get hit with a series of storms. Personally I would never do it at that time (The weather and the days are short). I live just south of San Francisco and February and March can be very wet. Not trying to be a naysayer... I just want you to know what your getting into. The leg from San Diego to Lee's Ferry would probably have better weather. Sounds like you have a Grand Canyon River run in line (I am on the waiting list).
Good Luck
Rob