Rio Grande do Sul under water

The rain began on 27 April and grew heavier through the following days. In the valleys of the Taquari, Caí and Jacuí rivers, in the hills north and west of Porto Alegre, the water rose metres in a few hours, washing away bridges and whole neighbourhoods. As the rivers drained into the Guaíba, the lake rose for days after the rain had eased. It passed its 1941 record; the floodgates and pumps that should have protected Porto Alegre leaked or failed, and the water poured into the city.S3

Weather in southern Brazil is often a meeting of cold air from the south and hot, moist air from the north. In 2024 the meeting stalled: a block of high pressure and heat over central Brazil held the front in place, while a stream of moist air from the Amazon flowed along the Andes into it.S2

ERA5's map of 1 May shows the band of heaviest rain across the centre of the state, from the Argentine border to the hills north of Porto Alegre. At its grid point near Santa Maria, at the head of the Jacuí, it puts 98 mm on that single day. Over the whole of May, the centre and east of the state received about three times their normal rain.

The water vapour map of 1 May shows the moist air reaching Rio Grande do Sul from the north-west, a corridor from the Amazon along the Andes, while the sea-level pressure lines show the front lying across the state. Aloft, the subtropical jet stream ran straight over southern Brazil, its strongest winds just to the south, lifting the air beneath it and keeping the storms going.