Will California’s Mosquito Fire, drought be affected by storm?


Beginning overnight Saturday, meteorologists predict rain will spatter San Francisco, lash the Sierra and soak much of the Central Valley, accumulating up to one and a quarter inches over three days, with a slight chance of thunderstorms. A low pressure system from Alaska will cause the flurry of wind and rain, which the National Weather service described in a tweet as “slow moving to stationary.”

This moderate but protracted cloudburst could slow the blazes already raging through California as summer turns to fall, though it it won’t end fire season altogether, said Emily Heller, a meteorologist with the National Weather Service in Sacramento.

Wet weather is so unusual this time of year that it’s inspired the portmanteau “Spring-tember.” Thursday’s cool temperatures and the torrent of rainfall expected Saturday will follow a record-breaking heat wave that scorched California’s interior and dried vegetation, likely contributing to this week’s explosive fires.

The rains will also will deliver slight relief from a drought that’s parched the state, left acres of farmland barren and lowered reservoirs, though meteorologists warn that the benefits could be temporary, and the dry period is far from over.

Storms that come early in the season are auspicious, said National Weather Service meteorologist Dalton Behringer, so long as the rain persists. But if it’s followed by warm weather — as likely will happen in California — it could have dangerous side effects. Rain allows foliage to grow thicker and taller, providing more fuel for future wildfires once the weather dries out.

“We do have a warmer than usual pattern coming after the rain,” Behringer said.

UCLA climate scientist Daniel Swain said in a tweet that powerful winds will blast through Northern California on Saturday and Sunday, which could cause wildfires to grow before they abate.

“If we can get through the (southwest) wind event, things will likely be looking much better (for the) Mosquito Fire by Monday,” Swain tweeted.

Forecasters foresee the coming showers falling on Placer and El Dorado counties, where the Mosquito Fire had charred 64,159 acres as of Thursday morning, by which point crews had it 20 % contained. Cal Fire officials noted in an incident report that the weather had cooled and the winds had calmed overnight, allowing smoke clouds to settle.

Light southwest gusts will continue to push out the smoke and leave a clear patch over the fire area, officials said, anticipating stronger winds ahead of this weekend’s gale.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan