SAN DIEGO -- A "red flag" warning, projecting critical wildfire conditions, will be in effect for San Diego County mountains and valleys Friday night through Sunday, the National Weather Service announced Thursday.

The advisory stems from forecasts for risky climatic conditions -- low humidity coupled with gusty winds -- over the weekend, though not as dicey as those that churned up last month's disastrous spate of wildfires.

The warning was issued Thursday afternoon, replacing a less urgent fire-weather watch.

"High pressure will develop in the Great Basin and bring low humidity to the area and locally gusty winds through and below mountain passes and canyons," according to an NWS advisory.

The northeast winds that will develop Friday night will blow at strengths of 15 to 25 mph, with "very isolated" gusts to near 45 mph, the advisory states.

The winds likely will be at their strongest between 6 a.m. and noon Saturday below the Cajon Pass and in the foothills of northeastern Orange County, according to the weather service.

Of even greater concern was the prospect of arid atmospheric conditions resulting from an expected "very dry air mass," according to the NWS.

"Relative humidities will decrease to around 10 percent before noon Saturday and could remain at those levels for up to 10 hours," according to the advisory. "After a modest recovery Saturday night, these low humidities will continue all day Sunday."

In San Diego County, the wildfire warning will cover the region's mountain areas, including the Palomar and Descanso Ranger Districts of Cleveland National Forest.

The alert also will be in effect in the mountains and valleys of Riverside and San Bernardino counties.

Meanwhile, only one of the nine conflagrations that ravaged San Diego County last month has yet to be fully contained.

The Poomacha Fire, which broke out Oct. 23 on La Jolla Indian Reservation, is 85 percent contained today after scorching 50,156 acres.

The deadliest of the blazes, the Harris Fire, was 100 percent contained Wednesday morning after claiming five lives and gobbling up 90,440 acres since it began Oct. 21. It is expected to be under control by Saturday.

Also fully contained yesterday was the Witch fire. The biggest of the county's wildland blazes, it scorched 197,990 acres, destroying 1,125 homes and 499 outbuildings, according to Cal Fire. It's expected to be under full control by Monday.

Citing the expected return of offshore winds, Gov. Arnold Schwarzenegger Wednesday directed state agencies and the National Guard to be prepared for a possible new round of blazes.

"... We're not out of the danger zone yet," he said at a news conference following a meeting in downtown Los Angeles with the California Association of Governments to discuss the state's response to the wildfires that began swallowing acreage and homes the night of Oct. 20.

"... We are watching the weather very closely ...," Schwarzenegger said. The governor said there were aircraft at the ready to respond if the winds pick up as expected and that emergency staff was pre-positioned in areas with a high risk of fire.