By mid May a complex range of atmospheric events often play a part in the development of storm activity. The afternoon of the 10th May 2006 was such a day when a large area of storms developed over the Chilterns and moved slowly west with  Thunderstorms and heavy rain, producing local flooding in places and Lightning damage reported in both Wiltshire and South Wales. Whilst it was touch and go whether storms developed, eventually local topography and convergence helped give the storms the boost they needed.

Basically the Atmosphere was pretty stable and for forecasters the development of storms was anything but certain. An area of high pressure covered much of the area to the north east of the UK and a rather slack easterly flow covered the UK.
The day started clear and fine across England & Wales. However upper air ascents at 06Z indicated the presence of a nose of warmer air aloft between 800-900mbs. A 'warm nose' such as this, stops Clouds from building up quickly until a trigger Temperature is met (in this case estimated as about 21C) which allows the Convection to develop rapidly and release masses of energy (technically it is called Convective Potential Energy -often abbreviated to CAPE).  However due to the basic atmospheric stability it was all very marginal and despite the trigger temperature being reached early in the afternoon  it required some extra element to really get the convection going.

In this case it was topographical assistance, always a good trigger mechanism- areas where there are hills, helped by the effect of a sea breeze convergence zone that developed in the area too.  Upper air ascents at midday suggested to forecasters that if  temperatures as high as 22C were reached then cloud could build to above 30'000ft and medium sized hail was also possible.

This is indeed what occurred. Satellite images showed large cumulonimbus clouds rapidly developing as thunderstorms broke out. By the late afternoon the storms moved west down the M4 Corridor in the light easterly flow. Walnut sized hail was reported in places as well as local flooding. There was also an unconfirmed report of a funnel cloud near Bracknell. Storms moved across Wiltshire and into Bristol area in the early evening and caused flights into the airport there to be diverted. It appears that the tops of the Cb's were around 36,000ft with temperatures as low as -70C. As diurnal heating diminished the storms finally dissipated overnight.

The storms provided a good example of how a 'loaded gun' keeps energy stored up until temperatures are high enough and forcing allos it to be released.