Paul Somerville, Chief Geoscientist, Risk Frontiers
Nearly 771,000 acres of largely unpopulated land have burned across California during the past week as dozens of lightning-sparked wildfires moved quickly through dry vegetation and threatened the edges of cities and towns. The fires have been most severe in the state’s northern and central regions, where about 600,000 acres have burned in the past week (Figure 1).
Evacuations surged on August 18 and 19 as authorities worried that high heat and gusty winds could cause the fires to spread rapidly. The resulting fires – and complexes of many small fires – have merged into major conflagrations in many parts of the state. By August 20, several of the major fires had more than doubled in size, in some cases jumping across major highways, as crews struggled to contain the blazes. By August 21, the two largest blazes, the SCU and LNU Lightning Complexes, had charred 340,000 and 325,000 acres respectively, becoming the second and third largest fires in California history (Table 1). The CZU Lightning Fire forced the evacuation of more than 64,000 people, some of whom may not be able to return to their homes for weeks. Five people have died and about 1,000 structures have burned.
The California wildfires, along with other blazes in the West, have sent a blanket of smoke across at st 10 states and southwestern Canada, with smoke extending over the Pacific Ocean as well (Figure 1, right panel). Air quality alerts are in effect for parts of California, where the tiny particles in the dense smoke are aggravating respiratory conditions and worsening preexisting health conditions that are already threatened by the coronavirus. The cloth masks that have now become a habit for many Californians when they venture outside are largely ineffective against the tiny smoke particles filling the air, and doctors recommend using N95 masks with vents. People are being asked to shelter in place, staying at home with their windows closed and ventilation systems set to recirculate air, which is difficult during a heatwave in areas such as San Francisco where many people do not have air conditioning.
A rare mix of ingredients came together in central and northern California to produce fast-moving, explosively growing wildfires that are powerful enough to create their own weather. Doppler radar revealed at least five tornado-strength rotational signatures inside the smoke plume in Lassen County, California. The record heat reached astonishing levels during the past two weeks as a massive “heat dome” parked itself over the West. On August 16, Death Valley, California, reached 130 degrees Fahrenheit (54 degrees Celsius). The combination of an intense, long-lasting heatwave, dry vegetation at the end of the summer, and a rare outbreak of August thunderstorms led to these blazes. Fueled by the heat, thunderstorms broke out on Sunday Aug 15 as a surge of tropical moisture pushed inland. The storms’ 20,000 lightning strikes (Fig. 3), including dry lightning storms, sparked more than two dozen blazes over a period of 3 days. An ancient stand of the world’s tallest trees has fallen victim to California’s raging wildfires. The CZU and SCU complex fires near Santa Cruz have ravaged Big Basin State Park, California’s oldest state park, some of whose giant redwoods are more than 50 feet around and 1,000 to 1,800 years old (Fig. 4).
This is just the beginning of the state’s wildfire season, something that has been a constant threat during the past four years of blazes, some sparked by downed powerlines, that have set records for size and lethality. Despite the familiarity, the current fires and their speed and thick smoke have presented a new terror amid a global pandemic – poor air quality and concerns about evacuating masses of people to crowded shelters, and that some might not heed the warnings. Tens of thousands of people have been asked to evacuate and make difficult decisions about where to go. In the past, they might have stayed with friends or family, but now they need to calculate the risk of exposure to the novel coronavirus. Wherever people go, they are likely to face other hardships. California has been enduring a record-breaking heatwave that has prompted rolling blackouts because of high electricity demands for air conditioning and other uses. Most of the area is also experiencing severe or moderate drought.
In Santa Cruz and San Mateo Counties, south of San Francisco, about 48,000 people were ordered to evacuate because of a fire, part of the CZU Lightning Complex, that is threatening communities there. The blaze has already burned 50 structures. On the evening of August 20, the University of California at Santa Cruz was under mandatory evacuation and had declared a state of emergency.
The largest of the lightning-related fires was north of San Francisco, covering Napa and Sonoma counties. On August 20, that mass of fires, the LNU Lightning Complex, had grown to 219,000 acres and was uncontained. Approximately 30,000 structures were at risk of burning and 480 had been destroyed.
The blaze near Vacaville, known as the Hennessey Fire and part of the LNU Lightning Complex, has been one of the most destructive, burning down homes and claiming the life of a PG&E worker who was assisting first responders. This blaze burned down the La Borgata Winery and Distillery in Vacaville. Mandatory evacuations remained in effect for the north part of the city on August 20, and CalFire reported three additional civilian fatalities associated with the LNU Lightning Complex.
CalFire is at normal staffing levels, with approximately 12,000 firefighters working on August 21. Additional firefighters are being sought from other states and from Australia. In Central California, a pilot on a firefighting flight near Fresno died when his helicopter crashed.
Overall losses include 5 deaths, 64,000 people evacuated, over one thousand structures burned, 31,000 structures threatened, and approximately one million acres burned as of August 21.
The 2019/20 bushfires in eastern Australia were fought under dire conditions, but the presence of the coronavirus in California has made fire-fighting conditions there even more dire, especially those relating to evacuation. There are 665,000 coronavirus cases in the state, growing by 5,000 a day, and 12,000 deaths, growing by 150 a day.
Table 1. 20 largest wildfires in California since 1932. Only 3 occurred before 2000. (Source: Updated from Cal Fire)
 SCU Lightning Complex Fire: Contra Costa, Alameda, Santa Clara, Stanislaus and San Joaquin counties
 LNU Lightning Complex Fire: Napa, Sonoma, Solano, Yolo and Lake counties
 CZU Lightning Complex Fire: San Mateo and Santa Cruz counties