In March 2004 the first-ever reported hurricane in the South Atlantic hit southern Brazil. Here we show that Catarina initiated as an extratropical cyclone in a frontal system, undergoing Tropical Transition two days later under persistent low vertical wind shear over near-average water temperatures. The trajectory derived from an automatic tracking scheme showed a rare loop before the cyclone approached the coast for a second time. The vertical structure presented anticyclonic relative vorticity above and a small 300 hPa warm core embedded in a cold area. A mid-to-high latitude-blocking index showed that the five days before the genesis were in the 0.6% first percentile of intensity considered over the last 25 years, followed by an unprecedented combination with low shear. The observed and predicted trends towards an increasingly positive phase of the Southern Annular Mode in global warming scenarios could favor similar conditions, increasing the probability of more Tropical Cyclones in the South Atlantic.