Spring is flood season for much of the U.S. From the Pacific Northwest to New England, melting snow combines with spring rains to raise water levels in rivers, lakes, and streams. Sometimes, a perfect storm of factors creates a major flood…. Rapid warmups and warm overnight temperatures can dramatically increase the rate of snowmelt and lead to flooding…. the second factor most likely to lead to rapid snowmelt is rain, expected to be a growing problem particularly in the Sierra Nevada Mountains of California and in the Pacific Northwest.