'Atmospheric river' to blast California with heavy rain, feet of snow

First a drought, and now a deluge.

A powerful "atmospheric river" was headed for California on Tuesday, bringing the threat of torrential downpours that could unleash destructive debris flows from wildfire burn scars as well as a massive dump of snow in the Sierra Nevada.

"Even though precipitation is greatly needed across the drought-stricken state, the storm will bring too much all at once and lead to serious flooding concerns," AccuWeather meteorologist Jake Sojda said. The storm will start late Tuesday and could last into Thursday.

Evacuation orders were in effect for areas of fire-scarred Santa Cruz County, and evacuation warnings were issued in San Mateo County.

“If you have not already heeded local county advice don’t wait any longer. This morning is the time to act and prepare if you`re near a burn area,” the National Weather Service office for the San Francisco Bay Area wrote.

Debris flows – torrents carrying massive boulders, soil, trees and other objects – are considered more dangerous than mudslides or landslides. The Jan. 9, 2018, debris flow that blasted the Santa Barbara County community of Montecito killed 23 people.

While coastal areas prepared for rain, snow will be the big story in the mountains. Up to 10 feet of snow is possible in some mountainous areas, forecasters warned, with whiteout conditions possible: "Ground travel will be difficult, if not impossible, at times, especially in the Sierra," the National Weather Service in Reno, Nevada, said in an online forecast. "Even a walk outside can become deadly in these conditions."

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Some valley areas that normally don't see that much snow are expected to get walloped: "This is looking like the biggest snowstorm in many years for the valleys. Bottom line: Be where you need to be by sundown today – you might be there for a couple of days," the weather service said Tuesday.

According the United States Drought Monitor, 78% of California is in moderate drought or worse, and nearly 40% is in severe or extreme drought. The storm will put a huge dent in the drought, AccuWeather said.

What are 'sky rivers'?

These rivers in the sky are responsible for up to 65% of the western USA's extreme rain and snow events, a 2017 study said.

Made visible by clouds, these ribbons of water vapor known as atmospheric rivers extend thousands of miles from the tropics to the western US. They provide the fuel for the massive rain and snowstorms and subsequent floods along the U.S. West Coast.

Though beneficial for water supplies, these events can wreak havoc on travel, bring deadly mudslides and cause catastrophic damage to life and property, the National Oceanic and Atmospheric Administration said.

One well-known nickname for an atmospheric river is the "Pineapple Express," which occurs when the source of the moisture is near Hawaii.

A single strong atmospheric river can transport up to 15 times the water vapor compared with the average flow of water at the mouth of the Mississippi River, according to NOAA's Earth System Research Laboratory.

This article originally appeared on USA TODAY: 'Atmospheric river' to blast California with heavy rain, snow