The development of tornadic storms on the cold side of a front favoured by local enhancement of moisture and CAPE

In the afternoon of 28 July 2005, a damaging F2 tornado in Birmingham, United Kingdom, was one of three tornadoes developing on the immediate cool side of a surface warm front. An analysis is performed to find out why the narrow zone on this side of the front was apparently so favourable for tornadoes in spite of lower surface temperatures. It is investigated how three ingredients for tornadogenesis are distributed and how these distributions evolve. These ingredients — the presence of a convective storm, low-level wind shear, and a low LCL height — and the presence of SBCAPE, which is a prerequisite for surface-based convective storms, are studied using surface, upper-air and satellite data, as well as a convection-permitting model. At some distance on the cool side of the front, strong wind shear existed across the lowest 1 km (around 15 m s−1 bulk shear), but no surface-based instability to support surface-based convection. In contrast, the air mass on the front's warm side exhibited weak SBCAPE (50–200 J kg−1) but only fairly weak low-level shear (0–1 km bulk shear ~ 6 m s−1). The analysis suggests that between these regions, within a narrow zone on the cold side of the front, surface-based instability was considerably higher (~ 450 J kg−1 of SBCAPE) and the 0- to 1-km bulk wind shear was estimated to be 10.5 m s−1. Moreover, the higher relative humidity in this zone resulted in a lower lifted condensation level (~ 200 m in this zone, compared to ~ 700 m on the warm side of the front). It is concluded that an overlap of all these four ingredients only existed within this zone that was only about 30 km wide.

Research Highlights
► We study a warm front related to multiple tornadoes.
► The tornadoes occurred on the immediate cold side of the front.
► There, a localized increase of low-level moisture created CAPE.
► Together with strong low-level wind shear and low LCL heights, this fostered tornadogenesis.