Concerns about dry lightning and the potential for more disastrous wildfires loomed over the Bay Area on Thursday, adding to the anxiety that already has enveloped the region amid a terrible fire season.
The culprit was a monsoon-type system moving up from the Southwest region of the United States.
“We have that strong southerly flow moving into the area,” meteorologist Gerry Diaz said. “It’s that monsoon moisture.”
According to forecasters, monsoon moisture rises high into the atmosphere, propelled by the hot air rising over the desert. The hot air then eventually meets with cold air higher in the atmosphere to cause stormy conditions, including thunder and lightning.
Precipitation is produced, but the Bay Area’s hot and dry conditions that are typical of late summer and early fall often prevent raindrops from reaching the surface, Diaz said.
“It actually evaporates before it makes it to the ground,” he said.
The dry lightning hazard and the possibility of strong wind gusts prompted the weather service to issue a red-flag warning set to last from 5 p.m. Thursday until 11 a.m. Friday. The East Bay hills and interior valley were the areas affected by the warning, as were the Diablo Range and the North Bay mountains.
The recent hot spell was also cause for the state’s Independent System Operator to issue a Flex Alert for energy conservation from 4 p.m. to 9 p.m. on Wednesday, asking consumers to lower their energy use to ease strain on the power grid. On Wednesday afternoon, the agency issued another Flex Alert for the same time frame Thursday.
“That’s the time frame we expect the highest probability,” Diaz said. “There’s just enough moisture moving northward that it starts to interact with the environment. When it arrives, portions of the North Bay in northern Sonoma and Napa counties is where it will start. If it hangs around long enough, you may see it in the East Bay.”
A widespread lightning storm could hurt the battle to stop California’s two biggest wildfires. The Dixie Fire had burned 922,192 acres and was 59% contained as of Tuesday morning, according to the California Department of Forestry and Fire Protection. The Caldor Fire had burned 217,569 acres and was 50% contained.
According to Cal Fire forecasters, hot and dry conditions were expected to remain around the Dixie Fire in Butte, Tehama, Plumas, Shasta and Lassen counties. There was only a slight chance of thunder and lightning on Thursday night.
In El Dorado County, higher temperatures and strong winds were threatening to increase fire activity, as was the possibility of lightning, according to Cal Fire.
Weather and climate expert Daniel Swain wrote on social media that the incoming weather was similar to thunderstorms at this time in previous years. Both the August Complex and CZU Lightning Complex fires last year were fed by thousands of lightning strikes across Northern California.
NorCal weather set-up for Thu/Fri is reminiscent of previous Sept thunderstorm events. Key ingredients include offshore low pressure system & mid-level subtropical moisture tap. Very light showers possible, but main concern remains dry (or nearly so) lightning. #CAfire #CAwx pic.twitter.com/dKmF3U082d
— Daniel Swain (@Weather_West) September 7, 2021
The August Complex Fire burned 1.033 million acres — making it the largest wildfire in California recorded history — destroyed 935 buildings and killed one firefighter. The CZU Lightning Complex Fires, in San Mateo and Santa Cruz counties, burned 86,509 acres over 36 days, destroyed 1,490 buildings and killed one person.
Diaz said the system on its way is not nearly as powerful as those that sparked the fires in 2020 and won’t have nearly the same lightning activity.
“Essentially, that system and its interaction with the environment was much more potent,” Diaz said. “The good news for this event is that we don’t have near the amount of strength or the volatility. It’s much less pronounced.”
The heat would remain powerful, Diaz said. Temperatures were expected to surpass 100 degrees again on Wednesday in such usual hot spots as Brentwood, Concord, Livermore and Walnut Creek, and they also were likely to touch 95 in areas of the Santa Clara Valley and 90 in downtown San Jose.
On Thursday, those figures were expected to dip by 7-10 degrees in the South Bay and 10-15 degrees everywhere else in the region. Diaz said those temperature drops would be replaced just as quickly by higher humidity and muggy, sticky conditions.
“In the time leading up to the storms, it’s going to get on the muggy side,” he said. “It’s very common on the East Coast. As the cold front arrives, it gets very muggy, and then after the storm moves through, the system that comes behind it is much colder, much dryer.”
The cooling relief is expected to arrive Friday morning, as the monsoonal moisture makes its departure, he said.
Until then, anxiety is likely to increase.
“Unfortunately,” Diaz said. “All it takes is one strike to spark something really bad.”