What is an atmospheric river? Powerful storm to drench California

A METEOROLOGICAL phenomenon known as an atmospheric river is about to hit California, with residents told to brace themselves for heavy rain.

Atmospheric rivers are relatively long, narrow regions in the atmosphere

A METEOROLOGICAL phenomenon known as an atmospheric river is about to hit California, with residents being warned to brace for heavy rain.

An atmospheric river is a huge plume of subtropical moisture, typically 400 to 600 kilometres in width.

“Atmospheric rivers are relatively long, narrow regions in the atmosphere — like rivers in the sky — that transport most of the water vapour outside of the tropics,” the National Oceanic and Atmospheric Administration (NOAA) says.

“When the atmospheric rivers make landfall, they often release this water vapour in the form of rain or snow,” wrote the agency.

They may also bring severe precipitation and destruction, though “most are weak systems that often provide beneficial rain or snow that is crucial to the water supply.”

Atmospheric rivers play a vital role in the global water cycle and are closely tied to both water supply and flood risks — particularly in the western United States.

“While atmospheric rivers are responsible for great quantities of rain that can produce flooding, they also contribute to beneficial increases in snowpack,” explained NOAA.

The science behind atmospheric rivers. Picture: NOAA

WHAT’S GOING ON WITH CALIFORNIA?

Cities in central and southern California could be slammed with rain. The National Weather Service (NWS) is predicting more than seven inches for Ojai and almost six inches for San Luis Obispo from Tuesday morning through Friday morning.

Where are the recent burn areas? Most concerning: Thomas, Whittier, La Tuna, Sherpa, Fish, and Sand burns. If you are near these burns, DEBRIS FLOWS ARE LIKELY, stay alert to the weather and heed messages/evacuations from emergency officials. #larain #cawx #thomasfire #Montecito pic.twitter.com/UiEIltKpGi

— NWS Los Angeles (@NWSLosAngeles) March 20, 2018

As many as 30,000 people have been ordered to leave communities by noon on the south coast of Santa Barbara County, where a Jan. 9 deluge unleashed deadly debris flows into Montecito.

An atmospheric river is set to affect #NorCal over the next several days. Expect wet weather and mountain snow. #CAwx pic.twitter.com/cGBFKj2hhd

— NWS Sacramento (@NWSSacramento) March 20, 2018

Neighbouring Ventura County has taken similar measures amid concerns in adjacent counties.

Northern California may also be affected by the atmospheric river: the NWS says to expect precipitation to begin Tuesday, with it coming down the hardest from Wednesday to Thursday.

The Associated Press contributed to this report.