The extraordinary El Niño Costero of 2017 has affected the Peruvian coast, mainly in the Piura region, with the flooding of the Piura river, impacting rural and urban areas in the middle and lower basin. This paper analyses the situation from the perspective of hydrology. Since 2002, the basin has had an early warning system (EWS) made up of a hydrometeorological network and a forecast model, NAXOS. The EWS predicted flows at the Los Ejidos hydrometric station, located immediately upstream of Piura City. The hydrometeorological network has been gradually losing its operation and NAXOS has been left uncalibrated. Its inaccurate forecasts contributed to poor decision-making in disaster prevention in 2017. With the hydro-meteorological information of that year, three models of deterministic forecast of the flow in Los Ejidos have been adjusted by means of multiple linear regression, with 1...