This week Melbourne, Australia had an intense, sudden storm, with huge hailstones (up to 10cm diameter observed) and enough rain to flood the downtown core. Although rare, this is not a unique event for Melbourne.
The cold southern oceans have what is called “infinite fetch” — because the ocean surrounding Antarctica is clear of any protruding landmasses, wind can drive waves higher and higher and higher without interrupting coastlines. This means you can get some nasty storms in the few places where land does start peeking into the flow — New Zealand, Tasmania and southern Australia, Cape Horn… — you can get some very nasty storms.
Melbourne is partly sheltered by Tasmania, by the shallow waters of the intervening continental shelf, and by the warm, large Port Philip Bay, but when a strong cold front comes in from the ocean and tangles with the hot, dry air from the interior, severe storms are born. (See pages 45-69 of The Cloudspotter’s Guide for more details on how cumulonimbus form — he writes such an elegant, beautiful description, I can’t hope to improve on it.) This means that sudden severe storms are not uncommon, with particularly severe events occurring approximately once a generation (the last flash flood in Melbourne was in 1972).
If you sliced open a hailstone, you’d see layers of ice, like coloured candy layers of a jawbreaker. A hailstone forms by being tossed up and down in the updrafts and downdrafts of convection within a storm, each trip adding a layer of ice and growing the hailstone. The larger a hailstone is, the more times it’s made the journey — the lemon-sized stones in Melbourne were tossed around quite a bit before pelting the city.