Flash Flooding and Rapid Hydrologic Response

Much of the CNRFC area is susceptible to flash floods. In addition, many of these flash flood-prone areas are among the most heavily populated in the country. The NWS defines a flash flood as a flood event which occurs within 6 hours of the causative event. The rapid and often localized development of these events requires real-time assessment and immediate public warning. As such, the responsibility for public flash flood watches and warnings rests with the continuously-staffed WFOs. In support of their efforts, the CNRFC provides flash flood guidance based on current watershed conditions.

The CNRFC contributions to the flash flood threat don’t end there. With the assistance of the CNRFC, numerous cities and counties have installed and actively maintain an automated local flood warning system. The CNRFC pioneered the concept, design, development, and application of ALERT (Automated Local Evaluation in Real Time). The systems consist of automated event-reporting rain and river gages and a computer system that analyzes current and developing hydrologic conditions. Data from the river and rain gages are generated in response to changing conditions (e.g. 0.04 inches of rain or 0.1 foot change in river level) and transmitted to the computer base station using line-of-sight radio operated on the Hydrologic Warning Frequency. Approximately 30 such systems currently exist within the CNRFC area. Between them, they monitor in excess of 700 precipitation and 275 river level gages. All data are automatically passed to the nearest WFO as well as the CNRFC. Collaboration among the operators of these systems is facilitated by the ALERT Users Group.