Dome-forming volcanic eruptions typically involve the slow extrusion of viscous lava onto a steep-sided volcano punctuated by collapse and the generation of hazardous pyroclastic flows. We show an unequivocal link between the onset of intense rainfall and lava dome collapse on short time scales (within a few hours) and develop a simple thermodynamical model to explain this behavior. The model is forced with rainfall observations from the Soufri¿re Hills Volcano, Montserrat, and suggests that when the dome is in a critical state, a minimum rainfall rate of approximately 15 mm hr-1 for 2--3 hr could trigger a dome collapse.