Powerful Storm Threatens to Swamp Hard-Hit California Towns - NBC 10 Philadelphia

Powerful Storm Threatens to Swamp Hard-Hit California Towns

Heavy showers are expected overnight in Santa Barbara and Ventura counties before the storm moves into Los Angeles County Thursday

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    NEWSLETTERS

    What is a Pineapple Express Storm?

    The storm system headed toward SoCal on Tuesday is commonly known as a "pineapple express." Meteorologist David Biggar explains just what that means on the NBC4 News at 5 p.m. on Monday, March 19, 2018. (Published Monday, March 19, 2018)

    Watch Today in LA at 4 a.m. for up-to-date information on the days-long storm.

    A storm that's bringing three days of rain is fueled by subtropical moisture and moving slowly into the region, raising the threat of flash flooding and debris flows as mandatory evacuations were ordered in areas near burn scars. The storm is expected to move across a widespread part of the Southern California through Thursday.

    March Storm Images: Click here to send us your weather photos or email isee@nbcla.com

    The dayslong soaker could bring the highest rain totals of the season. So what's fueling this storm?

    A look at when to expect rainfall in Southern California March 20 through 23, 2018.
    Photo credit: KNBC-TV

    About the Storm

    The system that gained strength off the coast, absorbing warm, tropical moisture, was ushered into California by an atmospheric river, which acts like a conveyor belt of moisture. The atmospheric river, also called a Pineapple Express, was one of the reasons for last winter's historic levels of precipitation in California.

    "Think of it as a river in the sky," said NBC4 forecaster Belen De Leon. "It can be as large as 200 miles."

    The storm's strength is largely due to tropical air, which can hold more water than cold air.

    "All this moist air extends all the way down to the tropics, pretty much to the Hawaiian Islands," said NBC4 forecaster David Biggar. "Every time we talk about tropical systems, we're talking about warm air that carries a lot of water, and that carries the potential for moderate to heavy rainfall."

    An estimated 36 hours of precipitation could add up to some significant rainfall amounts, approaching 4 to 6 inches in the foothills and 2 to 4 inches for coastal areas. The most rain is expected in the San Gabriel Mountains, and in Ventura and Santa Barbara counties.

    The first critical period for wildfire burn areas is expected to be from early Wednesday into Wednesday night, especially in the Thomas Fire burn scar area. The December wildfire, the largest on record in California, stripped hillsides of vegetation, contributing to the Jan. 9 Montecito mudflow.

    Heavy rain is expected in the area between midnight and 5 a.m.

    Rainfall will continue in Ventura and Santa Barbara counties at midday Wednesday, when showers will extend into Los Angeles County. 

    California Department of Water Resources

    "Then, we start to see a reall big increase in rain totals," said NBC4 forecaster David Biggar.

    Some of the heaviest showers on Wednesday are expected around 6 p.m.

    The rain will taper to showers Thursday afternoon with lingering scattered showers into Friday morning.