With a new automated precipitation collector we measured a remarkable decrease of 51‰ in the hydrogen isotope ratio (δ ²H) of precipitation over a 60-minute period during the landfall of an extratropical cyclone along the California coast on 21 March 2005. The rapid drop in δ ²H occurred as precipitation generation transitioned from a shallow to a much deeper cloud layer, in accord with synoptic-scale ascent and deep “seeder-feeder” precipitation. Such unexpected δ ²H variations can substantially impact widely used isotope-hydrograph methods. From extreme δ ²H values of −26 and −78‰, we calculate precipitation temperatures of 9.7 and −4.2°C using an adiabatic condensation isotope model, in good agreement with temperatures estimated from surface observations and radar data. This model indicates that 60 percent of the moisture was precipitated during ascent as temperature decreased from 15°C at the ocean surface to −4°C above the measurement site.