Windy ‘ inside slider’ grows Bay fire risk

A mass of cold air moving in from Western Canada and through the Great Basin is responsible for the extreme fire risk the Bay Area faces this weekend.

Gusts up to 100 mph on some North Bay peaks, combined with unusually dry vegetation, could easily turn a spark into a wildfire.

“What we have going on is a system that we like to call the inside slider,” said Brian Garcia, a meteorologist with the National Weather Service

Bay Area, using a baseball analogy. That is because the “low and inside” air is cold and coming from inland, as opposed to off the Pacific Ocean, Garcia said.

These are the feared Diablo winds, and they are created when that cold, highpressure air moving into the region contrasts to lowpressure systems nearer to the coast.

“The greater the difference in pressure, the stronger the wind,” Garcia said.

That cold, dry air moving through the Great Basin area in Nevada and other western states will then drain westward through the Sierra Nevada, gaining speed as it moves down into California’s Central Valley like “water spilling over the edge of the dam,” said UCLA climate scientist Daniel Swain.

Swain said the wind will accelerate and dry out as it moves down the mountain slopes toward lower elevations in the Bay Area, where it will create the conditions for red flag fire warnings that have been issued by the National Weather Service through the weekend.

The warnings will take effect Sunday and run through Tuesday morning and have raised the likelihood that PG& E will cut power to large parts of its service areas to avoid its equipment sparking a blaze.

Winds will likely be strongest overnight Sunday into Monday morning, said meteorologist Jan Null, although he added that wind events like this one are not unusual.

Null said while the region has seen a relatively normal amount of rain this year, what is unusual was the low rainfall the previous year, when many places in the Bay Area had close to half their normal rainfall.

“Vegetation is at record dryness levels for this time of this year,” and primed to burn, Swain said, comparing conditions to those during the Kincade Fire in Sonoma County last year and fires that ravaged the North Bay in 2017.

At the water’s edge, there is potential for 5% relative humidity, according to Garcia, the NWS meteorologist.

“To me, it’s a mindblowntype emoji,” he said.