Filamentary structure is a common feature of atmospheric water vapor transport; the filaments may be termed ''atmospheric rivers'' because some carry as much water as the Amazon [Newell et al., 1992>. An extratropical cyclone whose central pressure fall averages at least 1 hPa hr-1 for 24 hours is known in meteorology as a ''bomb'' [Sanders and Gyakum, 1980>. We report here an association between rivers and bombs. When a cyclonic system is penetrated by a river, the cyclonic center moves to be close to the position occupied by the leading edge of the river twelve hours previously and the central pressure falls. If the river then moves away from the cyclone, the central pressure rises. Based on a pilot study of pressure fall and water vapor flux convergence for two winter months, the cause of the explosive deepening appears to be latent heat liberation. This is substantiated by composite maps of seven Atlantic and seven Pacific bombs which show that the flux convergence near the bomb center has a comma cloud signature. The observed association may be useful in forecasting 12-hour direction of motion and pressure change of rapidly developing cyclonic systems; the incorporation of better moisture data into numerical forecasting models may be the reason for the reported increase of skill in the prediction of bombs in recent years. ¿American Geophysical Union 1994