During periods of storm activity and enhanced convection, the plasma density in theafternoon sector of the magnetosphere is highly dynamic due to the development of plasmasphericdrainage plume (PDP) structure. This significantly affects the local Alfvén speed and alters the propagationof ULF waves launched from the magnetopause. Therefore, it can be expected that the accessibility of ULFwave power for radiation belt energization is sensitively dependent on the recent history of magnetosphericconvection and the stage of development of the PDP. This is investigated using a 3-D model for ULF waveswithin the magnetosphere in which the plasma density distribution is evolved using an advection model forcold plasma, driven by a (VollandStern) convection electrostatic field (resulting in PDP structure). The wavemodel includes magnetic field day/night asymmetry and extends to a paraboloid dayside magnetopause,from which ULF waves are launched at various stages during the PDP development. We find that the plumestructure significantly alters the field line resonance location, and the turning point for MHD fast waves,introducing strong asymmetry in the ULF wave distribution across the noon meridian. Moreover, thedensity enhancement within the PDP creates a waveguide or local cavity for MHD fast waves, such thateigenmodes formed allow the penetration of ULF wave power to much lower L within the plume thanoutside, providing an avenue for electron energization.