A deep trough in the upper atmosphere is allowing cold air to surge southward from Washington and Southern California and eventually into the Southwest.Snow levels as low as 500 feet in the Bay Area,and as low as 1500 feet in the L.A.Basin can be expected as a significant blossoming of rain and snow arrives late tonight into Saturday.With strong winds aloft accompanying the Saturday system,blizzard conditions,at times,can be expected over the Southern California high country.