People in the Puget Sound region have experienced chilly weather this week, especially overnight. Temperatures have dipped into the low 30s in some places, with cool and partly sunny weather most days. That's going to continue pretty consistently through the weekend.
At the same time, furious winds have been stoking California's wildfires. KNKX weather expert Cliff Mass says the cold here is directly connected to the Diablo and Santa Ana winds that are fueling infernos there.
“It all starts with cold air, strangely enough,” said Mass, a professor of atmospheric sciences at the University of Washington – who also has a grant from the National Science Foundation to study these winds.
THE NORTHWEST CONNECTION
The cold air starts in areas north of California like Eastern Oregon and Nevada, but can extend as far north as Canada and the Arctic. It combines with high pressure and often passes through Washington as a cold front before it gets to California’s coast, where lower pressure dominates. The difference in temperature and pressure causes downslope, easterly flow.
As that air flows downward from the mountain peaks of the Sierra Nevada range, toward the coast, it compresses, warms up and gets drier as it moves into California. Eventually, it accelerates enough to become the kind of powerful, hot winds that can spark and fuel wildfires, especially as it hits areas with lots of dry grasses and chaparral.
These winds are known as the Diablo winds in the area where they hit around Northern California and San Francisco. Mass says they have been associated with some very major recent fires.
“The wine country fires back in 2017 – and last year’s Camp Fire,” he said. “And, in fact, we have had Diablo winds during the last week that were associated with the Kincaid fire, north of San Francisco.”
When the high pressure and the cold air from the north goes further south, the same general pattern causes what are known as the Santa Ana winds. They consistently hit Southern California, descending from the high desert into the Los Angeles basin and the area around Los Angeles.
“So they both have a lot in common – just subtle differences of where the high pressure is – that produces the Diablo winds in Northern California and the Santa Ana winds in Southern California,” Mass said.
WHY POWER OUTAGES?
These winds can be forecast with great accuracy several days in advance, which is why power companies in California have started shutting down the electric grid, in an effort to prevent wildfires.
“At least three or four days out, we could see it coming quite clearly,” so utilities are using this information, especially in areas where their infrastructure is vulnerable, says Mass.
“So the strong winds hits trees and branches, it hits the power lines, they start arcing and then that ignites the vegetation underneath it. And so the idea is to shut the power down so you don’t get those fires,” Mass said.
CLIMATE CHANGE IMPACTS?
Will global warming make these winds and the fires that they produce more intense, as things get hotter? Mass says it’s more complicated than you might think. It’s not clear that the Santa Ana and Diablo winds will get worse.
“It appears they’re going to weaken under climate change," Mass said, because the temperatures will be warmer in the northern areas that are their source regions.
“So the air won’t be as cold, that means the pressure won’t be as high," he said. "That means there’ll be less of a tendency for high pressure inland and low pressure near the coast. And that will tend to weaken both the Diablo winds and the Santa Ana winds."
He says that would appear to mean a lower risk of wildfires from these powerful winds.
“On the other hand, as the planet warms up, California will be probably a little bit drier and also it will be warmer," Mass said. "So that’s going to work to encourage fires."
The dueling effects – of weaker winds and warmer, drier conditions – create quite a bit of uncertainty about the ultimate effects of climate change on California's wildfires.
“So it’s really unclear what’s going to happen to the wildfires as we go later into the century,” he said.
Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.