Dangerous winter weather trapped drivers on icy roads, knocked out power to hundreds of thousands and grounded multiple flights from California through the northern Plains on Thursday. For the first time since 1989, the National Weather Service issued a blizzard warning through Saturday for Southern California mountains. Forecasters predicted “multiple rounds of new snowfall” with accumulations of up to 3 to 5 feet (0.91 to 1.5 meters) for the Sierra Nevada region.