Broadband satellites, operating at Ka band and above, are playing more and more important roles in future satellite networks. Meanwhile, rain attenuation is the dominant impairment in these bands. In this context, a dynamic power allocation scheme based on rain attenuation prediction is proposed. By this scheme, the system can dynamically adjust the allocated power according to the time-varying predicted rain attenuation. Extensive simulation results demonstrate the improvement of the dynamic scheme over the static allocation. It can be concluded that the allocated capacities match the traffic demands better by introducing such dynamic power allocation scheme and the waste of power resources is also avoided.