In Calif., La Nina sparks wildfire fears

In Calif., La Nina sparks wildfire fears

18 October 2010

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USA —  The strengthening of La Nina, the weather system known for bringing drier and warmer winters to Southern California, has water and fire officials bracing for shortages and wildfires.

In La Nina years since 1949, when tracking began, the average rainfall was about 70 percent of normal in the region. But combined with depleted groundwater supplies and reservoirs, and restrictions on water imports from northern California, it could make for an upcoming dry year, climatologists said.

“At best (rainfall) will be 80 percent of normal, but it could be as low as 50 percent. That’s not much,” said Bill Patzert, a long-range forecaster at NASA’s Jet Propulsion Laboratory in Pasadena.

This La Nina has been building since spring and has grown increasingly stronger. It is “at least large, if not bigger, which makes the impacts much more probable,” Patzert said.

It is not unusual for a La Nina pattern to follow an El Nino, the abnormal warming of surface water in the eastern tropical Pacific that can deliver heavy rains in the Southwest. Last year’s El Nino, which brought near-normal rain- and snowfall, had weather watchers cheering after three years of drought.

In La Nina years, trade winds grow stronger than normal and cold water that normally exists along the coast of South America extends to the equator in the Pacific. The system shifts weather patterns around the globe, bringing less rain along the coasts of North and South America, the equator and the far western Pacific.

With a La Nina, the Southwest typically is hotter and drier while the Pacific Northwest gets more rain and flooding. The southern and eastern United States turn hot and the chance of hurricanes in the Atlantic often increases.

One of the strongest La Nina events, in 1988, caused significant drought across North America. Another in 2008 fueled a ferocious hurricane season that included Hurricane Ike, which hit Texas and killed almost 200 people.

At Cal Fire in Riverside, Calif., meteorologist Tom Rolinski is calling for average to above-average precipitation this fall, but less in the spring. That will leave the area below normal at the end of the water year next Sept. 30.

Rolinski, whose weather and climate information helps fire officials allocate resources, is keeping an eye on the potential for Santa Ana winds, which, when combined with La Nina’s dryness, increases the region’s risk of a large incident in the next few weeks. Vegetation fed by late-season rains has dried out and is vulnerable to fire.

Since 2003, a string of major wildfires has hit the state, starting with that year’s siege that included the Old and Grand Prix fires in the San Bernardino Mountains. There was the deadly Esperanza fire near Idyllwild in 2006, followed by the Slide, Grass Valley and Butler fires in San Bernardino and the surrounding mountains in 2007.

“If we get a few weeks of dry weather, I think the fuels will still be ready to burn,” Rolinski said.

“I would suspect we’ll probably have some drought conditions returning to the area in six or seven months. The fuels would be drier than normal, the reservoirs would be lower, snowpack in the Sierras and the local mountains would be less than normal,” Rolinski said.

That means continued water cutbacks. Most local districts have continued conservation measures started in the midst of the drought that began in 2006, including tiered rates to discourage waste and limits on outside irrigation.

The unpredictability of La Nina’s impacts has kept some weather experts from worrying.

In the past, the pattern has brought everything from critically wet to dry weather, said Michael Anderson, the state’s climatologist at the Department of Water Resources in Sacramento.

Drought teams and flood responders are on standby, he said.

If a ridge builds up over the Pacific, tropical moisture could be routed to the state “like a fire hose,” he said. Those storms tend to be warm and bring flooding, but occasionally offshore flows mix with arctic air and bring cold storms. But if the flow blocks storms from hitting California, it ends up a dry year, Anderson said.

“We’re doing much better than we were last year at this time,” Anderson said. “But it’s still below average. We’re not out of the woods yet.”

Anderson said La Nina will last at least through next spring. “It really creates a lot of uncertainty. Where and how it forms and how the weather patterns revolve around it make a big difference in terms of what we get in water.”

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