I sent my question to Levi Cowan of Tropical Tidbits and got this response:
"Those striations are typical when a burst of thunderstorms pushes outflow outward at a fast rate, so you'll often see them right after a big burst of convection like we had yesterday [with hurricane Sally]. Their cause is not known for sure, but they are likely 'rolls' caused by strong shearing of the wind in the outflow layer."