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An extreme precipitation event took place during the second half of January of 2020 in the Atacama Desert. From Tacna, Perú to Iquique, Chile (18-21ºS) rainfall extended for several days producing floods, major damage to infrastructure, and affectation to the population in one of the driest deserts of the world. Analysis of surface-weather stations, sounding, satellite data and reanalysis suggests that the most intense precipitation occurred in precordillera (2,000 to 3,600 m.a.s.l). In a historical context, several weather stations between the pampas and precordillera recorded the highest daily rainfall intensity on record. The analysis of this EP event also suggests that at least four major factors were present to produce record-breaking precipitation in the Atacama Desert: (i) a low-level circulation off-shore Atacama Desert, potentially linked to the southward displacement of the Bolivian High and a high troposphere trough subtropical southeast Pacific Perú, (ii) the advection of humidity through an atmospheric river like structure, trapped to the coast in the front of the low-level cyclonic circulation, leading to increases in precipitable water vapor over the Atacama Desert, (iii) above normal sea surface temperatures that favor moist conditions in the boundary layer and (iv) a reinforced local circulation, where low level winds transport humid wet parcels toward the east, producing terrain-forced ascend at the foothill of the west slope of the Andes Cordillera, triggering precipitation and thunderstorm mostly in precordillera, but also extended to the coast, pampas and Altiplano. Analysis of previous austral summer precipitation days from 2008 to 2020 suggests that this thermo-dynamic mechanism is highly linked to the majority of the EP days in Southern Perú and Northern Chile, becoming in an important configuration to predict future EP events in the Atacama Desert.
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