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Michael Cabanatuan May 7, 2021Updated: May 8, 2021 4:18 p.m.Comments

With dry offshore winds and warm temperatures forecast for the weekend, the National Weather Service has issued a red flag warning — the first of the year and a possible portent of a long and catastrophic fire season.

The warning, which cautions that conditions could make it easier for wildfires to spark and spread, begins at 11 p.m. Friday and will last until 6 a.m. on Monday. It covers the North Bay mountains and East Bay hills as well as interior valleys.

It arrives as an unusually dry rainy season draws to an end, leaving an abundance of dry grasses and brush that could ignite easily and rage out of control quickly. California just concluded its driest April since 1985, according to the National Oceanographic and Atmospheric Agency, and the fifth driest since records have been kept.

High and dry winds — gusting up to 50 mph above 2,000 feet — are expected to arrive in the Napa County hills at around 11 p.m. Friday, sweeping into the East Bay hills overnight and into early Saturday morning. Bone-dry, breezy weather, with temperatures reaching into or near the 90s in Santa Rosa, Concord and Livermore, and 70s to near 80 are expected around the bay.More for you

Ryan Walbrun, a National Weather Service meteorologist for the Bay Area, said the red flag warning is arriving about a month earlier than usual and is “not unprecedented but rare.” The weather pattern — with dry winds sweeping in from the north — is not uncommon this time of year, Walbrun said. But two dry winters in a row have left grasses and vegetation at or near record dry levels, escalating the fire danger.

According to weather service records, this is only the second red flag warning ever issued in May with the other in 2013.

Red flag warnings have become common in Northern California in the fall — and an all-too-frequent sign that major wildfires are imminent. But Walbrun described this weekend’s warning a “moderate stage” alert.

“There’s a difference between a May red flag and a fall red flag,” he said. “The conditions are there for a fire to start and spread but the fuels are a lot drier in September and October.”

While some of the weekend fire danger is in remote areas in the North Bay and East Bay hills, Walbrun said populated areas in Santa Rosa, Napa Valley near Mount St. Helena and far eastern areas of Alameda and Contra Costa counties, near Mount Diablo and the Altamont Pass could be at increased risk.

With Mother’s Day weekend events, the warm weather and more people getting out and about as the pandemic eases and vaccinations rates increase, the weather service decided to issue the red-flag warning.

“We wanted to increase people’s awareness,” he said.

Cal Fire is making sure it’s adequately staffed where fire danger is highest, said Christine McMorrow, a spokeswoman, and is urging the public to be cautious, particularly with activities that are known to risk sparking wildfires such as towing a trailer, mowing lawns or cutting weeds and any types of activities in dry grassy areas.

“What a red flag warning signifies is low humidity, high temperatures and high winds — a combination that increases fire danger,” she said. “We know one small spark can create a fire that quickle becomes much bigger more quickly than normal.”Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: Twitter: @ctuan