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In-depth: Why La Niña could make wildfire season worse

Weather pattern typically means warm, dry fall
Posted at 6:10 AM, Sep 24, 2020
and last updated 2020-09-24 11:13:33-04

SAN DIEGO (KGTV) - The National Weather Service says that conditions are in place for a La Niña weather pattern in the fall and winter of 2020-21. That could bring warmer, drier than usual weather to San Diego over the next few months.

By definition, La Niña happens when the water along the equator is colder than usual. That pushes the jet stream farther north and directs storms away from the Pacific Southwest region of the United States.

Because of that, Southern California typically sees less rain during La Niña years.

"What that means for everyone is that our winters can be variable, meaning they can be all or nothing," explains National Weather Service Meteorologist Alex Tardy. "It doesn't necessarily result in more or less rain, though. You just have a very inconsistent winter in general with a La Niña pattern like we're seeing developed now."

The numbers bear that out. According to the San Diego County Water Authority, San Diego averages 10.34 inches of rain every year, with most of it falling from December through February.

But in La Niña years, the rainfall is usually below average. During La Niña in 2018, San Diego only saw 3.34 inches. A La Niña in 2017 brought 12.73 inches. La Niñas in 2017 and 2012 got 8.18 and 7.90 inches of rain, respectively.

"The bulk of the historical cases have been somewhat below average," says Dan Cayan from the Scripps Institution of Oceanography. "It's 10 to maybe 30% below what we would normally achieve in those in those core winter months."

That's bad news for firefighters across California. Already this year, the state has set records for wildfire disasters, with 7,982 wildfires burning more than 3.6 million acres. A dry, warm fall and winter will prolong wildfire season.

"That's not a real welcome signal for us here in Southern California, particularly after this extended dry period and heatwaves," says Cayan. "A wetter winter this next year would have been a welcome relief. But the roll of the statistical dice right now does not look like they're in our favor."

Tardy puts it more bluntly.

"We are a tinderbox," he says. "We're looking at fuel moisture that is not just dry the way it should be, but it is record dry."

La Niña weather also brings more Santa Ana winds, which can help wildfires spread. Tardy says we can expect this to last for several months.

"The prediction is for the fall is to be warmer than average," he says. "So a continuation of what we saw in August, and for mostly dry conditions as we go deep in the fall."