Bob Henson ·

Above: The Multi Spectral Imager of the European Space Agency’s Sentinel-2 satellite captured this false-color image of the burn scar and active burn areas of the Thomas Fire in Southern California on Tuesday, December 5, during its first phase of rapid growth. The fire has since burned much further beyond the left and top of the image. Active fires appear orange; the burn scar is brown. Unburned vegetation is green; developed areas are gray. The Sentinel-2 image is based on observations of visible, shortwave infrared, and near infrared light. The city of Ventura is at far lower right. https://go.nasa.gov/2B6Drov

More than 6000 firefighters were engaged in a furious effort on Monday to contain the Thomas Fire, the largest of the multiple wildfires that have pummeled Southern California over the past week. The Thomas Fire made an enormous westward surge on Sunday, consuming tens of thousands of acres in Santa Barbara County and reducing the fire’s total containment level from 15% to 10%. Incredibly, the 57,000 acres burned between Sunday and Monday morning far exceeded the entire coverage of October’s catastrophic Tubbs Fire in the Santa Rosa area (36,807 acres).­

As of Monday morning, Cal Fire reported that the Thomas Fire had officially consumed 230,000 acres and at least 790 structures, making it the fifth largest and tenth most destructive wildfire in state history. Two of the top 20 most damaging fires occurred in November, but none of the previous top 20 fires in terms of acreage occurred any later than October—much less in December, well beyond the typical tail end of wildfire season. It’s entirely possible this fire will burn till Christmas and beyond, and not out of the question it will roll past the Cedar Fire of 2003 (273,246 acres) to become California’s largest fire on record.

On Monday morning, evacuation zones extended to the northern and eastern outskirts of the city of Santa Barbara and included coastal communities from Summerland to Carpinteria (see Figure 1). The University of California, Santa Barbara, postponed its final exams till early January.

Figure 1. Evacuation zones in Santa Barbara County as of 8 am PST Monday, December 11, 2017. Mandatory evacuations (red) cover a huge swath of higher terrain. Voluntary evacuations (orange) extend even further west, while also including more than 10 miles of land along the coast from the eastern end of the city of Santa Barbara to east of Carpinteria. If you are in the region of the Thomas Fire, please consult local authorities for the latest evacuation guidance. Image credit: County of Santa Barbara, via Google My Maps.

Figure 2. Christmas decorations illuminated a house in Carpinteria, Calif., on Sunday, December 10, 2017, as the growing Thomas Fire advanced toward seaside communities in Santa Barbara County. Image credit: David McNew/Getty Images.

Stubborn offshore flow, dry air will continue to hamper firefighting

Crews have gotten the upper hand on the other major fires that broke out last week. All of them were at least 75% contained as of late Sunday. However, the Thomas Fire is a beast unto itself. Control efforts have been complicated by the fire’s sheer scope, the ruggedness of the terrain, and the ample fuel present in the form of thick brush. Although major fires have occurred in the rugged land of Santa Barbara County every few decades, some tracts of land in this area reportedly haven’t seen a major fire for more than a century.

If nothing else, fire weather across coastal Southern California will be less extreme this week than last week. The highly amplified upper-level ridge has weakened and shifted north, and the pressure contrasts pushing air from the Great Basin toward and over the coastal ranges will be less intense overall. On the down side, however, surface winds will continue to blow in a downslope/offshore direction, allowing the air to warm as it descends. As a result, temperatures will remain well above average, especially during the daytime, and relative humidity will continue to dip into the 5-15% range. Weaker winds in general will be a major help in firefighting, though any periods of intensified wind could help the Thomas Fire take advantage of the very dry atmosphere.

In its Monday morning outlook, the NOAA/NWS Storm Prediction Center put coastal southern California in “elevated” fire risk for both Monday and Tuesday. The center noted the possibility of an upgrade to “critical” (the second highest level) if winds increase somewhat on Tuesday, as models suggest.

In the longer range, forecast models continue to suggest a dry pattern across Southern California and the Southwest U.S. prevailing for at least the next 1-2 weeks.

We don't like to stretch limits of computer models, but we really need some precipitation in our future. This stubborn high pressure ridge (current - left) doesn't budge thru X-mas (right)! Stronger than ever from Alaska & down west coast. #ThomasFire #CAwx #SoCal pic.twitter.com/mHJJS5hCnd — NWS Los Angeles (@NWSLosAngeles) December 11, 2017

Factor by factor: how unusual is the Thomas Fire?

This month’s wildfires in Southern California have been historic even by the standards of this naturally fire-prone region. Huge fires can erupt across the Great Plains in winter during dry, windy conditions, but “I can't think of any western forest fires of size this far into winter,” said fire expert Stephen Pyne (Arizona State University) in an email. The lack of precedents for any wildfire this large so late in the fire season makes it natural to wonder what factors—including human-produced climate change—came together to produce the Thomas Fire.

Winds: This has been a strong and prolonged Santa Ana event, though not a record-breaker in the strength of its top winds. On November 30-December 1, 2011, a powerful Santa Ana closed the Los Angeles International Airport and produced a peak gust of 97 mph on Whitaker Peak, as reported by WU weather historian Christopher Burt.

As for duration, the NWS Los Angeles office said on Monday morning that offshore winds could persist through Friday, which would be the 13th consecutive day. That would put the streak among the 20 longest such streaks of offshore flow observed since 1948, according to the NWS, but well short of the record-long 24 consecutive days observed from December 1953 into January 1954.

Timing: This windstorm fell squarely in the peak season for Santa Ana events, which are typically most frequent in December. The wildfire threat from Santa Ana winds is usually highest in early and mid-autumn, the traditional intersection point when strong winds may arrive before the first substantial rains. In this case, the rains haven’t yet arrived. Los Angeles has seen just 0.11” since October 1. In records going back to 1877, only eight other water years have gotten off to such a dry start.

The region’s deadliest December fire came during one of those dry years: 1977, at the tail end of a vicious two-year drought. Only 0.08” of rain fell in Los Angeles from October 1 to December 11, 1977, and only 0.11” by December 20. On that day, powerful winds swept across interior and coastal Southern California, and a fast-growing fire ripped through Honda Canyon on Vandenberg Air Force Base after a power pole blew down. Four firefighters were killed by the blaze, one of them dying several days afterward. The fire ended up consuming 98,000 acres and affected 35 miles of coastline, according to a report by Joseph Valencia, who wrote a book on the disaster, “Beyond Tranquillon Ridge."

Figure 3. A man motorcycles through a landscape charred by the Thomas Fire on December 8, 2017 in Ojai, California. Image credit: Mario Tama/Getty Images.

Development: Increasing population around the wildland-urban interface allows more opportunity for human activity to start a fire. (The cause of the Thomas Fire had not been determined as of Monday morning.) Increased population in fire country can affect the size of a fire in another, indirect way, said Pyne: when firefighting resources are stretched, the focus tends to be on saving life and property rather than wildlands. “Given the threats to communities, firefighting is surely being concentrated on structure protection, which allows the flames to propagate through the wildlands, which means the fires grow,” said Pyne.

Preconditions: Not only is Southern California going through an exceptionally dry start to the wet season, but the landscape includes extensive dried-out brush resulting from the relatively moist 2015-16, when Los Angeles picked up 19” of rainfall between July 1 and June 30. That wet season was in turn preceded by a vicious five-year statewide drought.

Temperature, precipitation, and climate change: The closest tie between the Thomas Fire and long-term climate change is the sharp trend toward warmer conditions in California, which intensify the effects of any particular drought. Precipitation goes through natural year-to-year and decadal swings in California (and there is evidence of “megadroughts” prior to industrial times that were longer and more intense than anything observed in the last century, according to the Climate Science Special Report just released as part of the upcoming U.S. National Assessment).

If long-term drying does set in across California, it’ll most likely work its way from south to north, as models largely agree on an expansion of the low-latitude Hadley circulation that tends to produce sinking air and dry conditions in the subtropics. Figure 4 shows that the strongest multi-model projections for North American drying in the coming century are across Mexico and the southwest U.S., especially during springtime.

Figure 4. Projected change (%) in total seasonal precipitation from CMIP5 simulations (Climate Model Intercomparison Project) for 2070–2099. The values are weighted multimodel means and expressed as the percent change relative to the 1976–2005 average. These are results for the highest-emissions scenario, RCP8.5. Stippling indicates that changes are assessed to be large compared to natural variations. Hatching indicates that changes are assessed to be small compared to natural variations. Blank regions (if any) are where projections are assessed to be inconclusive. From Chapter 7, Climate Science Special Report, U.S. Global Change Research Program. Data source: World Climate Research Program’s (WCRP’s) Coupled Model Intercomparison Project. Figure source: NOAA NCEI.

Heat and drought: an incendiary mix

California is already experiencing an increase in “hot” droughts—those that occur with above-average temperatures and thus dry out the landscape much more readily. This autumn is a classic example of such a period. In data going back to 1895 for the climatic region encompassing the Southern California coast, NOAA/NCEI reports that October-November 2017 tied with 2014 for the warmest on record and tied with 1903 for the second driest on record.

Warmer temperatures may in fact become the main driver of increased fire risk across the U.S. as a whole by later this century, according to one 2010 study. This appears to be going on in California now. In a landmark 2015 paper, Noah Diffenbaugh (Stanford University) and colleagues concluded: “We find that human emissions have increased the probability that low-precipitation years are also warm, suggesting that anthropogenic warming is increasing the probability of the co-occurring warm–dry conditions that have created the [then-]current California drought.” In more recent work, the team found that the conditions associated with severely dry, hot years in California have increased—but not necessarily at the expense of patterns that lead to the occasional very wet year. This is consistent with a common theme in precipitation and climate change research: the intensification of both wet and dry extremes.

An attribution study on California’s extreme 2014 fire season, published in 2015 in the Bulletin of the American Meteorological Society, concluded: “Some measures of extreme fire risk are also expected to increase in the future despite the overall lack of change in the mean fire probability and annual precipitation simulated by climate models for the next 50 years. Our result…indicates that man-made global warming is likely one of the causes that will exacerbate the areal extent and frequency of extreme fire risk.”

It may be impossible to completely disentangle the roles of wildland management, firefighting strategy, short-term weather, and long-term climate change in shaping California’s fire future—but for the time being, it’s hard to see the combination pointing anywhere but up, toward a future of increased fire risk spreading over an ever-longer season.

Climate Signals has an extensive compilation of feature articles and peer-reviewed research on the increasing long-term wildfire threat for California related to climate change.