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Wildfires Rage across California; Tropics Quiet for Now

By: Bob Henson 5:06 PM GMT on September 14, 2015

The explosive fire behavior that many Californians have been fearing all summer came to fruition over the weekend about 70 miles north of San Francisco, as the Valley Fire metastasized from an estimated 400 acres on Saturday to 50,000 acres on Sunday (see timeline at bottom of this page). The fire roared across the community of Middletown on Saturday night, prompting hasty evacuations and apparently destroying large parts of the town. One death has been confirmed by the California Department of Forestry and Fire Protection, and four firefighters from a helicopter crew were hospitalized with second-degree burns. Officials have had trouble confirming the amount of damage as the fire continued to rage nearby, but the National Interagency Fire Center reported in its daily update on Monday that at least 412 structures had been lost. The fire was zero percent contained, and close to 20,000 people have had to evacuate.

Two other large wildland fires are afflicting central California. The Butte Fire, which has scorched 65,300 acres and destroyed at least 214 structures since Wednesday, is now 25 percent contained, with more than 4500 firefighters on the scene and many structures still threatened. The long-burning River Complex Fire, which has roamed across 76,614 acres of far northern California since July 30, is 50 percent contained.

Figure 1. Firefighters create a firebreak near a home in Middletown, California, early on Sunday, Sept. 13, 2015, just ahead of the fast-growing Valley Fire. Image credit: AP Photo/Elaine Thompson.

Figure 2. A kitchen stove sits among the remains of a home on Sunday, Sept. 13, 2015, destroyed by the Butte Fire near Mokelumne Hill, California. Image credit: AP Photo/Rich Pedroncelli.

Figure 3. The infrared signal from the Valley Fire was brighter and larger than Reno, NV, on this image collected early Sunday morning, September 13, 2015, by NASA’s Visible Infrared Imaging Radiometer Suite (VIIRS). Image credit: NWS/Sacramento.

As discussed in this blog a couple of weeks ago, September and October are often the worst months for wildfire in California. The region’s Mediterranean climate leaves it high and dry during summer, so the impact of any drought during the previous winter’s wet season becomes exacerbated by the heat of summer and by the strong winds brought by autumn frontal systems and offshore Santa Ana winds. California is in the midst of a four-year drought on par with anything in the century-plus precipitation record, and state temperatures this year are the warmest on record by far. A new climate.gov analysis by Tom Di Liberto illustrates how tough it will be for California to dig itself out of its multiyear precipitation deficit: every region in California would need record-smashing rainfall this winter in order to bring the five-year totals (2011-12 through 2015-16) merely up to average.

Strong El Niños often bring wet conditions to Southern California, and the intensity of the emerging El Niño event may be enough to extend those above-average rainfalls into the central part of the state, as noted by the NOAA Drought Task Force in a report last month. However, even unusually heavy precipitation may not yield an above-average snowpack in the Sierra Nevada that could help boost water supplies next summer. Snowpack will depend hugely on how cold it is when the biggest storms hit the Sierra, and temperatures are at record warmth both globally and regionally.

Did pyrocumulus play a role?
I asked Daniel Swain, a Stanford University doctoral student and author of the excellent California Weather Blog, to weigh in on the situation. He responded late Sunday night with some clues as to what might have made the Valley Fire behave so explosively.

“It has been really disconcerting to watch this event to watch unfold over a little more than 24 hours. It became pretty clear last night that a true firestorm was underway--50,000+ acres in less than 24 hours and 40,000+ acres in less than 12 hours! What's amazing is that weather conditions were not particularly severe from a fire danger perspective. While there were some gusty winds and warm temperatures, that's not at all unusual for that part of the world. One strange thing did happen, though: a mid-evening ‘heat burst’/gusty wind event seems to have occurred across the North Bay and Lake County area yesterday, possible due to downdrafts from very weak mid-level convective clouds in the area. I've been sent a couple of photographs that suggest that the pyrocumulus cloud from the fire itself may have played a role in this strange event (which I followed with some of the wunderground PWSs, actually!), which would not be that surprising given the magnitude of the fire event. Still, the fact that the fire spread as fast and as far as it did given ambient weather conditions is nothing short of extraordinary.”

Pyrocumulus are the bubbling, cumuliform clouds that often form above large, intense wildland fires. The most spectacular pyrocumulus are sometimes called pyrocumulonimbus for their close similarity to cumulinumbus clouds, sometimes including lightning and anvil-shaped cirrus clouds. Pyrocumulonimbus can even inject aerosols into the stratosphere, according to NASA expert Michael Fromm and colleagues in this open-access article published in 2010 by the Bulletin of the American Meteorological Society. A NASA Earth Observatory article documents spectacular pyrocumulus captured in July 2014 by NASA’s Terra and Aqua satellites during California wildfires.

Figure 4. This view of a developing pyrocumulus cloud above the Oregon Gulch fire, a part of the Beaver Complex fire, was taken from an Oregon Air National Guard F-15C on July 31, 2014, at 8:20 pm PDT. Image credit: James Haseltine, via NASA Earth Observatory.

The role of drought and heat
In his email, Swain also stressed the preconditioning role of this year’s unprecedented combination of drought and heat:

“in some ways, this fire is a realization of widespread fears that California's worst drought on record had created the potential for truly extreme fire behavior this summer and fall. CALFIRE, the state agency tasked with fighting wildfires in California, has been emphatically and explicitly stating that the kinds of extreme fire behavior being observed on the Valley Fire (and other California fires this year) is not something that has been previously observed, and can be attributed directly to the severity of the ongoing, multi-year drought. This kind of on-the-ground assessment is consistent with a number of recent studies suggesting that the observed combination of extremely low precipitation and record-high temperatures is unprecedented in modern California. It's also a very sobering reminder that while many of the impacts of a severe drought are not as conspicuous as those during a more acute meteorological disaster--like a flood or a tornado--sometimes they can be truly and immediately devastating.

As for the week ahead, Swain’s outlook isn’t exactly optimistic:

“The Valley Fire still has zero containment, and is still spreading quickly. Clouds clear tomorrow and winds will start to pick up Tuesday in advance of that weak trough. I'm a bit worried about these pre-frontal winds (could hit 30-40 mph in the fire area), even though the trough itself may drop some light showers in the region. Does look warmer and drier once again after that, so it would be a temporary reprieve at best. Meantime, it seems that Linda's remnants will bring some rain to Southern California early this week (though the models seem to be all over the map with location/amount). If the higher end totals pan out, it'll put a temporary damper on fire season south of Santa Barbara (for perhaps a week or so), but historically that amount of rainfall does very little to mitigate the coming Santa Ana season, which is when most of SoCal's major fires occur.”

A rare September weekend: No tropical cyclones on Earth
We’re just past the climatological peak of the Atlantic hurricane season, and close to the peaks of North Atlantic hurricane and typhoon production as well. Yet for most of this weekend, we had no officially classified tropical cyclones anywhere on the planet. The disturbance shown just east of Vietnam in Figure 5 evolved into short-lived, minimal-strength Tropical Storm Vamco, which was bringing heavy rains early Tuesday local time as it approached the central Vietnam coast (see Figure 6). Varco’s formation ended a 54-hour streak with no tropical cyclones on Earth, the longest such streak to occur in any September since 2009, according to WU blogger Phil Klotzbach (Colorado State University).

Figure 5. At 1400 GMT (10:00 am EDT) on Sunday, September 13, 2015, there were no tropical cyclones anywhere on the planet. The disturbance shown east of Vietnam later became Tropical Storm Vamco. Image credit: Brian McNoldy, University of Miami/RSMAS.

Figure 5. Tropical Storm Vamco (purple blob at center) was moving onto the coast of Vietnam at 1611 GMT (12:11 pm EDT) on Monday, September 14.

Figure 7. This METEOSAT-9 satellite image, collected over the eastern tropical Pacific at 1500 GMT on Monday, September 14, shows tiny Invest 93L (left) and much larger Invest 95L (right). Image credit: NOAA/NHC.

Once Vamco is officially declassified, we may get a second global break from tropical cyclone activity before the next Atlantic system spins up. The most likely candidate is Invest 93L, located several hundred miles west-southwest of the Cape Verde islands at midday Monday. The National Hurricane Center gives 80% odds that this system will become a tropical cyclone by Wednesday. Models are in fairly close agreement that this system will develop into Tropical Storm Ida by midweek, but they also agree it should be recurving sharply by that point, posing no threat to North America or the Caribbean. A much larger tropical wave closer to the African coast, newly designated Invest 95L, is worth watching for potential development later this week. NHC gives this wave 40% odds of becoming a tropical cyclone by Saturday as it enters the central tropical Atlantic. Invest 94L, centered just east of Tampico, Mexico, is associated with a large area of showers and thunderstorms extending into the central Gulf of Mexico at the tail end of a decaying cool front. This system has a chance of developing into a short-lived tropical cyclone over the next several days (NHC gives it 30% odds over the next five days). It would most likely end up tracking westward into Mexico, although its slow development and weak steering currents add some uncertainty to the long-term outlook.

Bob Henson

Video 1. This heart-stopping video was taken by residents evacuating the Anderson Springs area, about five miles northwest of Middletown, California, at 8:30 pm Saturday, September 12, as the Valley Fire roared into the area. Image credit: mulletFive.

Fire Hurricane

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