Why this 'atmospheric river' is causing mudslides and 'roof landslides' in California

"Atmospheric rivers" bring tropical water vapor to the western United States. This is a visualization of an atmospheric river event in 2018. (NOAA's National Environmental Satellite, Data, and Information Service (NEDIS))

Californians experienced some unusually severe winter weather this week, with an "atmospheric river" cutting through the state Bringing strong winds and heavy rain to most areas.

The storm arrived in northern California on Tuesday (February 12) night and lasted until Wednesday (February 13), causing the National Weather Service (NWS) to issue flash floods, mudslides and landslides in the area. and wind warnings. The National Weather Service (NWS) predicts that Thursday (February 14) will bring "excessive rainfall" to Southern California. [Weird Weather: 7 Rare Weather Events]

Atmospheric rivers are giant "rivers in the sky" that cause moisture in the tropics to flow north from California to Canada. According to local news outlet KQED, these giant weather systems can carry large amounts of fresh water flowing through the Mississippi River.

"They are the largest freshwater rivers on Earth," said F. Martin Ralph, director of the Western Weather and Extreme Water Resources Center in La Jolla, Calif., Ralph told KQED.

These rivers of condensed water vapor in the atmosphere can easily be 1,000 miles (1,600 kilometers) long and 300 miles (482 kilometers) wide. When an atmospheric river carries water vapor from Hawaii to the western United States, like the current storm, it's called the Pineapple Express.

Infrared image shows current atmospheric rivers carrying water vapor from Hawaii to California in February 2019. (National Oceanic and Atmospheric Administration (NOAA)

Atmospheric rivers can dump heavy rain or snow as they make landfall, bringing much-needed rain—or wreaking havoc. California's recent storms mean The current downpours are falling on water-logged soils that have also been scorched by summer wildfires, leaving areas of California more susceptible to flash flooding and debris, according to the National Weather Service. Up to 3 inches (7.6 centimeters) of rain fell in 24 hours in some parts of the northern Bay Area on Wednesday morning, with about 1.5 inches (3.8 centimeters) falling in San Francisco, along the Bay Area coast and in San Francisco, according to the National Weather Service. Residents in hilly areas could face winds of 25 to 35 miles per hour (40 to 56 kilometers per hour), with gusts up to 60 miles per hour (97 kilometers per hour). Social media was awash with reports of downed trees and flash flooding in Sieras. , the National Weather Service warned that atmospheric rivers could cause "roof snow," or the sudden release of snow from roofs that have accumulated snow, which could cause serious hazards

Earlier this month, Ralph and his colleagues developed a new scale to describe the strength of atmospheric rivers, described in the February Bulletin of the American Meteorological Society, that ranks these weather events in a "1 to 5" category. Category 1 represents "weak" storms and Category 5 represents "abnormal" storms, according to a statement. The ranking is based on the amount of water vapor a storm carries and how long it takes to release moisture over a given area. It also shows how beneficial the storm could be (for example, bringing much-needed rain to replenish reservoirs after a drought) or how dangerous it could be, causing flooding and mudslides. According to local news outlet CBS San Francisco, the current storm is a "Category 3." ,

Tia Ghose contributed reporting.

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Originally published in Live Science