Thirsty California pins hopes on El Niño's return

The arrival of spring Thursday ushered out the third-driest winter in Northern California history, leaving behind parched hills, shallow reservoirs and a higher-than-usual threat of fire.

But even as hope dims for a March miracle storm, climatologists say weather conditions could change this year if an El Niño takes shape. The U.S. Climate Prediction Center issued an El Niño watch this month, citing a 52 percent chance of Pacific Ocean waters warming and creating - possibly - a wetter-than-average winter.

Historically, El Niño conditions have been associated with the state's biggest rain years, including the winters of 1997-98 and 1982-83, which brought fatal mudslides to the Santa Cruz Mountains and devastating surf to the Southern California coast. In 1997-98, San Francisco was pounded by a record 47.2 inches of rain.

Turnaround not assured

But while El Niño boosts the odds of rain, it provides no guarantees, especially if the ocean warming isn't extreme.

Image 1 of / 8 Caption Close Thirsty California pins hopes on El Niño's return 1 / 8 Back to Gallery

"If it's only in the weak to moderate category, it doesn't really make me convinced that things will turn around," said state Department of Water Resources climatologist Mike Anderson, who plans to monitor the telltale phenomenon as fall approaches. "We've still got a long, hot summer to go."

To date, San Francisco has received just 8.6 inches of rain since July. That compares with the 20.3 inches that falls on average by this point in the rain year.

The story of below-average precipitation is the same across the state.

Gov. Jerry Brown issued an emergency drought declaration in January, asking residents to voluntarily scale back their water use by 20 percent. Many water agencies, particularly those in areas of the state that are the driest, have made water reductions mandatory.

In the Central Valley, the state and federal water projects that provide Sierra snowmelt to farmers say they're likely to have no water to give out this year.

The short-term forecast doesn't look good. The National Weather Service is calling for a small chance of rain in the Bay Area around the middle of next week, but no more than half an inch.

"We would need five times the amount of normal rainfall every day between now and June to get us back to normal levels," said Steve Anderson, a forecaster for the weather service.

Climatologists looking at the long-term picture, meanwhile, say ocean temperature readings and wind models suggest that an El Niño may form over the eastern tropics of the Pacific Ocean in the summer or fall.

"It's a long way from being a slam dunk," said Mike Halpert, acting director of the Climate Prediction Center.

El Niño's varying effects

The water and atmospheric conditions, Halpert said, have the potential to feed off each other and prompt major shifts in world weather patterns.

Typically, El Niño brings drier weather to the western Pacific, in places such as Australia and Indonesia, and wetter weather to the Americas, he said.

The effects vary considerably with the strength of El Niño - and can differ from place to place.

For example, weak to moderate El Niño conditions have brought more rain to Southern California, while doing little for the northern part of the state. But a strong El Niño historically has increased rainfall across the entire state.

"If that gets locked in place, it can lead to storm after storm after storm," said John Monteverdi, a meteorology professor at San Francisco State University.

With the state in its third straight drier-than-average year, rain in the northern half is particularly important. That's where most of the water supply is stored in the forms of snow, rivers and creeks.

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For more on the state's water issues, go to www.sfgate.com/drought.