The setup in the atmosphere is known as a strong pressure gradient. When an area of high pressure and low pressure are close to each other, they create a wind tunnel of extremely fast offshore winds.
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The position of low pressure over the Baja Peninsula and high pressure over the Pacific Ocean caused that wind tunnel to be located over Los Angeles County. We generally refer to those winds in Southern California as the Santa Ana Winds, although the National Weather Service is now not sure if the winds that stoked the fires in Los Angeles were actually Santa Ana Winds because of their direction.
Above those fast winds is a lot of cold air, which is very dense. The dense cold air can come crashing down to the surface, pulling those very strong winds down to the ground and accelerating them as they roll down the mountains and hills across California. Squeezing into canyons and valleys, those winds can easily exceed 60 mph.
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Had the high pressure been located farther north, that wind tunnel would have been placed across Northern California. We have seen that setup happen in the past, and when these extremely strong winds develop in Northern California, they are called Diablo Winds.
In 2017, across Napa and Sonoma Counties, we saw Diablo Winds 60 to 70-plus mph, which caused six major fires to occur. Among them were the Tubbs, Nuns and Atlas fires. At the time, it was the costliest group of wildfires in the state's history.
MAP: Track wildfires across California