Here’s how California’s worst-ever wildfire siege occurred

28 August 2020

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USA – One indication of how destructive the past two weeks have been in California lies in the terminology the state firefighting agency uses to refer to what has happened. CalFire refers to the period since Aug. 15 as a “fire siege,” a term that might otherwise be used in warfare, but it’s appropriate considering two of the state’s top 5 largest fires on record began and reached record sizes during this time frame.

The blazes are still burning, harming air quality and human health, but they’re no longer prompting urgent evacuations. In fact, residents are being allowed back into some areas as the state shifts from crisis mode to containment and recovery. However, the most dangerous part of the Southern California wildfire season still awaits in October and November, with more months of fire risks in Central and Northern California as well.

Blazing heat, extreme dryness and a dry lightning outbreak

Two indexes fire forecasters use to gauge the risk of rapid fire spread were at or near record levels for Central and Northern California at the time the blazes broke out. One of these indexes is known as the vapor pressure deficit, which is a proxy for how quickly vegetation dries out, and the other being the Evaporative Demand Drought Index, or EDDI, which measures the thirst of the atmosphere. These indexes showed that the region was primed for burning if the right trigger came along.

The siege resulted from a unique combination of factors: an intense heat wave that broke monthly temperature records, including a 130-degree reading in Death Valley, Calif., which is one of the hottest temperatures ever reliably recorded on Earth. The heat helped dry out already parched vegetation, providing ample fuel for fires once they got going. In addition, gusty winds helped spark extreme fire behavior, including a verified fire tornado Aug. 15 in Lassen County.

But the fires would not have happened had it not been for a rare outbreak of lightning, which focused its assault on the San Francisco Bay area northeastward to the border between California and Oregon.A vehicle drives along Knoxville Road with flames from the LNU Lightning Complex in the background on Aug. 18 in Napa County, Calif. (Noah Berger/AP)

The intense lightning with a short time period was extraordinarily unusual for the state, comprising about 11 percent of the average annual lightning activity, according to Chris Vagasky of Vaisala, a company that tracks lightning around the world. Much of the strikes occurred with little rain accompanying it, a phenomenon known as dry lightning. Such lightning strikes can easily start fires, and those fires can burn for days before being noticed if they’re located in remote areas.

According to Vagasky, the five-year average for the month of August is 30,089 cloud-to-ground strokes across the state. Yet for the Aug. 15 to 19 period this year, there were 20,203 cloud-to-ground strokes statewide, meaning that more than half the month’s typical lightning total occurred in just four days.

According to CalFire, since Aug. 15 there have been more than 700 new wildfire starts in California, which together have burned more than 1.32 million acres. This is larger than the state of Delaware. At least seven have died, and nearly 2,000 structures have been destroyed.

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