A ferocious storm system working its way down the West Coast pounded Southern California Friday with heavy rain and hurricane-force winds, causing flash floods and mudslides that closed roads, buried homes in mud and left tens of thousands without power.
As the storm moved south Friday, it was expected to dump up to four inches of rain on the Los Angeles area, according to AccuWeather. Rainfall from this one storm could be as much as the average rainfall for the entire month of December in the region.
Flash flood and winter storm warnings remain in effect through much of the southern part of the state Friday afternoon.
The storm, dubbed the “Pineapple Express,” is one of the strongest to hit the West Coast in years.
Pineapple Express is a non-technical term for a meteorological phenomenon characterized by a strong and persistent flow of atmospheric moisture and associated with heavy precipitation from the waters adjacent to the Hawaiian Islands and extending to any location along the Pacific coast of North America. A Pineapple Express is an example of an atmospheric river, which is a more general term for such narrow corridors of enhanced water vapor transport at mid-latitudes around the world.
A Pineapple Express is driven by a strong, southern branch of the polar jetstream and is marked by the presence of a surface frontal boundary which is typically either slow or stationary, with waves of low pressure traveling along its axis. Each of these low pressure systems brings enhanced rainfall.
The composition of moisture-laden air, atmospheric dynamics, and orographic enhancement resulting from the passage of this air over the mountain ranges of the western coast of North America causes some of the most torrential rains to occur in the region. Pineapple Express systems typically generate heavy snowfall in the mountains and Interior Plateau, which often melts rapidly because of the warming effect of the system. After being drained of their moisture, the tropical air masses reach the inland prairies as a Chinook wind or simply “a Chinook”, a term which is also synonymous in the Pacific Northwest with the Pineapple Express.