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The extreme wind events that have fueled rapidly growing wildfires and prompted deliberate power blackouts across huge portions of California this month are referred to by many names, some more dramatic than others.
Diablo. “Devil wind.” “Inside slider.” What do they mean?
“Basically they’re any winds that come up over a mountain range and dry and warm after they hit the valley,” National Weather Service forecaster Michelle Mead explained.
Mead says a more precise term for the phenomenon is a downslope wind event, also called a “drying” wind.
Diablo winds, named for Mt. Diablo in the East Bay Area, refer to northeasterly downslope wind storms that affect Central California and the Bay Area. These events carry the devilish nickname whether the originating winds pass over the mountain in Contra Costa County or not, as long as they originate from that direction.
Downslope winds in Southern California are known as Santa Ana winds, and in the north portion of the Sacramento Valley, weather experts often just call them “north winds,” Mead explained.
“What creates them is you get an area of high pressure or dry weather that is basically parked off the West Coast, including California most times, and you get weather systems that are coming down the backside of that high pressure ridge over the Pacific Northwest or Nevada,” Mead said. “They increase the pressure gradient, and they blow harder.”
Mead confirmed that this past weekend’s downslope wind event was the strongest California has experienced in 2019.
“It was just a pretty strong weather system that went down the back side of the pressure.”
The upcoming wind event that has led NWS to issue yet another red flag warning for Tuesday, indicating critical fire weather conditions, will be similar in severity to the one from the middle of last week. As severe winds loomed, PG&E cut power Wednesday to just under 180,000 customers across 17 Northern California counties. Later that night, the Kincade Fire ignited in Sonoma County, in the hills east of Geyserville.
Downslope wind patterns happen predominantly during the spring and the fall, but are most dangerous in the latter, especially if conditions remain dry.
“We haven’t had any rain yet, so that’s what’s creating these critical wildfire conditions,” Mead said.
Some media outlets, including SFGate, referred to the wind event from early October that prompted a then-record-setting PG&E blackout as an “inside slider” weather system.
UCLA’s White Mountain Research Center in a glossary page defines “inside slider” as “a weather system that moves into California from the northwest, with the bulk of the system’s energy moving inland toward the Sierra Nevada and the Great Basin. These systems usually bring cool, breezy weather to Northern and Central California.”
Because downslope winds typically run east to west, an “inside slider” could be seen as the mirror opposite of a true Diablo wind event.
But as erratic as winds have been, it can all be a bit nebulous. Radar forecasts posted by the NWS throughout the month show that extreme winds have blown into the valley and North Bay Area from the northwest, from the northeast and from true north - sometimes all at once.