AP Explains: Hot, dry Diablo Wind scorched wine country

Notorious breezes links between many of California’s worst wildfires are known by various names — Diablo, Santa Ana and Sundowner — but all share the common trait of being able to whip a trigger into a deadly inferno that seems to come out of nowhere.

Here’s a look at where and when these winds pass 😛 TAGEND

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DIABLO WIND

This wind fanned the deadly firestorms that turned swaths of Northern California wine country into an ashen moonscape on Monday.

The name is informally applied to a hot, dry wind in the San Francisco Bay region that blows from the interior toward the coast.

Known as an offshore wind — the direction the air is moving toward — it’s the reverse of the normal onshore flow of cool, moist air from the Pacific Ocean.

The wind is spawned as high pressure builds over the West and air flows toward areas of lower pressure along the coast.

The air is so dry that relative humidity levels plunge, drying out vegetation, constructing them ready to burn.

In forecasting the offshore gale event on Sunday, the National Weather Service noted that analysis by land management agencies showed that gasolines were at or near an all-time record for dryness. A bumper harvest of grasses produced by record wintertime rains combined with heavier vegetation stressed by years of extreme drought and disease, the climate service said.

During the event, breeze peaked at 79 mph( 127 kph) in northern Sonoma, and a Santa Rosa weather station recorded a temperature spike of 91 degrees at 4:30 a.m ., the weather service said.

The most infamous Diablo wind occurred in 1991 when remnants of a tiny blaze were whipped into an inferno in the hills of Oakland and Berkeley. The flame killed 25 people, injured 150 and destroyed more than 3,000 homes and apartments.

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SANTA ANA WIND

This name is given to offshore winds that sweep through Southern California, commonly during autumn, and fan fires like the one that erupted at midmorning Monday in Orange County suburbs southeast of Los Angeles.

A classic Santa Ana is formed when high pressure over the Great Basin causes air to flow out of the interior in a clockwise rotation and enter Southern California from the northeast.

Squeezing through the mountain ranges that separate the desert from coast, the air speeds up like a river through the narrow passes, loses moisture and heats up as it descends through pass and canyons.

The plunge in relative humidity saps moisture from vegetation. High wind velocities combined with any flame can create a blowtorch.

The origin of the wind’s name has never been settled. But most debates mention Santa Ana Canyon in Orange County.

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SUNDOWNER

The southern coast of Santa Barbara County is prone to dry breezes that blow down the slopes of the Santa Ynez Mountains and out to ocean, often in the late afternoon or early evening, sending temperatures and flame threat soaring.

Among the worst Sundowners was the notorious 1990 Painted Cave fire that speedily burned more than 400 homes and killed one person.

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TAKEAWAY

“For the last 25 years or so the Oakland mounds flame has been the seminal flame event that was seared into Bay Area residents’ psyche. For the current generation of North Bay residents, today’s firestorm will leave an indelible scar and for years to come we will all recall the Columbus Day firestorm.” — Meteorologist Ryan Wilbrun of the National Weather Service office for San Francisco/ Monterey.

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