Forecasting tornadoes in the United Kingdom

Abstract

This paper identifies the synoptic situations, and combinations of those situations, together with other influences such as topography and minor fronts (e.g. sea breeze and coastal fronts), that produce tornadoes in the United Kingdom (UK). This marks an important step towards improving the forecasting of tornadoes in the UK and the issuing of tornado warnings at suitable times. The Tornado and Storm Research Organisation (TORRO) has experimented with issuing tornado watches and pre-watch severe thunderstorm/tornado advisories to its UK network of observers since 1991. Complementing this work, this paper identifies those synoptic situations that statistically and historically have given rise to the most prolific outbreaks or strongest tornado events. These situations include a strong (ana) cold front associated with a rapidly deepening low; a zone of wind shear, especially when enhanced by topography; and a slack area of low pressure in the late spring and summer. The role of lee effects in which eddies are created downwind of islands, coastal promontories and high ground inland are highlighted too. Synoptic situations that produce tornadoes, but only rarely, include severe summer or supercell thunderstorms; the point of occlusion (triple point) associated with a deep depression, often incorporating the remnants of an ex-tropical storm; and returning polar maritime airstreams with an upper trough to the west of the UK. Generally, most UK tornadoes occur during an air-mass change and during ‘cold weather’ scenarios during the autumn and winter. This highlights that the synoptic situations that frequently produce tornadoes in the UK differ from those producing tornadoes in continental USA and in many other parts of the world. Instead, these findings suggest the UK tornado regime is similar to other west coast locations such as California, western France and, perhaps, Portugal.