The National Weather Service issued a red flag warning for high fire danger on Wednesday for areas that include the East Bay hills, the Diablo Range and into the South Bay.

The warning, primarily due to high winds, took effect at 11 p.m. and runs through 3 p.m. Friday, and signifies a higher risk for fire danger and more potential for larger fires in areas above 1,000 feet in Alameda, Contra Costa, Santa Clara and Santa Cruz counties, said National Weather Service Meteorologist Austin Cross.

It also affects mountains in the North Bay and Santa Cruz, Cross said. The warning comes as low humidity and strong winds blanket the East Bay and cause vegetation in the area to dry out quickly. Wind was a major factor in the four-day Morgan Fire on Mount Diablo earlier this month. The blaze began Sept. 8 just southeast of Clayton and torched more than 3,100 acres.

Fire Captain Kent Kirby of the Contra Costa Fire District said crews have been alerted to expect winds up to 35 mph in some higher elevations.

"It's the winds that really drive the fire over any other factor," Kirby said Thursday. "But we're keeping it monitored."

In even higher elevations in the north and East Bay hills -- those above 2,500 feet -- gusts of 45 to 55 mph are expected.

"Those are areas they'll really be keeping an eye on," Cross said. "We're a little more vigilant when theses type of warnings come out."

Contact Natalie Neysa Alund at 510-293-2469. Follow her at Twitter.com/nataliealund.