A life threatening atmospheric river event, also known as a Pineapple Express event, is forecast by the National Weather Service to smash into California tomorrow, bringing extremely heavy rain and snow to portions of central and southern California.
An atmospheric river refers to a narrow stream of concentrated moisture in the atmosphere. The most common atmospheric river event is nick-named “Pineapple Express”; in this type of situation, a robust stream of moisture near the Hawaiian islands flows north and east into western North America, dropping copious amounts of rain and snow as it interacts with the terrain there.
These long, narrow atmospheric rivers help transport moisture from the tropics, in water vapor form, to areas far from the tropics. The liquid equivalent of these moisture plumes could be comparable to the water flowing through the mouth of the Mississippi River, according to NOAA.
Heavy precipitation is expected to move across the San Francisco Bay area and the Central Coast of California tonight through Sunday, eventually reaching southern California Sunday into Monday. Some of the heaviest rain is expected on Sunday in areas around the Los Angeles metro area, especially to the north. This heavy rain will result in considerable flash, urban, and small stream flooding with debris flows and mudslides. The National Weather Service also warns that river flooding is also likely.
The precipitation will smash into the higher terrain as heavy wet snow. The heavy snow will spread across the Sierra Nevada tonight through Monday with accumulation rates of 2-3″/hour. Winds will also be strong, gusting to 60 mph at times; this will produce whiteout conditions and near impossible travel above 5’000 feet. Southern California ranges will see heavy snow later Sunday through to early Tuesday above 7,000 feet.
There will also be damaging winds and high surf at the coast. Powerful, damaging onshore winds will impact northern and central California tonight through Sunday, shifting to southern California for Sunday and Sunday night. Power outages and downed trees are possible. Potentially damaging high surf is expected for much of the California coast on Sunday into Monday.