The Pineapple Express is a non-technical, shorthand term popular in the news media for a meteorological phenomenon which is characterized by a strong and persistent flow of atmospheric moisture and associated heavy rainfall from the waters adjacent to the Hawaiian Islands and extending to the Pacific coast of North America. The Pineapple Express is driven by a strong, southern branch of the Polar jetstream and is usually 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 conditions are often created by the Madden-Julian oscillation, an equatorial rainfall pattern which feeds its moisture into this pattern. The combination of moisture laden air, atmospheric dynamics, and orographic enhancement resulting from the passage of this air over the mountain ranges of the West Coast causes some of the most torrential rains to occur in the region. Many Pineapple Express events follow or occur simultaneously with major arctic troughs in the Northwestern United States, often leading to major snowmelt flooding with warm, tropical rains falling on frozen, snow laden ground. Examples of this are the December 1964 Pacific Northwest flood and the Willamette Valley Flood of 1996.
The San Francisco Bay Area is occasionally affected by a Pineapple Express. When it visits, the heavy, persistent rainfall typically causes flooding of local streams as well as urban flooding. In the decades before about 1980, the local term for a Pineapple Express was Hawaiian Storm. During the second week of January, 1952, a series of Hawaiian storms swept into Central California, causing widespread flooding around the Bay Area. The same storms brought a blizzard of heavy, wet snow to the Sierra Nevada Mountains. The greatest flooding in Northern California since the 1800s occurred in 1955 as a result of a series of Hawaiian storms, with the greatest damage in the Sacramento Valley around Yuba City.
A Pineapple Express battered Southern California from January 7 through January 11, 2005. This storm was the biggest to hit Southern California since the El Niño of 1998. The storm caused mud slides and flooding, with one desert location just north of Morongo Valley receiving about 9 inches of rain, and some locations on south and southwest facing mountain slopes receiving spectacular totals. San Marcos Pass, in Santa Barbara County, received 24.57 inches, and Opid’s Camp in the San Gabriel Mountains of Los Angeles County was deluged with 31.61 inches of rain in the five day period.
The Puget Sound region from Olympia, Washington to Vancouver, BC received several inches of rain per day in November 2006 from a series of successive Pineapple Express storms that caused massive flooding in all major regional rivers and mudslides which closed the mountain passes. These storms included heavy winds which are not usually associated with the phenomenon. Regional dams opened their spillways to 100% as they had reached full capacity due to rain and snowmelt. Officials referred to the storm system as the worst in a decade on November 8, 2006. Portions of Oregon were also affected, including over 14 inches in one day at Lee’s Camp in the Coast Range.