

The Pineapple Express is expected to bring a surge of moisture in the form of rain and snow to the West Coast this week, as shown in this precipitable water forecast from the GFS. (weatherbell.com)

Update at 4:50 p.m. ET: A blizzard warning has been issued for the northern Sierra, including Lassen National Park, Donner Pass, Echo Summit, and Carson Pass. Heavy snow and winds gusting to 80 mph will lead to dangerous whiteout conditions. Passes could close for several hours between Wednesday and Friday, with the most dangerous conditions likely falling on Thursday.

A winter storm warning has been issued for the southern Sierra (south of and including Yosemite) for snow accumulation of one to two feet and wind gusts possibly in excess of 60 mph.



Watches, warnings, and advisories in effect for northern California as of 1:50 p.m. PT. ( weather.gov

Original post:

A river of atmospheric moisture is setting up between the tropics in the Pacific Ocean and the West Coast of the United States, forecast to deliver up to nine inches of rain to northern coastal California, and as much as three or four feet of snow to the Sierra around Lake Tahoe.

“In fact, if the forecast verifies,” writes Weather Underground’s Christopher C. Burt, “it may be the strongest and wettest storm to hit the region since October 2009 when the last major ‘pineapple express’ soaked the state.”

Latest @UWCIMSS TPW image shows plume of moisture (Atmospheric River) from ITCZ to the West Coast. pic.twitter.com/3RuiSPE675 — Rob Elvington WAAY 31 (@RobElvington) December 9, 2014

The Pacific storm will begin to impact the West Coast on Wednesday starting in Washington and Oregon, and work its way down the coast through the day. By Thursday morning, heavy rain will be falling across northern California as the Pineapple Express — another meteorological term for a river of moisture sourced from the tropical Pacific — gets cranking.

Precipitation across the western U.S. will last through Friday afternoon, eventually tapering off Friday night, providing three days of much-needed rain and snow for a region in the midst of historic drought.

Forecast graphics from the National Weather Service in Sacramento say it all: widespread rainfall totals over two inches across northern California, including the Bay Area and the Sacramento Valley, approaching as much as nine inches in coastal high elevations.

The northern Sierra Nevada can expect around two feet of snow, and as much as four feet in the highest elevations, which the Weather Service suggests for the summit west of South Lake Tahoe.

Flood and flash flood watches have been issued for much of northern and central California. Of particular concern are the burn scar areas of recent wildfires. “These areas have experienced severe tree and vegetation loss, and heavy rain is likely to produce excessive runoff,” writes the Weather Service.

A winter storm watch is in effect for most of the Sierra Nevada through Friday afternoon for snow accumulation of one to three feet possible in the higher elevations. Between the strong winds and the heavy snow, it is likely that highway passes will close at points, potentially for hours at a time, between Wednesday and Friday.

High wind watches and warnings are also in effect across northern California, parts of Oregon, and the northern Sierra, including Donner Pass and Echo Summit for wind gusts nearing 80 mph. At lower elevations toward the coast, including the Bay Area, wind gusts of 50 to 60 mph won’t be uncommon.

While the recent Pacific storms are a step in the right direction to ease the drought, California’s snow pack, which play the critical role of replenishing the reservoirs that are tapped in the dry season, is still running in deficit this week. Weather Underground’s Jeff Masters writes: