Understanding the Value of Radar Rainfall Nowcasts in Flood Forecasting and Warning in Flashy Catchments

Providing flood forecasts in flashy catchments poses significant challenges to the hydrologist. This is particularly the case when the prediction of high intensity rainfall at small spatial scales is difficult. Radar rainfall nowcasts, such as those provided by the Met Office Nimrod system, provide short range predictions at these spatial scales, and can be used as an input to hydrological models for the prediction of flood flows. Such short term forecasts are, however, considerably uncertain, and this uncertainty will influence the reliability of hydrological forecasts used in flood warning and forecasting. In this paper the value and benefit of the use of radar rainfall nowcasts in three small catchments in central Scotland is assessed through the evaluation of a large sample of forecasts. Both the reliability of the catchment rainfall predictions and the forecast flows are assessed. Whilst it is demonstrated that the rainfall predictions provided by Nimrod are uncertain and at times biased, it is also shown that there is considerable benefit in their use for flood forecasting when compared to the alternative of using no future prediction of rainfall. To deal with the uncertainty in the forecast, a method is shown that can help the hydrologist and forecaster to understand the structure of the uncertainties, allowing them to use the guidance provided by the forecasts more effectively in the provision of flood warnings. Copyright © 2009 Royal Meteorological Society