Boston, Rabi'II 30, 1436, Feb 19, 2015, SPA -- A polar air mass is moving across the United States and into its normally warmer southern tier, prompting warnings Thursday from the National Weather Service of a "dangerous" and historic chill. The Arctic front will create a "deep layer of bitterly cold air," it warned. Buffalo, New York, was bracing for temperatures of minus 19 Celsius, but high winds could make it feel like minus 40 degrees by Friday, the Buffalo News newspaper reported. Parts of the south-east, mid-Atlantic and Appalachian Mountains could experience some of the coldest weather since the mid-1990s, the Weather Service said. Northern Florida was bracing for temperatures as low as minus 7 Celsius and a "hard freeze" that will "kill crops and other sensitive vegetation," the prediction said. It was snowing again Thursday in Boston, which has been hit by one snow storm after another. At least 2.5 metres of snow have fallen there since late January, the second highest accumulation on record. Below-freezing temperatures have prevented melt, turned Boston's city streets into one-way canyons and crippled the public transport system. Transport officials said this week they expect it will take at leat 30 days to restore operations to normal. --SPA 22:35 LOCAL TIME 19:35 GMT www.spa.gov.sa/w