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A satellite image shows an atmospheric river, which brought rain and strong winds to the U.S. West Coast on Thursday. (National Oceanic and Atmospheric Administration)

As city streets flood and power lines go down, don’t blame El Niño. The epic storm that hit California early Thursday morning is caused by a water vapor filled low-pressure system called an atmospheric river.

Long and narrow, it carries ten Mississippi Rivers’ worth of water.

Originating near Hawaii, this ‘Pineapple Express’ brings atmospheric moisture up to the West Coast. As it condenses, water vapor in the river falls as rain – or snow – at higher elevations.

A handful of storms during California’s rainy season are atmospheric rivers that release large amounts of water in a short period of time. Although a megastorm can last more than a week, smaller atmospheric rivers usually last a few days. Annually, atmospheric rivers account for 30 to 50 percent of California’s precipitation.

These streams in the sky form near mid-latitudes. The last large one to hit the Bay Area was in February.

With 80 percent of California in an extreme drought, the deluge could bring some areas back to their average annual rainfall totals, according to Marty Ralph, a meteorologist with the Scripps Institution of Oceanography at UC San Diego.

“This will have a potentially useful impact on water supply during this drought,” said Ralph.”Some reservoirs will get a real benefit out of this,” he added.

In fact, some Bay Area counties are expected to receive more than a foot of water. Guerneville in Sonoma County has received nearly five inches of water in less than 24 hours. A blizzard warning is also in effect for the western slope of the northern Sierra Nevada, which could receive up to 3 feet of snow.

National Oceanic and Atmospheric Administration (NOAA) electronics engineer Tom Ayers conducting an atmospheric river observation at the Bodega Marine Laboratory in Bodega Bay, CA, on March 19, 2013.
National Oceanic and Atmospheric Administration (NOAA) electronics engineer Tom Ayers conducting an atmospheric river observation at the Bodega Marine Laboratory in Bodega Bay on March 19, 2013. (California Department of Water Resources)

After three years of little rain, some reservoirs like Uvas near Gilroy are at less than five percent of capacity. Fuller reservoirs may be one result of this storm, in addition to power outages, mudslides and blizzard conditions in the mountains.

Expect “numerous trees down” due to gusts of wind measuring 50 miles per hour, said meteorologist Steve Anderson of the National Weather Service.  “Most of the trees are in drought stress…so it won’t take much, if anything, to push them all over,” he said.

Damage from the storm, such as downed trees, power lines and mudslides, may extend from Mendocino to Monterey.

For meteorologist Marty Ralph, who studies extreme atmospheric events, this storm is a chance to understand how the pressure system works, which could help prepare for future powerful storm events.

At the UC Davis Bodega Marine Laboratory in Bodega Bay, he measures soil moisture networks and uses rain gauges to track precipitation.”This is part of the beauty of science, we’re studying how this all works…in real time,” he said.

The storm is expected to clear tomorrow, as the atmospheric river of rain continues moving down the coast to San Diego, losing strength along the way.

‘Pineapple Express’ Soaks Bay Area 26 May,2015Lindsey Hoshaw

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Lindsey Hoshaw

Lindsey Hoshaw is an interactive producer for KQED Science. Before joining KQED, Lindsey was a science correspondent for The New York Times, The Boston Globe, Forbes and Scientific American. On Twitter @lindseyhoshaw

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