Reuters: Dense clouds of migrating monarch butterflies used to snap branches and cast shadows across the forests of central Mexico, but severe weather is posing a new threat to the annual phenomenon. The yearly 2,000-mile journey, which takes four generations of butterflies to complete, starts in Canada and ends in the Mexican state of Michoacan, which normally enjoys mild weather from November to March. Millions of the insects swarm to these arid hills each year, their orange-and-black …