.California Flood Threat: Pineapple Express to Bring Heavy Rain, Sierra Snow, High Winds

Thanks to an atmospheric river known as the Pineapple Express, more soaking rain and mountain snow is on the way for California, the epicenter of the nation's most widespread exceptional drought.
Last week, Los Angeles County dams and spreading ground captured 1.8 billion gallons of water, enough to meet the needs of 44,000 people in one year. That's a relative drop in the multi-year drought bucket, but a start.
While this all sounds like good news, more flooding, rock/mudslides, debris flows and even damaging winds are possible with this upcoming storm. The National Weather Service in Monterey, California, said Monday that this storm is "expected to be one of the strongest storms in terms of wind and rain intensity" since storms in October 2009 and January 2008.

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Storm Setup

Forecast depiction of the "Pineapple Express", a plume of deep moisture that will be directed into the West Coast this week.

Setup: An Atmospheric River

What has finally opened the door to drought relief in California?
Last winter, the Pacific jet stream was diverted well to the north of the U.S. West Coast by blocking high pressure in the upper-levels of the atmosphere.
(MORE: 2013 Was California's Record Driest Year)
Now, an energetic Pacific jet stream will point squarely over the West Coast this week. This Pacific jet will tap a narrow, deep plume of tropical moisture over 3,000 miles long known as an atmospheric river.
According to NOAA's Earth System Research Laboratory, these atmospheric rivers (ARs) are important:
- About 30-50 percent of annual precipitation in the West Coast states occurs in just a few AR events.
- The strongest ARs can transport as water vapor up to 15 times the average flow of liquid water at the mouth of the Mississippi River, triggering flooding when stalling or making landfall.
In this case, an AR known as the Pineapple Express, so named due to the origin of the moist plume in the central Pacific Ocean near Hawaii, will slide down the West Coast through the week, wringing out the heaviest rainfall from western Washington to northern California.

Wednesday's Forecast

                   

 

 

Forecast: More Heavy Rain, Flooding

While an initial round of rain will soak primarily parts of western Washington through Tuesday, the bulk of the heavy rain event will arrive on the West Coast beginning Wednesday.
A band of heavy rain accompanying the initial cold front will sweep ashore from western Washington to northwest California during the day Wednesday.
Wednesday night and Thursday, the Pineapple Express will sag slowly south through northern and central California as the southward dip in the vigorous Pacific jet stream swings into the Golden State.
(FORECAST: S.F./San Jose/Oakland | Sacramento)
It's at that time that the heaviest rain and, therefore, highest threat of flash flooding will be in play in those areas.
(MAPS: Weekly Planner Forecast)
Late Thursday night and Friday, the weakening atmospheric river and corresponding southward dip in the jet stream will swing over Southern California, providing a quick burst of rain for the Southland in time for Friday morning's rush hour.
(FORECAST: Los Angeles | San Diego)
Overall, the heaviest rainfall totals are expected from western Washington to northern California, with numerous 3-to 5-inch rainfall totals expected. This includes much of the San Francisco Bay Area and Sacramento. Locally higher amounts of up to 8 inches, or more, can be expected in the Sierra foothills below snow level, the coastal ranges and windward locations of the Olympic Mountains.
Flash flooding, mud/rockslides and debris flows are most likely in these areas, particularly given saturated ground from last week's soaking.
In fact, the National Weather Service in Monterey, California mentioned the last time the city of San Francisco picked up a four-inch-plus rainfall total in two days or less was on Feb. 3, 1998, during a strong El Nino, no less. (There is no El Nino, as yet.)
Over an inch of rain is possible in parts of Southern California late Thursday into Friday, particularly in the L.A. Basin. While these totals are not nearly as impressive, they may be sufficient to produce additional debris flows in burn areas, and may prompt more evacuations.
http://www.weather.com/forecast/regional/news/california-rain-flood-threat-drought-relief-middec2014

A cyclist rides by a police officer in a flooded section of roadway on December 3, 2014 in Mill Valley, California. (Justin Sullivan/Getty Images)