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 12 hours of anticipation for maximum events, starting from daily precipitation and upstream flows, coming from conventional and automatic stations. In moderate flows, it is even possible to have 18 hours of anticipation. The model provides an adequate approximation of the hydrographs of summer 2017. For the maximum flow, 3468 m3/s, forecast values were obtained with an error of less than 5%. The model has also been validated with data from the year 2019, obtaining satisfactory values. This model can be used by the authorities, in view of the possible occurrence of future events, for the timely adoption of preventive measures.