The third hot spell in five weeks is expected to begin scorching the county Friday, prompting fire officials and lifeguards to beef up their staffs.

With inland temperatures projected to hit 100 degrees, low humidity levels and winds in some canyons and passes, fire-weather conditions are “borderline critical,” said National Weather Service forecaster Brandt Maxwell.

“With this weather coming, everybody needs to be on their toes,” Cal Fire Capt. Nick Schuler said. “There were several fires in the northern part of the state (Thursday). That's probably an indication of what we'll see in Southern California.”

Small brush fires also flared up Thursday along Interstate 15 north of state Route 56, in Valley Center and on the Mexican side of the border near Otay Mesa. The blazes were quickly extinguished.

Cal Fire will staff an extra five engines over the weekend, raising the number of firefighters almost to peak summer levels. An additional air tanker also will be available at the Ramona air base, Schuler said.

The beaches might escape the worst of the heat, but some coastal spots could still hit the low 90s Friday. San Diego lifeguard Lt. John Greenhalgh said the city will augment its normal springtime staffing in anticipation of big crowds. The coast should be warm again Saturday and then cool slightly Sunday.

In the inland valleys, Saturday and Sunday could remain in the upper 90s. The mountains and deserts could peak Sunday, with Borrego Springs expected to hit 107.

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