Forecast: Heat wave this week will intensify fire danger for Bay Area and Northern California
A pre-Memorial Day heat wave will prime the Bay Area for another dry fire season, roasting the region's landscape with some of the hottest weather so far in 2022 and pushing temperatures in some cities close to 100 degrees.
A month ahead of the official start of summer, high temperatures could climb 5 to 20 degrees above normal today and Wednesday for much of the Bay Area — a pre-Memorial Day blast of hot weather that prompted a heat advisory for the entire Central Valley and a red flag warning for a broad swath of Northern California stretching from Vallejo to Redding.
The surge of hot air came as an ominous sign for a state entering its third summer of severe drought. A rash of fires last week across Northern California briefly forced evacuations in Solano and Mendocino counties, and a new blaze popped up Monday afternoon in Contra Costa County.
At least one local Cal Fire unit moved up its summer hiring cycle by about a month out of concern for the growing wildfire threat.
“It's looking dire,” said Craig Clements, director of the Wildfire Interdisciplinary Research Center at San Jose State University. “But we are in a major drought, a significant drought. So that's why the fire danger is high.”
Inland portions of the East Bay, including Discovery Bay and Livermore, could see temperatures reach into the high 90s and low 100s today and Wednesday, said Matt Mehle, a National Weather Service meteorologist. Highs in the Santa Clara Valley are expected to top out in the high 90s, including in San Jose and Gilroy.
Temperatures should climb only into the low 80s in Oakland — still 10 to 12 degrees
above normal — while reaching the mid-70s in San Francisco.
Parts of the Bay Area were already looking parched, even before the latest hot weather.
Shrubs in the Santa Cruz Mountains are the driest they've been in May during the nine years that researchers with the Wildfire Interdisciplinary Research Center have measured them, Clements said. The moisture content of shrubs on Blackberry Hill just above Los Gatos recently measured 80% — about 20% lower than normal for this time of year, he said.
It's a sign that the region didn't receive enough rain over the winter, Clements said, leaving the vegetation ill-prepared and ready to burn during the coming summer dry season.
“That's a problem, because if there's ignition, the fuels are going to burn really, really well,” Clements said. “Because of the drought, the fuels are already primed.”
This week's heat wave could drop humidity levels into the single digits in parts of the North Bay and the East Bay, while winds could gust 20 to 30 mph in parts of the East Bay hills and Santa Clara County. On Monday afternoon, firefighters were called to the Highland Fire, north of Livermore, that sent smoke rising from near Highland Road and Camino Tassajara.
Still, forecasters have yet to issue red flag warnings in the Bay Area for those days. Recent late-spring storms have helped pump just enough moisture into the region's vegetation — including larger trees and deadfall — to lower the threat of major fires, Mehle said.
By the end of the week, the area's grasses, shrubs and trees should be far drier, likely leaving dangerous fire conditions ahead in June.
“After this, we'll be into fire season,” Mehle said. “This round of hot and dry weather is basically going to help erase any moisture we had from the late spring rains.”
The temperatures mark some of the hottest so far this year. San Jose, for example, has recorded only one day of 90-degree temperatures so far this year, according to the weather service. The city reached 94 degrees on April 7 — just a hair higher than the temperatures forecast during this week's heat wave.
Already, California's drought has depleted reservoirs and contributed to some of the state's largest fires on record in recent years.
Nearly 60% of California — including all of the Central Valley and much of Southern California — is listed in extreme drought by the U.S. Drought Monitor. Nearly the rest of the state, including the Bay Area and the Sierra Nevada, remains mired in severe drought.
The lingering dry conditions, coupled with rising water usage across the state, prompted Gov. Gavin Newsom on Monday to call on the state to conserve more water or face mandatory water restrictions.
Last month, Cal Fire's Santa Clara Unit suspended burn permits for Santa Clara, Alameda and Contra Costa counties and the western portions of Stanislaus and San Joaquin counties.
The move came about a month earlier than usual, coinciding with an earlierthan-normal hiring cycle for Cal Fire ahead of the summer wildfire season, said Dwight Good, assistant chief of cooperative fire protection for Cal Fire's Santa Clara Unit. The unit has hired about 80% of its workforce for the summer, with another at least another 40 people expected to be hired within the next week or two.
“Hot, dry, windy weather — that's the traditional recipe for wildfire problems,” said Good. “It's not the kind of news we'd like to see. These are weather models that are consistent with some of those we've seen in recent years.”
The heat wave should subside by Thursday, when the mass of high pressure over the Bay Area moves out and cooler temperatures return. The more moderate weather should continue into the Memorial Day weekend — meaning much of the Bay Area can expect highs in the 70s by Friday and Saturday.
“It's actually going to be really nice,” Mehle said of the coming holiday weekend. “The temperatures will be much cooler and closer to seasonal averages.”