From mid July the weather over the British Isles was dominated by a sluggish high pressure block over Western Europe, this allowed record breaking heat to affect many parts, especially over England where the all time record for July was broken at Wisley (36.5C on the 19th).  From the 21st a longwave trough was anchored to the west of the UK and this pattern gave rise to thunderstorms over parts of the UK on the 22nd of July. The more widespread storms occurred on the 22nd (however, as detailed in a further article, storms on the 26th and 27th, whilst not as widespread, were also severe and generated flooding as well as reports of lightning damage and hail).

From the 15th to the 22nd high pressure was in overall control of the weather over the British Isles bar some weak incursions into the NW. Heat gradually built on a daily basis due to a mixture of insolation, little air mixing and slow warm advection from the continent. The peak temperatures occurred on Wednesday 19th July when 36.5C was recorded at Wisley (Surrey) and 36.3C at Charlwood (Surrey). The hot air was pushed east for a time on Thursday 20th, but it only got as far as the low countries before being pushed back westward on Friday and Saturday.  At the same time cooler air above 500mbs was pushing into the west assoc with the longwave trough, the scene was being set for the atmosphere to become more unstable and to release some energy.

The synoptic picture at 00Z on the 22nd is seen in Fig 1 along with the 12Z 300mb analysis (Fig 2). It can be seen that at the surface there was a very flabby pressure pattern with an approaching low pressure system to the west of the UK and declining high pressure over Denmark; aloft however it was a bit more organised. A fairly well organised low pressure system was located to the west of Ireland with the upper trough and assocociated vorticity advection racing east and running ahead of the surface trough to the west of Ireland and then splitting. Overnight on the 21st into the 22nd, a minor shortwave moved NE and allowed storms to develop across central southern England and move NE (with 9mm in one hour reported at Hurn overnight). By 09Z the storms were over Lincolnshire and were still generating some impressive rain and wind speeds. For example, a gust to 54knts (62mph) was recorded at RAF Cranwell between 08Z and 09Z.

Elsewhere, a warm high Theta-W 850mb plume had moved north from France and was located across the UK, running from S Scotland down through Central England to Northern France (temperatures greater than 16-17C Theta-W); beneath this the surface air was from the E-SE and fairly dry. These points are noted in the 22/00Z Herstmonceux Ascent and shown in Fig 3, where the dry warm boundary layer is capped by an elevated moist layer between 850mbs and 700mbs, with steep lapse rates above, which had been previously cooled and would be cooled /steepened further by the approach of the shortwave upper trough from the SW.  Forecasters noted that the upper trough to the SW was set to move over this moist warm layer; the ascent created ahead of the trough axis would likely then be enough to destabilise the underlying plume and allow initially elevated storms to develop from the south west, however with continual moistening of the boundary layer, storms eventually could erupt from the boundary layer itself.

The surface situation at 09Z is shown in Fig 4. A surface trough had developed in the SW approaches and was moving NE. This was co-located under the 850mb plume and a weak surface low pressure centre had developed in response near Portland Bill. Over England clearing skies were allowing temperatures to rise with a light E-SE wind. During the remainder of the morning the trough to the SW continued to move NE and shortly after 1030Z a large cell burst into life over the English Channel. This storm was generated by the destabilisation of the plume by the sharp upper trough moving NE and grew upwards from 800-850mbs right up to the tropopause (35,000ft). MSG indicated the cell had tops to over -50C. Water Vapour imagery is shown in Fig 5 for 1030Z; the dark stripe from SW Ireland to Normandy indicating the location of the upper trough with the vorticity advection located on the forward side. The cell over the Channel can be seen growing. It  continued to grow and made landfall over the South coast near Poole Bay. A heavy thunderstorm was reported in Bournemouth between 11-12Z with flooding reported in the town centre and a gust to 33knts at Hurn.  This cell then moved north-east whilst other cells were breaking out further along the coast in Hampshire and Sussex; these were also elevated.

Noon MSG high resolution visible imagery is shown in Fig 6 and enhanced infra red imagery in Fig 7 opposite, which clearly illustrates the size of the developing storms. . 

Inflow into the storms was from the SE and NW with daughter cells moving NE in the Channel, whilst other storms developed over Wiltshire and Gloucestershire. By 1230Z the storm was located over CS England (enhanced radar imagery from 1215Z over the area is shown in fig 8). The storms intensified over Hampshire with reports of flooding; 23mm was reported in the hour to 13Z at Boscombe Down along with a 44 kt wind gust. 

During the afternoon the storms merged into a large Mesoscale Convective System (MCS) which tracked north east across the Midlands into Eastern England. There was a notable gust front on the leading edge of storms with gusts to 37kts at Cottesmore and 39 kts at Cranfield for example. As the storms moved NE there were also some very notable temperatures falls including:-