Wildfire — North Bay Mountains, California

2020-08-17 to 2020-08-31 · North Bay Mountains, California

Event narrative

A second round of thunderstorms moved through the region into the morning of August 17th producing additional lightning strikes over the Bay Area. On the early morning of the 17th lightning produced a fire in Napa County that became the Hennessey Fire. Eventually, this fire merged with several other fires across Napa, Sonoma, Solano, Yolo, and Lake Counties (Gamble, 15-10, Spanish, Markley, 13-4, 11-16, Walbridge) to form the LNU Lightning Complex https://www.fire.ca.gov/incidents/2020/8/17/lnu-lightning-complex-includes-hennessey-gamble-15-10-spanish-markley-13-4-11-16-walbridge/. Over 350,000 acres burned with over 200 structures damaged and over 1400 destroyed along with 5 fatalities (3 in Napa County and 2 in Solano County) including civilian and fire personnel across all counties. The LNU Lightning Complex Fire is now one of the top 5 largest wildfires in California state history. The fire was still burning at the end of August.

Wider weather episode

A prolonged and oppressive heat wave swept the Central Coast and Bay Area for almost a week from August 14th to August 19th with widespread record breaking temperatures observed across the region. This was caused by a strong high pressure system over the Desert Southwest that expanded westward into California. This dome of heat brought hot temperatures to the area for several days. Multiple days of triple digit afternoon highs were recorded inland with some coastal locations even reaching the mid 90s. Several days of hot and dry weather further dried fuels over the area increasing fire danger. During this event, a surge of monsoonal and tropical moisture from a former Tropical Storm advected northward with sufficient instability to generate multiple high based and dry thunderstorms that produced several thousand lightning strikes over the Greater Bay Area. Many locations saw wind gusts of 40-50 mph with isolated areas seeing gusts of 60-75 mph. This prompted the San Francisco Bay Area forecast office to issue a rare severe thunderstorm warning. These lightning strikes in combination with gusty and erratic outflow winds sparked hundreds of wildfires across the state of California. Several smaller fires combined to form complexes some of which are now among the largest wildfires in state history. Most of which were still actively burning at the end of August. Hundreds of thousands of acres have been burned with several hundred structures destroyed as well as a handful of deaths and injuries. Tens of thousands of residents were also forced to evacuate. Additionally, all of these wildfires burning simultaneously across the state gave the Bay Area the worst air quality in the world at one point.

Source: NOAA Storm Events Database, event_id 912887. Narrative written by the NWS forecast office that issued the report.