Rip currents - strong, isolated, offshore-directed flows - can occur in the absence of bathymetric features such as rip channels. No consensus exists regarding the cause of these impressive but life-threatening features, sometimes called "flash" rip currents. In a new model, flash rip currents self-organize because of feedbacks resulting from a newly hypothesized interaction between waves and currents. Robust predictions arise from this numerical model: Flash rip currents become less prevalent with increasing beach slope and increasing variations in incident-wave heights. The results of field tests of these predictions support this model, but are not consistent with some other models, as applied to the flash rip currents on beaches such as those in Southern California.