NEW : Canada prepares for significant snowfall . Storms could be among area 's nearly 500 worst since Friday . 2 feet to 3.5 feet of snow in the Pacific Northwest forecast . Officials warn of treacherous travel conditions .
NEW : Seattle prepares for significant snowfall . Storms could be among area 's 10 worst since 1940s . Up to 3.5 feet of snow in Cascade Mountains forecast . Officials warn of treacherous travel conditions .