The National Weather Service's Seattle office explained the Pineapple Express Wednesday as "a humorous yet memorable phrase that is used to describe weather systems that have their moisture source in the tropics. For us, the source is close to Hawaii, which has a lot of pineapples!"
These systems tap into tropical moisture and can cause heavy rain and major flooding in our area. It is important to note that heavy rainfall helps generate significant flooding in the Pacific (Northwest) -- not mountain snow melt," the weather service said.
If conditions are just right, tropical moisture is injected into the upper atmosphere and carried to the pacific by a strong subtropical jet stream creating the Pineapple Express.
The system brought heavy rain, major flooding and overnight landslides to Southern California canyons as the storm moved through the area, authorities said.
Police were urging residents of Laguna Beach to remain in their homes and avoid the downtown area, parts of which were under several feet of water, the Los Angeles Times reported Wednesday.
The torrential rain, hail, gusty winds and the threat of tornadoes prompted officials to order evacuations in mudslide-prone areas near Los Angeles. Slides blocked the Pacific Coast Highway in Malibu.
San Diego Gas & Electric Co. said more than 13,000 customers were without power in San Diego and southern Orange County due to the storm.
The powerful storm -- following storms that have pummeled much of California since Friday -- would likely drop 0.75 to 1.5 inches of rain an hour for 6 to 8 hours, flooding neighborhoods all over Los Angeles County as well as the county's suburban foothills and mountains, National Weather Service meteorologist Stuart Seto said.
In northwestern Arizona, flooding destroyed four unoccupied homes along Beaver Dam Wash in Littlefield and put the Beaver Dam Resort, a senior citizens community and RV park, partly underwater, The Arizona Republic said. Officials set up a shelter across the state line in Mesquite, Nev., for those who evacuated.
The coast and valleys could be pelted with 2 to 4 inches of rain Wednesday, and the foothills and mountains could see 4 to 8 additional inches, Seto told the Los Angeles Times.
The relentless deluge prompted Gov. Arnold Schwarzenegger to declare a state of emergency for six counties in the Los Angeles area, while officials evacuated several thousand residents.
Wednesday's storm is the most intense since rain and snow started hammering much of the state Friday, the weather service said.
In the Sierra Nevada, the rain became snow with 13 feet at Mammoth Mountain ski resort in the Sierra Nevada, MSNBC reported. Lake Tahoe, Calif., reported nearly 9 feet of snow.
The California storm may make its way across the country, bringing heavy snow to the East Coast on Christmas or the day after, Accuweather.com forecast.