Characteristics and environments of flash-flood-producing storms in the UK

A set of 103 surface-water flooding events in the UK over the period 2015 – 2021 has been identified using impact reports from a variety of sources. For inclusion, flooding impacts had to meet basic severity criteria (broadly comprising internal flooding of multiple properties, or disruption to key infrastructure such as hospitals) and the flooding had to be associated with deep, moist convection (assessed by analysis of radar data in conjunction with the flood reports). The synoptic-scale, mesoscale and storm-scale environments of the identified events were explored using ERA5 reanalysis data, radar data and surface meso-analysis, the latter incorporating bias-corrected observations from a network of home automatic weather stations. One of the main findings is that surface-water flooding events in the UK often occurred in environments of relatively weak vertical wind shear (median effective bulk wind difference = 12.8 knots) and small buoyant instability (median MUCAPE = 561 J kg^-1), though a few cases exhibited much larger vertical wind shear and CAPE. Mesoscale features such as convergence lines were often important in focussing repeated convection initiation over a small area. In this presentation, results of the work will be described in more detail, and some conclusions drawn about the typical characteristics of surface-water-flood-producing storms in the UK.