Hot, Dry Diablo Wind Fanned Deadly Wine Country Firestorms

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

Notorious winds linked to 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 spark into a deadly inferno that seems to come out of nowhere.

Here's a look at where and when these winds occur.

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, making them ready to burn.

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

During the event, wind 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 fire killed 25 people, injured 150 and destroyed more than 3,000 homes and apartments.

This name is given to offshore winds that sweep through Southern California, commonly during fall, 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 passes and canyons.

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

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

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

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

"For the last 25 years or so the Oakland hills fire has been the seminal fire 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," said meteorologist Ryan Wilbrun of the National Weather Service office for San Francisco/Monterey.

Copyright AP - Associated Press
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