Tuesday’s weather forecast contains a sentence not often heard in San Diego County: We’re going to have a day of full-on winter. A distinct river of uncommonly cold, moist air will flow through the region, bringing heavy snow to parts of Mount Laguna and Palomar Mountain, and about half as much to some lower elevations. The mountain peaks could wind up with a foot of snow between late Monday and late Tuesday, while areas like Julian could get a half-foot, said the National Weather Service.