A powerful storm barreling toward California from the tropical Pacific threatens to trigger widespread river flooding throughout the state as warm rain melts a record accumulation of snowpack and sends runoff surging down mountains and into streams and reservoirs.

Although state officials insist they are prepared to manage runoff from what is now the 10th atmospheric river of a deadly rainy season, at least one expert described the combination of warm rain, epic snowpack and moist soils as “bad news.”