California’s drought crept in slowly, but it could end with a torrent of winter storms that stream across the Pacific, dumping much of the year’s rain and snow in a few fast-moving and potentially catastrophic downpours.
Powerful storms known as atmospheric rivers, ribbons of water vapor that extend for thousands of miles, pulling moisture from the tropics and delivering it to the West Coast, have broken 40% of California droughts since 1950, recent research shows.
“These atmospheric rivers — their absence or their presence — really determine whether California is in drought or not and whether floods are going to occur,” said F. Martin Ralph, a research meteorologist who directs the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography at UC San Diego.
The storms, which flow like massive rivers in the sky, can carry 15 times as much water as the Mississippi and deliver up to half of the state’s annual precipitation between December and February, scientists say. Though atmospheric rivers are unlikely to end California’s drought this year, if they bring enough rain to erase the state’s huge precipitation deficit, they could wreak havoc by unleashing floods and landslides.
Scientists using a new type of satellite data discovered atmospheric rivers in the 1990s, and studies since then have revealed the phenomenon’s strong influence on California’s water supply and extreme weather.
This month, a group of government and university scientists, including Ralph, are launching a major field experiment to better understand atmospheric rivers as they develop over the Pacific. Through the end of February, some researchers will fly airplanes above storms as they pass through, while others will monitor them from ships hundreds of miles off California. As the storms make landfall, the scientists will collect data with ground-based instruments.
California usually needs about five good atmospheric rivers each winter to fill reservoirs, stimulate spring vegetation growth and build snowpack to healthy levels, said Michael Anderson, a climatologist for the California Department of Water Resources. But how much the storms boost the state’s water supply depends on the characteristics of each one, including how cold it is, whether it makes landfall toward the north or south, and whether the precipitation falls mostly as rain near the coast or as snow in the mountains.
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