Tens of thousands of people were ordered to flee their homes across Los Angeles County on Tuesday as multiple out-of-control wildfires, fueled by the strongest Santa Ana wind event in over a decade, threatened communities including Pacific Palisades and Altadena.
Even in a state where fires can occur year-round, in January, the sight of a massive plume of smoke looming above the Santa Monica skyline, and of the traffic snarled along the Pacific Coast Highway for miles by people evacuating, was mind-boggling.
Santa Anas are notorious for spreading wildfire flames, and they most often occur in colder months. By January, though, their impacts are often less dramatic, as the landscape is typically less flammable after rains in the fall and early winter. But this year, the rains have not come, leaving most of Southern California extremely dry.
Most locations south of Ventura County have only recorded about a quarter-inch of rain or less in the last eight months. The Los Angeles area has received only sprinklings of rain since April. San Diego and Riverside have logged even less.
“The sort of dryness we’re seeing in a lot of the plant species right now mirrors what you would typically expect in October or early November when the rainy season starts,” said Jonathan O’Brien, a meteorologist with the U.S. Forest Service.
Isaac Sanchez, the deputy chief of communications for Cal Fire, put it more plainly: “The landscape in all of Southern California is ready to burn.”
The moisture-starved vegetation and the strong winds are two key ingredients that led to the sort of devastating wildfires that erupted on Tuesday and continued to burn on Wednesday. Low relative humidity levels and Southern California’s landscape of mountains, canyons and valleys are also part of the recipe.
“It’s the same story that we talk about every time we see a big fire,” Mr. Sanchez said. “It’s the fuel, it’s the weather and it’s the topography. All three things are in alignment, and all it takes is a small ignition source to start a fire.”
A tale of two Californias: wet in the north and dry in the south
Wildfires are a year-round threat in California, but before the start of winter, a good soaking rain usually puts an end to the peak part of the wildfire season, when the most destructive wildfires occur.
Large wildfires are unusual in California in January, when the landscape is moist and less flammable. The state’s most destructive and deadliest wildfires have all occurred between July and December.
About two inches of rain is required to saturate the ground and vegetation and significantly lower wildfire risk. Northern California has received that much and a lot more. An atmospheric river that stalled over the region in November brought an astonishing 20 inches of rain alone to parts of Sonoma County, just north of San Francisco. Several rivers saw levels jump 20 feet before winter had even begun, and storms continued to drench the region in December. The snowpack in the Sierra Nevada, a key source of water for California, benefited from the rush of storms and is robust and piled high.
It’s a different story in Southern California, which has usually received a shot or two of rain by the end of December, and certainly by early January. But this fall and winter have only brought a few sprinklings and some drizzle.
“A lot of our storms move north to south and gradually make their way to our area,” said Ryan Kitrell, a forecaster for the National Weather Service’s office in Oxnard. “Usually they weaken, but they do get here. We’ve had those systems in Northern California that have stalled — and not come down. They just stall and then go away. Every single one of them.”
Southern California trends drier than the north, but this year the disparity is unusually extreme. Most of Northern California has received well over 100 percent of its average precipitation to date as of Oct. 1, and most of Southern California has received less than 20 percent, according to the Department of Water Resources’s Water Watch.
Southern California’s dry start to winter comes after a summer marked by prolonged, record-breaking heat that baked the landscape and cooked the trees and brush.
The National Weather Service gauge in downtown Los Angeles, a good indicator for rainfall in Southern California, has recorded only 0.29 inches of rain since May 1. This is the second-lowest amount of rain ever measured in that roughly eight-month time period, with records going back to 1877. The San Diego area is also seeing some of the driest conditions in recorded history.
A ridge of high pressure is deflecting storms from Southern California
The extremely wet conditions in Northern California, and the extremely dry ones in Southern California, are the result of a climate pattern that has been locked into place for a few months. The jet stream — the winds high in the atmosphere that carry storms across the country — has been directing atmospheric rivers across the Pacific Northwest and Northern California. Meanwhile, an area of high pressure has sat generally below the jet stream and over Mexico and Southern California and deflecting storms.
“Southern California is drier than Northern California, on average, said Mike Anderson, a state climatologist with the California Department of Water Resources. “This is just kind of an accentuation of that largely due to the strength in that high pressure system and how it moves and grows and shrinks relative to the other storms moving across the Pacific.”
One big issue with this pattern is that it prevents storms from passing over Southern California. Another problem is that the jet stream often guides the storms inland into the Great Basin, propelling airflow into Southern California.
“That causes more Santa Ana winds,” said Mr. Anderson. “As one of my colleagues down south says, that’s when we actually find a way to make it drier than it already is.”
The vegetation was already dry on the ground before the Santa Anas picked up, fanning the landscape and sucking any remaining moisture out of the ground and trees and plants.
The post A Dangerously Dry Southern California Was Ready to Burn appeared first on New York Times.
