Extreme Holiday-Season Fire Threat Puts Southern California on Edge

December 4, 2017, 7:02 PM EST

 
Above: Christmas stockings hang on Saturday, Dec. 2, 2017, from a fireplace at a home destroyed in a ferocious October wildfire in the Coffey Park area of Santa Rosa, Calif. Image credit: AP Photo/Rich Pedroncelli.

From Santa Barbara to San Diego, California’s coastal cities and mountains are on high alert this week, as an unusually prolonged bout of Santa Ana winds blowing toward the coast will lead to a multi-day period of extremely dangerous fire weather. Downslope winds are predicted to gust into the 45 - 60 mph range at lower elevations from Monday night into Tuesday, with localized gusts to 60 – 80 mph possible at higher elevations. Very low humidities and a tinder-dry landscape will exacerbate the threat. Update (6:10 am PST Tuesday): More than 27,000 people have been forced to evacuate from a massive fire in Ventura County that had burned some 31,000 acres overnight. At least 150 structures have been reportedly lost, and the fire was moving into the city of Ventura on Tuesday morning.

“Extreme fire behavior is possible with every spark,” warned the National Weather Service office in Oxnard, which serves the L.A. region.

As of Monday afternoon, the NOAA/NWS Storm Prediction Center had flagged most of the coastal zone mentioned above for critical fire weather conditions from late Monday through Tuesday. The center warned that these critical conditions could extend into Friday. Tuesday is a particular concern, as SPC has placed parts of five coastal counties in an “extremely critical” threat area—the highest risk possible.

SPC issues several “extremely critical” threat outlooks for some part of the U.S. in a typical year. The current one appears to be only the second such outlook issued during December since at least 2002. Big fires themselves are rare in California during December: the last major one was the Pfeiffer Fire, which burned 917 acres near Big Sur on Dec. 15-16, 2013. That blaze destroyed 38 structures, including the home of Big Sur’s volunteer fire chief.

NASA/MODIS image of Pfeiffer Fire near Big Sur, California on December 16, 2013, at 1:05 pm PST.
Figure 1. The MODIS (Moderate Resolution Imaging Spectroradiometer) instrument aboard NASA's Aqua satellite captured this image of smoke and detected the heat from the Pfeiffer Fire near Big Sur, California on December 16, 2013, at 1:05 pm PST. The red-outlined area represents the heat from the fire, which started on the previous night. Image credit: NASA Earth Observatory.

A bone-dry autumn sets the stage for December fire threat

“The upcoming (relatively) cool but extremely dry Santa Ana event poses a particularly high risk of fast-moving wildfires due to the incredibly dry start to the rainy season down in Southern California this year,” Daniel Swain (California Weather Blog) told me in a Twitter message.

“Vegetation in most places is near peak summer dryness since it simply hasn't rained yet in any meaningful sense, and the very strong winds combined with the extremely dry air mass associated with this offshore wind event is a very serious fire weather setup,” Swain said. “Of course, if there are no fire ignitions, we may dodge a bullet. But as we saw in Santa Rosa back in October, late-season dry offshore wind patterns are the ones to watch for when it comes to dangerous California wildfires.”

The state’s North Bay region is still reeling from the multiple fires that grew to catastrophic levels during an overnight bout of very high winds. The Tubbs Fire alone caused more than $1 billion in damage and killed 22 people. All told, the wildfires were the deadliest and most destructive in modern U.S. history.

A flipped, burned-out car sits among debris and rubble in a fire-destroyed neighborhood in Santa Rosa, California on Oct. 20, 2017
Figure 2. A flipped, burned-out car sits among debris and rubble in a fire-destroyed neighborhood in Santa Rosa, California on Oct. 20, 2017. Image credit: Josh Edelson/AFP/Getty Images.

One might expect this week’s fire risk to be lessened by the cooler temperatures that prevail in December as opposed to late summer or early autumn. Highs this week should range from around 75°F to the low 80s in Los Angeles and San Diego, which is only a few degrees above average for this time of year. However, the air being pushed into the region by the Santa Ana winds will originate as very cold, dry air aloft. The relative humidity (RH) will drop further as the air is forced downslope, which compresses and warms it. RH is predicted to drop into the 5 – 10% range over the fire threat areas. And when it comes to fire risk, RH trumps temperature, according to wildfire expert Stephen Pyne (Arizona State University), author of “Between Two Fires: A History of Contemporary America.”

“Everything I've read (and experienced) points to RH as the more critical factor, and temperature as influential largely through RH,” Pyne told me in an email. “A fire burns so hot that a few degrees of ambient temperature won't make much difference. But fuel moisture—particularly in the ‘fine fuels,’ like dead grass, pine needles, and fine branches on shrubs—translates immediately into how well the landscape will burn.

“Such fuels can respond within a few hours to changes in RH, which is why most fires die down at night, and can even go out, and why a rise in RH can dampen burning even in the absence of rain.”

Parched start to the wet season in SoCal

California’s fire risk tends to peak in late summer and early autumn, after vegetation has dried out in summer heat but before cool-season rains kick in. Between October 1 and December 3, downtown Los Angeles averages 1.90” of rain, and San Diego averages 1.70” (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 Sunday, Dec. 3, downtown Los Angeles had picked up 0.11” of rain for the water year starting October 1. That’s the 14th 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, the water-year total (Oct. 1 to present) is also 0.02”, which puts this year in a tie with 1962 for fourth-driest water year in records going all the way back to 1850.

San Diego precipitation for the period from October 1 to December 3, 1987-2017.
Figure 3. Wide year-to-year swings are evident in San Diego precipitation for the period from October 1 to December 3, 1987-2017. This year’s 0.02” is even more scant than the 0.04” recorded in 1999 and the 0.12” in 2009. Image credit: NOAA Regional Climate Centers.
3-7 day forecast for NoAm 500-mb flow from GEFS, 18Z 12/4/2017

Figure 4. A very amplified pattern will take hold over the next week across North America, with strong upper-level ridging across western North America and deep troughing over the eastern Pacific and eastern North America. This pattern favors dry, mild conditions along the Pacific Coast, with frequent shots of cold air across the eastern U.S. and Canada. Black contours depict the flow at 500 millibars (about four miles high), averaged across the 20 members of the GFS model ensemble for the 3-to-7-day forecast period from 1 pm EST Wednesday, Dec. 6, to Sunday, Dec. 10. Image credit: tropicaltidbits.com.

There’s no sign of any thirst-slaking moisture for Southern California in the foreseeable future. The upper atmosphere over North America is locking into a pattern with a very strong ridge near the Pacific Coast, a strengthening trough in the East, and a freight train of northwest flow and cold air funneling into the eastern half of the U.S. and Canada. This pattern will force any potential rainmakers well north of Southern California—and even the San Francisco Bay Area, if the latest models are any indication—for at least the next week or two.

Is the RRR rearing its head again?

California weather watchers may be getting just a bit of déjà vu. A powerful upper ridge recurred along and near the West Coast for three winters in a row during the state’s five-year drought (2011-2016). The feature came to be known as the Ridiculously Resilient Ridge, a name coined by Swain that stuck. New research by Daniel Swain and colleagues is shedding more light on the RRR and its connections to sea-surface temperatures in the Pacific. Swain covers the current North American circulation pattern in the context of these new findings in a Monday post at California Weather Blog.

Swain's entire post is well worth reading, but the closing paragraph is what resonates the most:

“Tropical warmth (in the West Pacific) and coolness (in the East Pacific) are both linked to different patterns of North Pacific winter ridging, and may offer an early warning of seasons with an elevated risk of dry conditions in California. Interestingly, tropical Pacific Ocean temperatures during autumn 2017 were warm in the west and cool in the east amidst a modest (and ongoing) La Niña event—a combination that suggests a substantially elevated likelihood of West Coast ridging this winter. To date, Southern California has experienced one of its driest starts to the Water Year on record, and strikingly persistent West Coast ridging is now expected to last at least two weeks. It will certainly be interesting to see how this winter plays out in the context of these new research findings.”

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Bob Henson

WU meteorologist Bob Henson, co-editor of Category 6, is the author of "Meteorology Today" and "The Thinking Person's Guide to Climate Change." Before joining WU, he was a longtime writer and editor at the University Corporation for Atmospheric Research in Boulder, CO.

bob.henson@weather.com

@bhensonweather

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