LOS ANGELES (AP) They're called "atmospheric rivers" and they can dump massive quantities of Pacific Ocean water on California, carrying it through the air from as far away as Hawaii.
Here's a look at the weather phenomenon that has swelled the state's rivers, flooded vineyards, dumped snow and rain on the Sierra Nevada and raised the risk of mudslides on hills scorched by last summer's wildfires.
Last weekend's atmospheric river that sent rains pummeling California through Monday interacted with a low pressure system pushed by a cold jet stream from the northern Pacific, said Bill Patzert, a climatologist at NASA's Jet Propulsion Laboratory.
[...] the powerful ones can transport an amount of water vapor equal to 15 times the average flow of water that flows out of the Mississippi River's mouth, according to NOAA's Earth System Research Laboratory.
When the moisture-laden air moves over mountain ranges like the Sierra Nevada along the California-Nevada border, the water vapor rises and cools, becoming heavy precipitation that falls as rain or snow, according to NOAA.
While traditional cold winter storms out of the north Pacific build Sierra snowpack vital to the state's water supply, Patzert said atmospheric rivers tend to be warm.