It was the latest wild turn in this most extreme of Australian summers. Large hailstones have rained down on Sydney, Melbourne and Canberra over the past two days, destroying vehicles, punching holes in roofs and blanketing the lawn in front of Parliament. Suddenly, a season of suffocating heat, bone-dry skies and voracious fires has given way in southeastern Australia to widespread thunderstorms. Skies once darkened by smoke are now brooding with clouds and rain — at least for a few days.