A blizzard engulfed Michigan's Upper Peninsula with nearly two feet of snow and 61-m.p.h. wind gusts on the last day of winter, while a tornado snatched cars off a Georgia highway and slammed into a housing development, injuring 15 people. Spring begins at 2:03 p.m. today. In the Northeast, rain and high winds knocked branches off trees and the wind toppled a wall and killed a construction worker in Buffalo, N.Y., authorities said. Heavy thunderstorms stretching across the South whipped up a tornado in Marietta, Ga. The twister damaged buildings and tossed vehicles through the air.