The Pineapple Express is a Pacific Ocean subtropical jet stream that brings warm moist air from Hawaii (where pineapples are grown) to the U.S. West Coast states of California, Oregon, and Washington, as well as the Canadian province of British Columbia.
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 major West Coast flooding events, such as the 1995 floods in Southern California, came about by way of this weather phenomenon.
The most recent Pineapple Express battered Southern California from January 7 through January 11, 2005. This storm was the biggest to hit Southern California since El Nino of 1998. The storm caused mud slides and flooding, with one normally dry desert location near Morongo Valley receiving 7.64 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.
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