What caused freak hailstorm in Argentina?

Dear Tom,
Argentina just had a record storm recently, dropping a phenomenal amount of hail within minutes. What caused this freak storm?
— Carol Rosenzweig, Chicago

Dear Carol,
In late October severe thunderstorms dropped both large and massive amounts of small hail in central Argentina. The city of Formosa was hit with softball-size hail on Oct. 25, causing considerable damage to structures, cars and vegetation. The following day a violent barrage of small hail buried the town of La Cruz with nearly five feet of hail that fell in just 15 minutes. While both events were exceptionally severe, Argentina is in the heart of its spring severe weather season and the atmosphere was explosively unstable as frontal boundaries lay across the area. Similar hail storms occur in the U.S., especially in the central Rockies and high plains.