Moisture Sources and Convective Processes Driving Extreme Rainfall in Arequipa, Peru, on 19 and 22 February 2026

Ephemeral channels in Arequipa (Peru) convey floods and lahars during the rainy season. On 19 and 22 February 2026, tributaries of the Chullo channel on Chachani volcano’s southern flank became active, but the origin of the triggering rainfall was unknown. This study characterises both episodes from the continental to the local scale and assesses whether they shared a common moisture origin. We combined CHIRPS v3 precipitation, NCEP/NCAR R1 reanalysis, 54 backward trajectories with moisture bookkeeping, GOES-19 infrared imagery and Air Mass RGB composites. February 2026 rainfall (412.8 mm) was 145% above the 1991–2020 mean, and seven-day antecedent rainfall was larger before 22 February. Air arrived from the east on 19 February and from the southeast on 22 February, yet in both episodes most arriving moisture (63% and 96%) was gained over the preceding five days along mainly continental paths. Air Mass imagery showed a continuous tropical air mass from the tropical Atlantic to the southern Andes and dry air aloft over the adjacent Pacific. The second episode had stronger vapour transport and colder cloud tops (−74.2 vs. −55.2 °C). The Chullo activation reflected afternoon convection over wet antecedent conditions, fed by continental moisture compatible with a remote Atlantic origin.