The Hydrology Program of the National Weather Service (NWS) provides forecasts, watches and warnings, and other services regarding the nation's waterways. As part of the Advanced Weather Interactive Processing System (AWIPS) currently being deployed at NWS field offices, the Weather Forecast Office (WFO) Hydrologic Forecast System (WHFS) software is making dramatic improvements to the capabilities of the Hydrology Program. A recent release of the WHFS includes a component system to assist WFO forecasters in monitoring short-fused flood events.
This paper will provide an overview of this monitoring system and discuss the technical and operational issues associated with it. The system computes estimates of the potential for flooding in small basins based on comparisons of observed and forecast precipitation with Flash Flood Guidance (FFG) values, which represent the forecast amount of precipitation that an area can receive before it experiences flooding. Specifically, it computes precipitation as a percentage of FFG, as a difference from FFG, and as a rate value, for various durations ranging from one to 24 hours. This information is then presented in graphical and tabular form for display and interpretation by the forecaster.
There are three primary data sets used in the system. First, the precipitation data are the most important data set as short-fused flood events are driven primarily by the rainfall intensity. The precipitation data includes gridded radar estimates, point gage observations, and areal forecasts; each has differences in their timeliness and in their spatial and temporal resolution. Second, the FFG estimates are provided to the WFOs by the NWS River Forecast Center (RFC) models. The FFG values have limitations, in terms of accuracy and spatial resolution. Lastly, the basin boundary data defines the geographic areas for the analyses; the resolution of these areas define the scale of the system. The characteristics of each of these data sets plays a crucial role in how effective the comparisons are in assessing the flood threat. This monitoring system is designed to minimize these data issues and to provides a powerful operational tool for quickly and effectively dealing with short-fused flood events