A winter storm packing powerful winds, heavy rain and potentially several feet of snow in the Sierra Nevada shut down mountain highways, toppled trees and triggered flood watches and avalanche warnings on Saturday from the coast of Northern California to Lake Tahoe. The powerful storm system is expected to move east over the next several days, forecasters said. More than 250 miles of the Sierra remained under a winter storm warning at least until Sunday night or early Monday from north of Reno to south of Yosemite National Park. As much as 4 feet of snow is expected to fall by the end of the weekend in the upper elevations around Lake Tahoe, and as much as 6 feet in more remote parts of the Sierra to the north and south. A 70-mile stretch of eastbound U.S. Interstate 80 was closed "due to zero visibility" from Colfax, California to the Nevada state line, transportation officials said. Chains were required on much of the rest of I-80 in the mountains from Reno toward Sacramento. A stret