Unusual weather system produces destructive winds

The winds reached 97 mph at one mountain peak. More than 380,000 homes lost power. Thousands of trees snapped, blocking roads and damaging property. Scores of schools were closed, as was Griffith Park. And motorists battled gridlock caused by broken traffic signals and blowing debris.

The storm, which produced some of the strongest wind gusts in more than a decade, was caused by a highly unusual weather system that even had experts marveling at its power.

While Santa Ana winds are common this time of year, this storm was anything but.

PHOTOS: Sana Ana wind damage

The winds were produced by two separate weather systems that channeled cold air from the north into the Los Angeles area.

A clockwise high-pressure system was parked over Northern California and the Great Basin as a counter-clockwise low-pressure system hovered over Arizona.

Like two massive gears spinning in opposite directions, the systems funneled the winds.

"In some places we've seen gusts over hurricane force, which for the Southwest part of the country is not something that usually happens," said Brian Edwards, a meteorologist for AccuWeather.com. "This is a one-every-10-years kind of thing."

Indeed, the blustery conditions extended across the Southwest, including Utah, Nevada, Wyoming, Arizona and New Mexico. In some places, including Utah, wind gusts topped 100 mph.

Experts said one reason for the extensive damage was that the winds were remarkably choppy and unpredictable.

In some places, winds suddenly shifted from 10 mph or 20 mph to more than 80 mph. The shift made trees as well as roofs and power lines vulnerable.

"Everything lined up perfectly," said Bill Patzert, a climatologist for Jet Propulsion Laboratory in La Ca?ada-Flintridge.

Trees were no match for the winds, especially those with heavy canopies. Patzert noted that trees in urban Southern California neighborhoods don't have the strong root systems found in more natural environments.

"L.A. trees don't have deep roots. The urban forest is artificial and is primarily watered by lawn sprinklers," Patzert said. "So what keeps our urban forest alive is people watering their lawns, which are not natural, so you don't have deep root systems. So our trees are very vulnerable to Santa Ana events."

Walter Warriner, a Santa Monica arborist and community forester, agreed, adding that the large canopies of many local trees lack strong foundations.

"When you look at a tree above ground there's a ratio of 20 to 1 compared to below ground, so there's not that many roots holding our big trees in place," he said.

While damage was reported across the Southland, communities in the western San Gabriel Valley were particularly hard-hit, including Pasadena, South Pasadena, San Marino, Altadena and La Ca?ada-Flintridge.

National Weather Service meteorologist Eric Boldt said this, too, was unusual.

Typically, the San Fernando Valley and Ventura County get the brunt of such windstorms.

Source: http://feeds.latimes.com/~r/latimes/news/science/~3/CsB_27JgesY/la-me-winds-science-20111202,0,1785597.story

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