San Diego’s mountains have turned into a winter wonderland thanks to a storm system that dumped a layer of snow on the county’s peaks to start the week. The winter storm swept across San Diego County overnight Sunday and left snowfall on Julian, Mount Laguna and Palomar mountains. By 9 a.m. Monday, at least 4 inches fell at Mount Laguna and more than 4 inches on Birch Hill near Palomar Mountain, and 3 inches were recorded in Julian, the National Weather Service said.