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'Firestorm' conditions weakening but still a danger

  • Story Highlights
  • Top gusts, at 100 mph Tuesday, could diminish to 50 mph Wednesday afternoon
  • Weather service: Advisories, warnings remain in effect Wednesday
  • Santa Ana winds expected to die down as pressure system shifts eastward
  • Weather service advises firefighters to be cautious despite improving conditions
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(CNN) -- The hot, dry Santa Ana winds were waning Wednesday, a boon for firefighters and Southern Californians who hope the lull in "devil winds" is more than a respite.

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The Santa Ana winds stoke a wildfire burning near California's Lake Arrowhead on Tuesday.

The winds, which brought top gusts of 100 mph Tuesday, were expected to fizzle Wednesday afternoon to between 15 mph and 35 mph, with maximum gusts in the region predicted to be cut in half.

Even diluted, the winds still pose a danger, as they can carry wildfire embers into other vulnerable patches of parched woodland.

"This will be the last morning of offshore winds for this week's powerful Santa Ana event," said a morning wind advisory from the National Weather Service.

"Advisory level wind gusts will peak to 50 mph around dawn over the windiest mountain and valley areas. These winds will diminish further below advisory criteria by afternoon." Map of affected areas »

Wind advisories will remain in effect until noon (3 p.m. ET) in parts of the region, and red-flag warnings will remain in effect in some areas until 6 p.m. (9 p.m. ET) but could be extended until the humidity in the area recovers.

The humidity is expected to climb in coastal areas but could take until Thursday or Friday in some of the areas farther inland, the weather service said.

According to the weather service, the improving conditions can be attributed to an eastward shift in the high pressure over the Great Basin, the desert region that comprises most of Nevada and spawns the Santa Ana winds.

However, winds are unpredictable, and firefighters and residents had warnings to exercise caution, especially as the Santa Anas collide with the strengthening sea breeze coming ashore as the inland pressure heads east.

"Erratic, shifting winds are possible near the coast as the sea breeze and weakening Santa Ana winds interact," the weather service cautioned. "Fire crews need to stay alert to possible wind reversals." Video Watch how winds dying down might help those battling the blazes »

The Santa Anas fall into the category of katabatic winds, called chinooks in the Rocky Mountain regions of the United States and foehns or mistrals in parts of Europe.

The term essentially refers to a downhill wind. In the case of the Santa Anas, the winds generally form in autumn and winter when temperatures begin to drop in the Great Basin. The high pressure over the Great Basin causes the cold air to sink, making it denser, according to the University of California-Los Angeles meteorology Web site.

The dry, heavy air begins to descend from the basin's plains -- parts of which are more than a mile above sea level -- into the valleys and canyons of Southern California, according to the American Meteorological Society.

As the draught flows down from the plateau, warm air compresses and heats the wind, causing it to pick up speed, much like East Coast hurricanes and tropical storms gather strength over the balmy Gulf of Mexico and Atlantic Ocean.

The wind picks up even more speed as it's channeled through the canyon passes and into the heavily populated coast of Southern California.

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Los Angeles County fire inspector Sam Padilla said the union of low humidity, high winds and dry heat -- temperatures that in some cities have reached into the upper 90s -- make a combustible combination. Photo Scenes from the path of devastation »

"This is a windstorm with fire," Padilla said Tuesday. "That is why they call it a firestorm. Once the fire starts, it gives it that ignition, then it's on. Everything around it is dry and ready to go." E-mail to a friend E-mail to a friend

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