Hours before they were expected to fade away, powerful Santa Ana winds swept San Diego County from the mountains to the sea Thursday, stoking new wildfires — one near UC San Diego, the other on Otay Mountain near the U.S.-Mexico border. The winds exploded out of the east, dropping into Gilman Canyon and onto Otay Mountain and interrupting the Farmers Insurance Open golf tournament at Torrey Pines. Santa Ana winds gusted to 91 mph in the mountains and 30 mph near the coast Thursday in San Diego County, in what was expected to be the longed-for finale of weeks of perilous windstorms. Soon after the Otay Mountain fire erupted early Thursday afternoon, it grew to more than 500 acres and threatened the Otay Mountain Repeater Site — home to critical telecommunications infrastructure, officials said. As that fire got moving, a fire also erupted near UC San Diego in La Jolla, prompting evacuations just south of campus. But fast-working crews — 175 firefighters — kept the Gilman fire to three acres, a San Diego Fire-Rescue spokesperson said. And by Thursday evening, high winds led San Diego Gas & Electric to shut off power to nearly 20,000 customers living mostly in rural and backcountry communities, in an effort to prevent high winds from causing power lines to fall to the ground and potentially igniting a wildfire. There was the potential for another 64,000 customers to lose power should conditions worsen. Adding to the risk of wildfires: bone-dry conditions. Across the region, humidity ranged from 3% to 9%. Casey Oswant, a forecaster at the National Weather Service, said it got down to 3% at McClellan-Palomar Airport in Carlsbad — “really unusual for a place so close to the ocean.” Friday could see some easing. “The winds probably won’t reach the coast on Friday, and they’ll only blow about 35 mph in the mountains,” Oswant said. And, he said, “the relative humidity won’t be as bad.” There’s also rain in the near-term forecast. “A cold low pressure system from the north is expected to bring cooling with widespread showers and mountain snow for Sunday through Tuesday,” the weather service said in a statement. “There is even a slight chance of thunderstorms for Sunday, and potentially Monday.” The Otay Mountain fire is seen from the hills of Tijuana on Thursday. (Alejandro Tamayo / The San Diego Union-Tribune) Those could prove a mixed blessing. Thunderstorms can produce lightning capable of starting a wildfire if it hits the ground — and the ground is unusually dry, especially in the backcountry. Most of the county has had less than 0.25 inches of rain since July 1, and much of it has recently slipped into extreme drought, according to the U.S. Drought Monitor. Forecasters say areas west of Interstate 15 could get about 0.25 inches of rain over the weekend and into Monday, while twice as much could fall across inland valleys and up to 1 inch in the mountains. Still, the storm is not expected to bring an end to wildfire season. And crews have a big job tackling the fire on Otay Mountain, on Bureau of Land Management lands not far north of the international border. The National Weather Service in San Diego reported winds in the area to be around 20 miles per hour with 5% humidity when it started around 2 p.m. The fire appeared to be located in steep and rocky terrain east of multiple structures, including the Otay Mesa Detention Center, Calpine Otay Mesa power plant, the East Mesa Juvenile Detention facility and Richard J. Donovan Detention Facility. Aircraft could be seen dropping flame retardant on the mountainsides. Ground crews faced terrain that was difficult to navigate. As of late Thursday, they had not been able to draw a containment line around any of it. Cal Fire said the blaze was burning at “a moderate rate of spread.” The fire generated a column of smoke that could be seen from Rancho San Diego, San Carlos and into North County.   Smoke from the fire in La Jolla could also be seen from afar, rattling nerves as evacuations were ordered for neighborhoods between North Torrey Pines Road, Gilman Drive and La Jolla Parkway. Areas farther west were warned to be ready to evacuate. Among those in the evacuation zone who fled were Jewish community members at Hillel of San Diego, immediately south of the UC San Diego campus. “We evacuated right away,” said Karen Parry, the center’s executive director. “We took Torahs with us.” That fire broke out around 2 p.m. near Gilman Drive and Via Alicante and within 30 minutes had charred an acre, said Jose Ysea, a spokesperson for the San Diego Fire-Rescue Department. Just before 3:30 p.m., UC San Diego told the campus community in an email that forward spread had been stopped. On Thursday morning, crews also doused a small fire in the Grantville neighborhood. A San Diego police official said one person had been arrested in connection with the fire. Thursday’s fires follow a busy week for fire crews, who have put out blazes in Mission Valley, Poway and other areas of the county amid dangerous fire conditions. The cause of those fires is under investigation. Staff writers Caleb Lunetta, Rob Nikolewski, Karen Kucher and Sam Schulz contributed to this report.