The effectiveness of extreme rainfall alerts in predicting surface water flooding in England and Wales

The extreme rainfall alert (ERA) pilot was a response to the Pitt Review's recommendation that the Met Office and Environment Agency (EA) provide an early assessment of the costs, benefits and feasibility of techniques which can predict where rain will fall and where surface water flooding will occur. In April 2009, the ERA became an operational service provided to the EA professional partners and Category 1 and 2 respondersorganizations with legal responsibilities to respond to flooding. ERAs are intended as a first step towards warnings for surface water flooding (SWF). This article assesses the effectiveness of the ERA pilot at predicting SWF. ERAs relating to three case study areas and issued during the pilot were compared with reports of SWF, identified from the data available relating to all sources of flooding. While data were found to be lacking in quality and consistency, it is possible to provide an initial assessment of the effectiveness of the ERA pilot. The existing ERA rainfall thresholds do not relate directly to SWF in all areas. As the ERA service specifically intends to warn for rainfall likely to cause severe SWF in urban areas, it is impossible to draw strong conclusions without further investigating the magnitude of flooding which occurred both when an ERA was and was not issued. As SWF magnitude and extent data were extremely sparse, it is recommended that systematic collection of comprehensive data relating to flood events should be implemented nationally. This will facilitate more accurate investigations of the links between rainfall intensity and SWF, which it is vital to understand in order to provide the most effective surface water flood warnings possible. Copyright (c) 2011 Royal Meteorological Society