We document the yearly occurrence of a monsoon jump of approximately 20 degrees latitude during the onset of the summer rainy season over the Greater Horn of Africa (GHA). This is in contrast with a simple model of a smoothly varying ITCZ over the region. The rainfall jump is observed annually during April and May in three precipitation datasets and regional climate model (RCM) simulations. The RCM simulations show the rainfall jump to be roughly coincident with abrupt circulation changes that occur as the Somali jet develops during April, May and June. In particular, the cross-equatorial (meridional) branch of the Somali jet forms along the East African coast in April, bringing moisture (and rainfall) northward to the southern slopes of the Ethiopian plateau. This meridional branch forms well before the northern zonal branch of the jet, which diverts moisture eastward from southern Ethiopia and feeds the Indian monsoon. These results establish a framework for understanding the precipitation cycle over the GHA, and provide a foundation for improving sub-seasonal forecasts over drought-prone regions of eastern Africa.