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Mudslides, snow, and flash floods: an atmospheric river has soaked California

The storm system stretched hundreds of miles wide and carried as much water as 15 Mississippi Rivers.

A satellite image of the atmospheric river of weather that is channeling moisture near Hawaii toward the West Coast, bringing immense rain and snow.
An atmospheric river is channeling moisture near Hawaii toward the West Coast, bringing immense rain and snow.
Umair Irfan is a correspondent at Vox writing about climate change, Covid-19, and energy policy. Irfan is also a regular contributor to the radio program Science Friday. Prior to Vox, he was a reporter for ClimateWire at E&E News.

A massive, gushing river in the sky — a mile high, more than 300 miles wide, and carrying up to 15 times the amount of water in the Mississippi River — has drenched huge swaths of the West Coast. This phenomenon, known as an atmospheric river, brought more snow to northern California in a day than parts of New England have seen all winter. Redding, a city in California’s Central Valley, got more than 13 inches of snow.

A winter weather advisory remains in effect for Southern California through noon Saturday with expectations of more rain and around 1 inch of snow. Snow and rain will likely continue soaking the state throughout the weekend. The National Weather Service is also warning of possible flash floods in the region.

The current storm system is not a Pineapple Express, an atmospheric river that originates over Hawaii (hence “pineapple”). Instead, it originated a bit further south and east of Hawaii, so it doesn’t get to share the same branding. (Last year, two Pineapple Expresses made stops in California.)

However, the mechanism behind the Pineapple Express and the current atmospheric river is similar. Such rivers occur when plumes of moisture over the Pacific Ocean start to mesh with a larger storm system. Sometimes years can go by without an atmospheric river, and sometimes several can occur in a single season.

A massive volume of precipitation soaking the West Coast all at once can be dangerous. On Wednesday, Sacramento broke a daily rainfall record with a 1.6-inch downpour over 24 hours. The prior record was 1.22 inches. There are also mandatory evacuations in Orange County.

As Capital Weather Gang reported Friday, “Palomar Observatory, located on a mountaintop 60-plus miles north-northeast of San Diego, took in 10.1 inches of rain – its wettest day ever recorded. Palm Springs posted 3.68 inches, its wettest February day on record and the third heaviest 24-hour rainfall in any month.”

A key concern for California right now is mudslides. After back-to-back years of expansive, record-breaking wildfires, many areas of the state are denuded of the vegetation that would ordinarily anchor the soil in place. But even areas that haven’t seen fires recently are vulnerable. Sausalito, California, saw five inches of rain in the past 24 hours, which triggered a mudslide in the hilly Bay Area town and forced people to evacuate at least 50 homes.

The National Weather Service is expecting more than seven inches of precipitation in Southern California and the Sierra Nevada through Saturday, as a new storm system rolls in.

Over the long term, researchers expect that atmospheric rivers will grow more intense as the climate changes. As average temperatures go up, more water evaporates and the atmosphere can hold on to more moisture to dish out in storms.

“There is now emerging evidence that these atmospheric rivers, at least along the West Coast of the US, have become more intense over the last few decades, and there’s certainly an expectation that they will become more intense as the climate continues to warm,” Daniel Swain, a University of California Los Angeles climate scientist, told Vox last year.

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