This article shows the design and development of a reactive early warning model as part of a reactive early warning system for urban flash flood management in the city of Barranquilla, Colombia. The system is divided into four components: (i) measuring equipment with rainfall and flow rate gauges, datalogging and transmission, (ii) a system for database processing and a web interface, (iii) a numerical rainfall-runoff model in PCSWMM calibrated and validated, and (iv) a reactive early warning model for determining the hazard level, and for making decisions of alertness along the main streets. Twelve (12) rainfall gauges were installed in the city, covering around 150 Km2. The numerical model in PCSWMM was calibrated and validated with rainfall and flow rate data measured in a typical subcatchment of the city. A time of concentration equation was developed from the characteristics of the subcatchment such as slope and length. A pilot urban watershed was selected to test the system, from which hazard alerts every minute with forecasting of up to 40 minutes ahead on the lower parts of the watershed were determined. That time may allow people to make prompt decisions regarding their mobility over the city and reducing accidents caused by flash floods.