The Mosquito Fire was a large wildfire that burned in California's Placer and El Dorado counties as the state's largest wildfire of 2022. The fire began on September 6, above Oxbow Reservoir in the Middle Fork American River drainage on the western slope of the Sierra Nevada. The Mosquito Fire went on to burn , destroying 78 structures in the small, rural communities of Michigan Bluff, Foresthill, and Volcanoville. It was declared 100% contained on October 22, more than a month and a half later, well after the fire's progression was halted.
During the first week of the fire's spread, more than 11,000 people were placed under evacuation orders and more than 9,000 structures were threatened. Officials warned of "great potential" for continued fire spread, but the fire was moderated by an unusually early Pacific storm that brought wetting rains to the area in mid-September, after which the fire grew little and containment steadily increased.
The fire suppression effort cost more than $180 million, and at its peak involved more than 3,700 firefighters under the direction of the Type 1 California Incident Management Team (IMT) 5. Cal Fire, the U.S. Forest Service, Placer and El Dorado counties, and the Foresthill Fire Protection District were all part of the Mosquito Fire's unified command. The cause of the fire has not been established, but the potential role of Pacific Gas & Electric Company (PG&E) equipment is the subject of multiple civil lawsuits and a U.S. Forest Service criminal investigation.
Background factors
Before and during the early days of the Mosquito Fire's spread, much of California experienced a record-breaking heat wave. Downtown Sacramento, 50 miles to the southwest, recorded its all-time highest temperature of 116 degrees Fahrenheit. UCLA climate scientist Daniel Swain described the heat wave as "the worst September heat wave on record, certainly in Northern California."
In the weeks before the Mosquito Fire began, vegetation moisture levels in the northern Sierra Nevada were already nearing record lows. The heatwave created a "flash drying effect", drying out vegetation even further. That the heat wave occurred unusually late in the summer, when fuels are typically at their driest but after temperatures have usually peaked, exacerbated the flammability of vegetation across the state. On September 2, in advance of the heat wave, the National Interagency Fire Center issued a "Fuels and Fire Behavior Advisory" for most of Northern California, warning of elevated fire weather concerns with "conditions conducive to long range spotting, rapid fire growth, and high resistance to control."
Fire progression
Ignition
The Mosquito Fire was first reported at approximately 6:27 p.m on September 6, above the northern shore of Oxbow Reservoir near Mosquito Ridge Road (for which it was named). Aircraft were overhead to fight the fire by 6:47 p.m, and they reported that the fire was 4 to 5 acres in size, burning in brush and timber with a moderate rate of spread and some spotting. Steep terrain with tough access hindered the firefight: an old landslide on the road near the fire made it difficult for fire engines to pass, and the fire was burning on the shadowed side of a canyon slope that made it difficult for larger air tankers to drop water or fire retardant on it. Air attack was initially hopeful that ground crews alone could handle the fire once night began to fall and firefighting aircraft were forced to return at roughly 7:41 p.m, less than an hour after arriving on scene.
At around 9:00 p.m, the fire was fanned by outflow winds from nearby thunderstorms, and crews reported being challenged by the gusty winds and a now-moderate rate of spread. As the fire began to spot ahead, evacuations were ordered for Michigan Bluff and surrounding roads. The fire jumped the North Fork of the Middle Fork American River and continued to spread, reaching 100 acres before midnight and producing a smoke plume visible from as far away as Auburn, 20 miles to the southwest. The fire continued to burn actively through the night, moving northwest up the side of the Middle Fork American River canyon.