Firefighters are on edge as a strong high-pressure system moves over Northern California on Wednesday, delivering gusty winds that could flare up fires smoldering after thousands of lightning strikes in the past week.

The wild weather system that created more than 20,000 lightning strikes over the state in the past week -- including dozens in the Bay Area -- is over. Recent clouds, rain and drama will be replaced by cobalt blue skies and temperatures soaring to 105 degrees in the inland areas.

But it left behind a new worry: so-called "holdover fires."

"Lightning can hit a tree and just hang out," particularly after rain, said Brenda Belongie, a meteorologist with the U.S. Forest Service in Redding, which tracks fires. "It can smolder for several weeks. Think of a long, slow, glowing ember.

"Then, when it warms up and dries, a fire emerges," she said.

Lightning is the leading cause of wildfires in California, and by Sunday the Forest Service says "more new fire starts are possible" with more thunderstorms in the forecast for the state.

Such violent weather is unusual in the Bay Area because our gloomy coastal fog tends to keep our air stable. Thunderheads -- those towering clouds that are created by moisture, atmospheric instability and a lifting mechanism, like a mountain range -- are far more common in places like the Sierra.

But a spinning high-pressure system conceived over America's desert Southwest -- dubbed "The Four Corners High" -- shifted unusually far north Monday. Then, upon arriving in the Bay Area, it released a splash of warm, fat raindrops carried all the way from the Gulf of Mexico.


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Lightning struck O.co Coliseum in Oakland on Tuesday morning, causing it briefly to lose power.

But it brought real danger to Klamath and Modoc counties, where 10 percent of the past week's lightning strikes triggered fire -- despite receiving 1.5 inches of rain. In just one location, Plumas National Forest, more than 40 fires were ignited by lightning in the past week, said Belongie. Most were detected, attacked and extinguished.

The lightning caused the Bully Fire in Shasta County, which has burned 12,500 acres and is costing $20 million to extinguish, according to the California Department of Forestry and Fire Protection.

In 2008, lightning sparked more than 2,000 wildfires from Monterey County to the Oregon border. The biggest blaze, called the Inyo Complex fire, scorched the mountainous Big Sur coast, forcing evacuations and closing Highway 1.

On Wednesday, concern is shifting to the gusty southwestern winds likely to develop behind this warming system.

"It could significantly fan any ongoing or new fire starts," warns the U.S. Forest Service.

Three years of drought have caused the state to become very dry. Firefighters are using aircraft to monitor sites identified by the nation's "lightning detection system," which uses radio signals on antennae to report a lightning strike within 15 seconds.