Robert S. Davis National Weather Service, Pittsburgh, PA, NOAA, DOC
The advantages of using a warning by basin methodology based on the Flash Flood Monitoring and Prediction (FFMP) program will be explored as an alternative to the current warning by polygon approach. Flash flood warnings and advisories are currently issued by the National Weather Service Forecast Offices (NWSFO) using the Warning Generation (WARNGEN) program. In WARNGEN the areal extent of the warning or advisory is defined by drawing a polygon around the area to be impacted by the flooding. The advantages of using a warning by basin approach in WARNGEN, based on the FFMP stream basin database, will be detailed.
Selecting pre-defined stream watersheds directly from FFMP for WARNGEN would automatically create the required polygon more quickly and much more accurately than hand drawing the polygon. In addition, using the FFMP watersheds would make the downstream extent of the flooding much easier to determine.
An aggregated basin layer of basins 5 to 50 square miles would have to be created from the FFMP small stream database by the National Basin Delineation team to support warning by basin. The current FFMP small stream database contains too many stream segments to be effectively used in text based warnings. By creating a shape file of the aggregated basins for each NWSFO, the quality control of the aggregated basins would be restored to the local forecast office. The reduced number of streams in the aggregated basin layer would make stream naming and other corrections or modifications much easier for the local offices.
The increased speed and areal accuracy of flash flood warning issuance would be the biggest advantage to warning by basin. By specifying the individual stream basins to be impacted in the warning, the user will be able to react more quickly to the specific flash flood threat. Giving the NWSFO local control over the aggregated basin layer will also lead to improved flood warning services.