Pineapple Express

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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, Washington, Alaska, as well as the Canadian province of British Columbia.

Madden-Julian oscillation drives Pineapple Express.

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 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 normally dry desert location near Morongo Valley receiving 7.64 inches (194 mm) 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 (624 mm), and Opid's Camp in the San Gabriel Mountains of Los Angeles County was deluged with 31.61 inches (803 mm) of rain in the five day period.

The unusually intense rain storms that hit south-central Alaska in August of 2006 were termed "Pineapple Express" rains locally.

November 2006 flood, Granite Falls on the Stillaguamish River

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 which 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, and the normally arid and sheltered Interior of British Columbia received heavy coastal-style rains.

In British Columbia especially, 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 Canadian Prairies as a Chinook wind or simply "a Chinook", a term which is also synonymous on the Coast with the Pineapple Express.