Just an hour after Chris Ashurst finished a morning of frigid cross-country skiing, an atmospheric river descended upon Haida Gwaii from the south, swinging the temperature 15 degrees “almost into T-shirt weather” and setting off a massive melt that nearly led to calamity on the craggy archipelago off B.C.’s north coast.That was Sunday morning. By the evening, Mr. Ashurst, a volunteer emergency co-ordinator for the North Coast Regional District, was one of more than 2,000 residents stranded on the north half of the main island when the lone highway was washed out by a flood.The...