by Jeff Masters, via the WunderBlog
A hailstone with the diameter of roughly that of a grapefruit that hit Oahu on March 9, 2012, has been confirmed as the largest hailstone on record for the state of Hawaii, according to NOAA.
The record-setting hailstone was dropped by a “supercell” thunderstorm on the windward side of Oahu. There were numerous reports of hail with diameters of 2 to 3 inches and greater. Hail the size of a penny (diameter of 3/4 inch) or quarter (diameter of one inch) has been reported in Hawaii only eight times since records began, and there is no record of hail larger than 1 inch in diameter. Hail the size of golf balls and baseballs can only form within intense thunderstorms called supercells. These supercells need warm, moist air to rise into progressively colder, drier air, as well as winds changing direction and increasing speed with increasing height off the ground.
For both sets of conditions to exist at the same time in Hawaii is extremely rare, but that did occur on March 9. Conditions that day were ideal for a supercell to form, and the storm looked very much like supercell thunderstorms common in the Central U.S. during spring. Supercells can also produce tornadoes, another rarity in Hawaii. The same hail-producing supercell produced a confirmed EF-0 tornado with winds of 60-70 mph in Lanikai and Enchanted Lakes on Oahu.
Jeff Masters is the co-founder of the Weather Underground, where this piece was originally published.