What is the principal cause of flooding along Northwest rivers?
Many factors affect flooding; however, the principal cause of flooding in Western Washington is intense rainfall associated with storm systems originating over the Pacific ocean. Typically from October through March, low pressure systems form in the Aleutian area of the Pacific and draw warm moisture laden air from the tropical areas into the Pacific Northwest. This warm moist air has come to be known as the “Pineapple Express” as its origin can often be traced to the vicinity of the Hawaiian Islands. Heavy rain occurs when the warm moist air is cooled and lifted as it passes over the Olympic and Cascade mountains.
