N EW ORLEANS — Powerful storms were threatening communities across the country Tuesday with weather ranging from fire in the Southern High Plains to blizzards in the Midwest. Forecasts also predicted dust storms in the southwest, tornadoes in the South and blizzard conditions in the Central Plains, and were forcing forcing some changes to Mardi Gras in New Orleans. The city moved up and shortened the celebration's two biggest parades. Police were also expected to help keep the parades moving to