Charts show rain gave Northern California a temporary ‘shield’ from wildfire risk

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Last weekend’s storms provided a welcome respite from the scorching heat wave and a healthy amount of rain to some parts of the parched Bay Area and Northern California – helping temporarily reduce the risk from wildfires by giving them less dry fuel to feed on.

The precipitation boosted levels of moisture in brush and other vegetation – known as fuel moisture percentages – across Northern California to well above average for this time of year, said Chronicle newsroom meteorologist Gerry Díaz.

“The good news is that these recent rains brought our moisture levels up for some of the biggest vegetation,” he said. “In the Sierra and Santa Cruz mountains, they are running at almost twice the average for this time of the year.”

In the Bay Area and North Bay, fuel moisture levels are at “double their average” for this time of year as well, Diaz said.

“This means that vegetation is shielded by a coat of water that makes it harder to burn,” he said. “This will keep the risk of natural wildfires down for the next few weeks, helping to delay some of the worst impacts from the fire season.”

The rain over the weekend helped firefighters double containment of the Mosquito Fire, which has been burning in the Sierra foothills in rural Placer and El Dorado County since Sept. 6. Containment on Thursday was at 60%.

Bay Area

The 100-hour fuel moisture levels from the beginning of the year until now in the Bay Area region.

Northern California Geographic Coordination Center

North Bay

The 100-hour fuel moisture levels from the beginning of the year until now in the mid-coast area that includes the North Bay.

The 100-hour fuel moisture levels from the beginning of the year until now in the mid-coast area that includes the North Bay.

Northern California Geographic Coordination Center

The fuel moisture index uses a threshold called a time lag, expressed in 10-, 100- or 1,000-hour periods, based on how long it would take for two-thirds of the dead fuel in an area to respond to atmospheric moisture, according to the National Centers for Environmental Information.

The bigger the fuel, the greater the index time period — in Northern California, 1,000-hour fuels are used often for the oldest trees like the redwoods, while 100-hour fuels are more representative of the general forests in the area, Díaz said.

Surveyors coordinated by state and federal fire agencies measure changing moisture levels across the state. In the charts above and below, the blue line refers to current moisture levels, while the grays are the average and the reds are the minimums.

Northern Sierra

The 100-hour fuel moisture levels from the beginning of the year until now for the Northern Sierra region.

The 100-hour fuel moisture levels from the beginning of the year until now for the Northern Sierra region.

Northern California Geographic Coordination Center

Diablo and Santa Cruz Mountains

The 100-hour fuel moisture levels from the beginning of the year until now for the Diablo and Santa Cruz mountain region.

The 100-hour fuel moisture levels from the beginning of the year until now for the Diablo and Santa Cruz mountain region.

Northern California Geographic Coordination Center

Díaz noted that Diablo winds are in the forecast this weekend for some of the highest peaks in the North Bay highlands and in the Diablo Mountains in the East Bay. These hot, dry winds, which typically occur during fall and winter, sweep over the Sierra Nevada and coastal mountains, gaining heat and speed as they approach the ocean – helping whip up ferocious wildfires in recent years.

However, this time around, “fire risk is low thanks to the coat of water on our vegetation,” Díaz said.

That is promising news at the start of the fall season, when fuels are normally very dry after having had all summer to dry out.

But while the higher moisture levels may help right now, Cal Fire battalion chief Issac Sanchez said he is cautious about how much of a difference a single storm system will make in the longer term.

“Before we say (the wildfire) threat is mitigated, or at least reduced, we need repeated event after event with measurable rain to have an impact,” he said. “We still have the rest of September…We don’t expect to have a significant long-term effect unless this is followed up by repeated rain events.”

Even with the current reduced wildfire risk, Sanchez said Californians should stay on guard. He stressed the importance of personal and defensible space preparedness for potential wildfires that could spark in the coming weeks or months.

Kellie Hwang is a San Francisco Chronicle staff writer. Email: kellie.hwang@sfchronicle.com Twitter: @KellieHwang

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