Bob Henson ·

Above: The red glow of the Thomas Fire is reflected on the beach at Ventura, Calif., on Tuesday, Dec. 5, 2017. Raked by ferocious Santa Ana winds, explosive wildfires northwest of Los Angeles and in the city's foothills burned a psychiatric hospital and scores of homes Tuesday and forced the evacuation of tens of thousands of people. Image credit: AP Photo/Jae C. Hong.

One of the most widespread fire outbreaks in many years across Greater Los Angeles ratcheted to new levels on Wednesday, and conditions will become even more fire-supportive on Thursday all the way from L.A. to San Diego. By Wednesday afternoon, five separate blazes had already scorched more than 83,000 acres and at least 150 structures. The fires were being fed by a prolonged round of warm, dry Santa Ana winds ripping across a bone-dry landscape.

Unfortunately, the winds will only intensify from late Wednesday through Thursday. The National Weather Service is predicting gusts of 50 – 60 mph at lower elevations and as high as 75 mph across hillsides for up to 18 – 24 hours, an unusually long interval for Santa Ana winds of that strength. (More typically, Santa Ana winds peak overnight and weaken by day, as was the case on Tuesday). Relative humidity is expected to dip well below 10%, especially as temperatures rise during the day Thursday.

BREAKING Fire chief Ralph Terrazas says Los Angeles faces the highest risk of wildfire tomorrow that he’s seen in his entire 31 year career. — James Cook (@BBCJamesCook) December 7, 2017

The biggest fire development on Wednesday was the growth of the Skirball Fire in the mountains between central Los Angeles and the San Fernando Valley, along the east side of Interstate 405. Several homes in the affluent Bel Air neighborhood had been lost by Wednesday afternoon, according to the Los Angeles Times. The blaze was less than 5 miles from the campus of the University of California, Los Angeles, where classes were cancelled on Wednesday afternoon and a men’s basketball game for Wednesday night was cancelled.

Figure 1. Firefighters survey a home consumed by a wildfire in the Bel Air district of Los Angeles, Wednesday, Dec. 6, 2017. A dangerous new wildfire erupted in the tony Bel Air area of Los Angeles early Wednesday as firefighters battled three other destructive blazes across Southern California. Image credit: AP Photo/Jae C. Hong.

With most of the blazes virtually uncontained, and some close to highly populated areas, there was real concern that Thursday’s high winds could lead to spotting—the lofting and tossing of embers well ahead of a main fire line. Spotting was a key factor in the explosive growth of the deadly Tubbs Fire north of San Francisco in October. “Fire spotting is one of the major ways that fires spread and homes are ignited and destroyed in wildland/urban interface fires,” warns the National Fire Protection Association. “Firebrands can come down on and ignite combustible roofs, combustible items stored adjacent to homes, and other nearby combustible fuels.”

“Extremely critical” fire weather expected

More than 8 million people live in the coastal swath where NOAA/NWS Storm Prediction Center is calling for “extremely critical” fire weather conditions on Thursday. This designation—the most dire issued by NOAA—is surrounded by a “critical” threat area that includes all of downtown Los Angeles and San Diego, including more than 11 million residents.

Figure 2. True-color MODIS satellite image of smoke from the fires in Southern California as seen on Wednesday morning, December 6, 2017. Image credit: NASA.

Air quality a growing concern

As of Wednesday, the bulk of smoke from the Southern California fires was blowing offshore from the Thomas Fire in Ventura County, affecting residents of the city of Ventura but avoiding the bulk of the Los Angeles metro area. However, smoke from the large Creek Fire and the smaller Rye Fire was blowing across the San Fernando Valley. Air quality across the valley and adjacent parts of Ventura County was in the “unhealthy for sensitive groups” range on Wednesday afternoon, according to data from the South Coast Air Quality Management District (see Figure 3).

Figure 3. Air quality values as of 1 pm PST Wednesday, December 6, 2017, across Greater Los Angeles. Numbers denote the various air quality reporting districts. Across the East San Fernando Valley (area 7), the 24-hour average value of fine particulate matter (PM2.5) was 147, near the top end of the “unhealthy for sensitive groups” rating. Image credit: SCAQMD.

Why is this happening in December?

Large fires are more common in California’s more remote terrain than in Greater Los Angeles, where small fires are often caught before they have a chance to take off. It’s especially unusual to have multiple major fires burning in the metro area—and even more noteworthy that it’s happening in December. According to a compilation from Cal Fire, all of California’s 20 most widespread fires occurred between June and October. There are signs that the wildfire season is expanding, though: more than 26,000 acres were burned in San Diego County during May 2014.

December is actually the most common month for the notorious Santa Ana winds, which push downslope from the coastal hills of Southern California toward the coast. The Santa Ana pattern requires high pressure to the east of the coastal ranges, forcing air toward the sea, and strong highs are much more frequent across the Great Basin in winter than in summer. Because of compressional heating, the Santa Ana winds get warmer as they descend, and the relative humidity drops. The strongest, longest-lasting Santa Ana events occur when both large-scale dynamics and local temperature contrasts are significant, according to a recent model-based 65-year reanalysis led by Janin Guzman-Morales (Scripps Institution of Oceanography). Both large-scale and local factors appear to be at play in this week’s event.

So why are major fires so uncommon over Southern California in December, when the Santa Ana winds so often blow? One factor may be fewer people inadvertently setting fires in winter, given the reduced outdoor recreation this time of year. Also, dry lightning storms aren’t a trigger, as they are in much of California during the summer months. Moreover, the landscape has usually gotten at least some significant rain by now. Between October 1 and December 5, downtown Los Angeles averages 1.96” of rain, and San Diego averages 1.79” (based on the 1981-2010 climatological period). But those averages mask a lot of variability, both wet and dry. This autumn, both cities have wound up on the dry end of the stick.

—As of Wednesday, Dec. 6, downtown Los Angeles had picked up 0.11” of rain for the water year starting October 1. That’s the 11th driest start to the water year in 141 years of recordkeeping in downtown LA.

—San Diego received just 0.02” last month, putting it in the top-ten driest Novembers in data going back to 1850. Given that October saw only a trace of rain, and December has seen none thus far, the water-year total (Oct. 1 to Dec. 6) is also 0.02”, putting it in a tie with 1962. In records going all the way back to 1850, only two other water years in San Diego were drier at this point: 1929 (a trace) and 1872 (no precipitation).

Figure 4. The charred remains of actor Burt Lancaster's palatial home, which was destroyed in the Bel Air Fire of November 1961. Image credit: Photo by Hulton Archive/Getty Images.

“A tragedy trimmed in mink”: The 1961 Bel Air Fire

The Skirball Fire is located close to the footprint of the Bel Air Fire of November 5, 1961. That blaze led to L.A.’s single most destructive fire in the 70-plus years since the city’s massive expansion got under way after World War II. No deaths were reported in the 1961 fire, but celebrities from Zsa Zsa Gabor to Burt Lancaster lost homes as the fire consumed more than 16,000 acres and close to 500 structures in one of the city’s ritziest areas—“a tragedy trimmed in mink,” as LIFE magazine put it. The Los Angeles Times has a photo retrospective on the fire.

Marshall Shepherd (University of Georgia) posted a nifty explainer on Santa Ana winds at Forbes’ WhoaScience blog.