San Franciscans with cellphones were abruptly awakened at 5:50 am Saturday by a blaring tornado warning, barely a week after we received a similar blaring warning about a tsunami that never arrived.Was that tornado warning, the first of its kind ever issued for San Francisco, really necessary? Chronicle meteorologist Greg Porter explains that while our weather is typically dominated by a strong marine layer which would normally inhibit tornado activity, early this morning conditions were different."A strong cold frontal boundary served as a focal point for converging winds from different directions, leading to the conditions that triggered the warning," Porter writes.A National Weather Service radar detected wind rotation happening within a storm cell just off the San Mateo coast, and the cell was pointing directly at San Francisco. It was never clear that any tornado had formed, and Porter explains that because this cell was "rain wrapped," it made that detection more difficult.Within twenty minutes, the tornado threat had ended.High winds still arrived in the city, blowing down trees and knocking out power. The PG&E outage map shows a few hundred customers in the southern Mission District and Bernal Heights still without power Saturday morning at 10 am. Another area with outages is the Outer Sunset.As Mission Local reports, around 11,000 customers in the city were without power at one point, but PG&E has been out repairing lines. The San Franisco Fire Department reportedly fielded almost 50 calls about downed trees and other damage between 6 am and 7:40 am.And Mission Local has photos of downed ficus trees around the Mission neighborhood, including a large one at 24th and Shotwell.The Chronicle quotes a Glen Park resident, Matt Franz, saying, "The trees were just thrashing violently," and there was a white Jeep in that neighborhood that was completely totaled by a fallen tree.Another local resident, Oliver Leach. who was finishing a night shift near the Panhandle when the tornado warning hit tells the Chronicle, after the recent tsunami warning, "This might be our new normal. It does feel vaguely biblical. I'm just wondering what's next.”Photo: Greg Johnson