LOS ANGELES (AP) — Forecasters warned Thursday of an onslaught of gusty, dry conditions that will dramatically raise fire danger in large swaths of California as surface high pressure builds into the Pacific Northwest.

In Southern California, the National Weather Service predicted a siege of Santa Ana winds with powerful offshore gusts and very low humidity levels beginning Thursday night and lasting through the weekend.

"This could be one of the more significant Santa Ana events we've seen in many years," said Eric Boldt, a Weather Service meteorologist. "You can expect widespread single-digit humidity, from Ventura County down to the Mexico border."

An array of red flag warnings have been issued for the greater Los Angeles region, the inland counties and south to San Diego.

Temperatures will spike into the 90s across the Los Angeles basin, Boldt said. Wind speeds between 50 mph and 60 mph will be common in mountain and valley areas, with peak winds reaching 70 mph in the San Fernando and Santa Clarita valleys, he said.

In the San Francisco region, a similar forecast has prompted warnings for the East Bay Hills, Diablo Range, North Bay and Santa Cruz mountains, and parts of the Mendocino National Forest. The drying winds will also bring fire danger to the Sacramento Valley and areas to the west.

"We are prepared for what could be a very busy few days," said Daniel Berlant, spokesman for California's Department of Forestry and Fire Protection.

Reserve fire engines were ready and water-dropping aircraft fueled up, he said. Fire officials also ramped up weekend staffing, even bringing in firefighters from their days off.

"Even though the calendar says October, this is when conditions are at their driest across the state this year," Berlant said. "We just haven't had enough rain."