The skiing should be great in the upper Sierra this weekend " if you can get there.A snow advisory remains in effect through 4 p.m. today, and mountain motorists should expect plenty of snow at 7,000 feet and higher, according to the National Weather Service. Showers are predicted for the Nevada County foothills from today through Sunday, so traffic should be smooth through the lower elevations. The storms will dump heavy snow on the upper mountains Saturday and Sunday, the weather service said, and Donner Pass ski resorts should get another 10 to 20 inches."As long as the winds stay …