An observational study of the exceptional ‘Ottery St Mary’ thunderstorm of 30 October 2008

Abstract

AbstractA small but intense convective system affected parts of east Devon during the early hours of 30 October 2008, producing an estimated 200 mm of precipitation, including locally over 20 cm of hail, within a 3 h period. This exceptional storm and its environment are studied here from an observational perspective. Upper air and satellite observations show that the storm developed within an environment characterized by a trend for increasing instability and dynamic forcing for ascent, associated with an approaching upper level cold pool and small positive potential vorticity anomaly. Analysis of surface observations reveals that convective development was strongly focussed by localized, persistent low‐level convergence and moisture convergence in the vicinity of a mesoscale surface low pressure area, which rapidly deepened before becoming slow moving over east Devon. Radar, wind profiler and other available observations are used to describe the development and evolution of the convective system, and to identify some additional factors contributing to the exceptional rain‐ and hail‐fall totals. Finally, the role that real‐time, simultaneous analysis of these various observation types could play in nowcasting of localized, extreme rainfall events is discussed. © Crown Copyright 2010. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd