Produced by an atmospheric trough over the eastern Pacific, the Pineapple Express is a plume of deep, tropical moisture that is swept into the western edge of North America. Developing ahead the the trough's leading cold front and reinforced by circulation around high pressure to the southeast, this flow of warm, humid air, which originates near Hawaii, is forced to rise as it invades the Continent, producing heavy valley rains and dropping copious snow across higher terrain.
In most cases, the plume first strikes the Pacific Northwest; as the trough advances eastward and the angle of its cold front changes, the stream of moisture moves southward, through California and into Mexico. Should the front become stationary, as it did earlier this week, the onshore flow of tropical moisture may continue for several days, producing floods, mud slides, avalanches and great skiing conditions from the Coastal Ranges to the Rockies.
Once the trough moves across the western edge of North America, the Pineapple Express is shut off and cooler, drier air invades from the northwest. While other cold fronts may follow in its wake, they cannot tap the tropical moisture until the original trough moves further eastward and a ridge of high pressure reprimes the atmosphere.
