The National Weather Service has issued an unusual December red-flag fire weather warning for the Bay Area this weekend, a move that has prompted many local agencies to issue their own alerts to residents.

Forecasters say a combination of high winds, extremely low humidity and lack of rain in recent weeks will raise fire danger to critical levels.

The Bay Area red-flag warning covers higher elevations throughout the region, including the Santa Cruz Mountains, the East Bay and North Bay hills, Mount Diablo and the Diablo Range.

The forecast for areas above 1,000 feet is for winds of 15 mph to 30 mph and gusts up to 50 mph. Daytime humidity is expected to fall to the 10 percent to 20 percent range, recovering at night to about 30 percent.

The alert prompted several Bay Area public safety agencies to issue their own advisories. Fire officials in Alameda, Marin and Napa counties issued a ban on outdoor burning for the weekend. Sonoma County, which suffered the worst damage from the devastating October fires that swept the North Bay, advised residents to avoid outdoor fires for the next two days.

In Berkeley, the Fire Department urged residents in the city's hills neighborhoods to park in driveways and garages and leave streets clear for emergency vehicles.

Cal Fire has increased staffing and resource levels in the region ahead of the forecast winds, agency spokeswoman Janet Upton said in an email Friday. That includes 42 Cal Fire engines, 13 bulldozers, and 26 fire crews of 12 to 15 people.

Upton said the agency is also paying for 44 more local government fire engines to be staffed with either Cal Fire or local fire department personnel.

Asked whether she had ever seen such an intensive staffing effort so late in the year, Upton said by email, "Not in all my 31 years with Cal Fire."

Red-flag warnings are also in place for much of Southern California, the Sierra Nevada foothills and the northern two-thirds of the Central Valley.

The greater Los Angeles area has been under a red-flag warning for a record 12 straight days.

Gusty winds and low humidity continue to pose a challenge for the 8,400 firefighters struggling to contain the Thomas Fire in Ventura and Santa Barbara counties. The blaze has burned more than 250,000 acres, or nearly 400 square miles, destroyed more than 900 structures and killed two people.

The fire weather warnings are a result of a long-lasting ridge of high pressure just off the West Coast that has blocked storms from reaching California and is responsible for dry northerly or northeasterly winds.

Winter fire weather warnings are unusual but not unprecedented in the Bay Area. The National Weather Service issued three such warnings for the region during December 2013 and January 2014.