Piura, in northwestern Peru, is one of the places most affected by the recurrent El Niņo events, which produce heavy precipitation on this otherwise arid region. Monthly rainfall has been found to relate strongly to the high nearby sea surface temperature (SST). However, rainfall tends to occur in discrete intense events and an understanding of the physical conditions favouring a particular day over others with heavy rainfall, considering that SST changes little from day to day, can be useful in rainfall forecast and contribute to the preparedness of the population in the region.

In this work, we consider the anomalously warm period of March-April 2002 as representative of weak to moderate El Nino conditions and search for systematic differences in the atmospheric circulation that may be favouring some days over others for the occurence of rainfall on the western slopes of the Andes in the region of Piura. Composites of vertical profiles of winds measured by a boundary layer/troposphere radar (BLTR), as well as composites of NCEP Reanalysis wind fields, suggest that rainy days are associated with an enhanced onshore westerly flow, which we propose to be the triggering mechanism of convection by orographic lifting.