NSSL and CIMMS scientists complete project to enhance flash-flood warning decisions

Although floods and flash floods on average cause more fatalities and property damage annually than any other weather-related hazard, tools to assist National Weather Service forecasters in flash-flood warning decisions have historically been lacking. To address this problem, a simple yet effective flash-flood monitoring tool has been developed during the past few years. The Flash Flood Monitoring and Prediction (FFMP) system, which is part of the Advanced Weather Interactive Processing System (AWIPS), computes average basin rainfall accumulations and rates based on WSR-88D precipitation estimates. These calculations allow NWS forecasters to monitor precipitation in flash-flood-scale basins, potentially improving their ability to make accurate and timely flash-flood warning decisions.

The success of the FFMP is largely dependent on the accuracy of the precipitation estimates and the accuracy of the flash-flood-scale basin definitions. In an attempt to maintain consistency and centralize the task of defining small basins for the nation, the National Basin Delineation Project (NBDP) was undertaken by NSSL and CIMMS. Initiated in 2000 and completed in the summer of 2002, the purpose of the NBDP was to define flash-flood-scale basins for the conterminous U.S., Puerto Rico, and Guam. The project was accomplished through a partnership with the U.S. Geological Survey Earth Resources Observation Systems (EROS) Data Center (EDC). The EDC's plans for creation of a dataset of Elevation Derivatives for National Applications (EDNA) coincided with the NBDP. The similar objectives of the NBDP and EDNA facilitated a partnership to eliminate duplication, ensure sound technique, and maintain consistency in hydrologic derivatives on a national level.

Based on the one arc-second (approximately 30-meter) resolution raster digital terrain data in the EDC's National Elevation Dataset (NED), hydrologically-conditioned elevation grids, flow-direction grids, flow-accumulation grids, synthetic streams, and flash-flood-scale basin boundaries were derived using Geographic Information System (GIS) software. Special techniques were applied in coastal areas, natural sinks, and closed basins to ensure the accuracy of derived basins and streams. Edge matching between basin boundaries and codifying each basin with a unique identifier produced a seamless dataset for use in FFMP and other national applications. FFMP basin datasets consisting of synthetic streams, basin boundaries, radar bin center points, and other reference data were assembled for 143 WSR-88D coverage areas and delivered to 115 Weather Forecast Offices.

The resulting NED-derived NBDP and EDNA datasets are currently being used by state and federal agencies throughout the country in a variety of applications, and they have many other potential uses in various operational and scientific communities. It is important for any user to be aware that the accuracy of basins and streams derived through an automated delineation process is directly related to the accuracy of the base elevation data. Through local customization at forecast offices and other agencies, the basin dataset will be refined to ensure an acceptable level of accuracy for each intended application.

By Ami Arthur