Nickname of a Pacific winter weather phenomenon wherein the jet stream settles into a long, straight, fast flow (thus, express) bringing tropical air northeast from near Hawaii (hence, pineapple) to the West Coast of the United States. It's not a regular occurence, but it's not uncommon. When it hits cold air masses, the moisture drops upslope snow in the Sierra Nevada or Cascades, or even, if conditions are cold enough, in downtown Seattle. More often, however, the warm, moist air arrives as a storm, or series of storms, with heavy rain melting the snow pack and causing severe flooding from the runoff.

Once or twice a decade, conditions push the Pineapple Express south, and when the storm system hits Southern California, it can dump the equivalent of half of Los Angeles' precipitation in just a few hours. (Surprisingly, it is not any spot in a monsoon belt, but the San Gabriel mountains, which ring the LA basin, which holds the world record for both the most rain dropped in one minute (0.64 inches, April 6, 1926) and formerly held the one day record (26.08 inches).

Mike Davis, Ecology of Fear, (New York: Metropolitan Books, 1998)
"The Weather Notebook," http://www.weathernotebook.org/transcripts/2001/01/17.html
Jenny Bachman, Julia Cohen, and Derek Inokuchi, The Geological, Atmospheric, and Human Influences of the 1997 Floods, http://www-geology.ucdavis.edu/~GEL242/earthsci.html

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