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

Lahars in the ephemeral channels of Arequipa (Peru) are triggered by summer rainfall, the causes of which are poorly known. On 19 and 22 February 2026, extreme rainfall activated the tributaries of the Chullo channel on the southern flank of the Chachani volcano. We aimed to identify its causes: moisture sources, convective environment, and antecedent conditions. We combined European Centre for Medium-Range Weather Forecasts Reanalysis v5 (ERA5) backward trajectories with Lagrangian moisture accounting, vapour-transport diagnostics, Geostationary Operational Environmental Satellite 19 (GOES-19) cloud tops, Climate Hazards Group Infrared Precipitation with Station data (CHIRPS), and Integrated Multi-Satellite Retrievals for GPM (IMERG) precipitation contrasted with Servicio Nacional de Meteorología e Hidrología del Perú (SENAMHI) rain gauges and catchment curve number runoff. Neither episode received direct Atlantic moisture within ten days (≤0.8%). The largest share of the arriving moisture was gained in the free troposphere and could not be assigned to a surface source; identifiable surface uptake was mainly continental on 19 February and mainly from the coastal Pacific boundary layer on 22 February (about one fifth of the arrival humidity). The second episode was moister and, in the early afternoon, more convergent with stronger ascent, but had weaker transport, with colder cloud tops (−74.2 vs. −55.2 °C). All precipitation sources showed larger seven-day antecedent rainfall before 22 February, and rainfall peaked in the late afternoon. The evidence is consistent with afternoon deep convection in a moist, convergent environment over catchments wetted by antecedent rainfall, rather than with a direct Atlantic moisture supply.