From anomalous SST gradients in the Indian Ocean to floods and tornadoes in southern Brazil: the multi-scale nature of a historical precipitation event.

From late April to early May 2024 a persistent episode of high precipitation in Brazil’s southernmost state of Rio Grande do Sul (RS) produced a regional-scale flood of historical proportions, representing the largest geo-hydrological disaster in the country’s history in terms of affected area. Adding to the copious amount of rain, tornadoes were reported in RS during the same period, while a downslope windstorm was observed in central RS in the week following the high precipitation event. This multitude of hazards shared a setup that resulted from a combination of mechanisms belonging to a vast range of atmospheric scales. This article provides a view from researchers who experienced a direct hit from the historical weather event in RS, describing its extension in a general timeline from April 27th to May 8th 2024. Ranging from the planetary scale to the smaller scales, the forcing mechanisms included: interbasin SST gradients in the southern oceans, anomalous deep convection east of Africa, planetary Rossby wave trains across the Southern Hemisphere, persistent blocking high over the Southeastern Pacific, migratory troughs crossing the southern Andes, and frequent lee cyclogenesis over northern Argentina. This cascade of oceanic and atmospheric features culminated in a persistent northerly low-level jet that continuously supplied RS state with unseasonably large amounts of moisture, acting as an excpetionally strong atmospheric river. Finally, this same feature also created favorable conditions to the formation of tornadic storms, and to a downslope windstorm in central RS following the extreme rainfall event.