USP NEWS: CALIFORNIA WILDFIRES A USA CA
Sarah Ashton, left, gets a hug from family friend Robert Rex while Ashton, her sister, Tara Asciutto, and their dad Stephen Ashton got a first look at their family home and vineyard after the Nuns Fire destroyed the property in Glen Ellen, Calif.

The winds from hell have returned.

Santa Ana winds, one of the nation's most notorious wind events, have fueled the destructive wildfires across southern California that have already charred tens of thousands of acres.

This week's windstorm is "the strongest and longest Santa Ana event so far this season," the National Weather Service in Los Angeles said.

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"Moderate to strong Santa Ana winds will likely continue to bring very dry conditions with extreme fire danger through Friday for much of Southwest California," the weather service warned. 

Gusts between 40 and 50 mph in the canyons and passes and up to 70 mph in the mountains will be a daily occurrence this week, AccuWeather said.

This event differs from a normal Santa Ana windstorm, according to AccuWeather meteorologist Ken Clark. "Usually, there is a 12- to 24-hour period of the highest winds and then the winds decrease," he said. "This one will be a four-day event, which will make fighting ongoing fires much more difficult. The risk for additional fast-moving fires is quite high."

More: New brush fire sets homes ablaze in Los Angeles' toniest neighborhoods

Santa Anas are an annual weather hazard in southern California.

"Nowhere else do such winds impact so many people with so much force and possess such extensive opportunity for damage and destruction," the weather service said. 

The winds, which occur most often in the fall and winter, push dry air from over the inland deserts of California and the Southwest. Santa Anas blow over the mountains between coastal California and the deserts. As the wind comes down the mountains, it's compressed and warms up.

As the air warms, its relative humidity drops, sometimes to less than 20% or even less than 10%. The extremely low humidity helps dry out vegetation, making it a better fuel for fires.

This year's wildfire season in California has been much worse than usual, with at least 1.1 million acres burned, about twice the average, according to CalFire.

The threat of a catastrophic wildfire season in California was, ironically, exacerbated by the heavy, drought-ending rains that the state received last winter, according to meteorologist Mark Bove of insurance firm Munich Re. "The rain caused a period of rapid vegetation growth, especially in brush and grasses that cover the state’s hillsides," he said. 

"But the rain stopped by spring, and the new vegetation slowly dried out, becoming ample kindling and fuel for wildfires," Bove added.

La Niña conditions are currently present in the tropical Pacific, which typically leads to a drier California winter, meaning that prime wildfire conditions may continue into 2018, he said. 

Over the past decade, the U.S. has seen an uptick in the frequency of wildfires that have caused significant insured property losses, Bove said, noting that this is mostly due to new development in areas that are highly vulnerable to wildfires.

"However, California and the American Southwest have become drier over the past century as well due to Earth’s changing climate, increasing the probability of severe wildfires occurring," Bove said.