Supercell Thursday - 28th June 2012
For many people in the UK, including meteorologists and weather enthusiasts, June 28th 2012 was a day to remember. In the afternoon, violent thunderstorms and supercells broke out across the Midlands and northern England that produced flash flooding, large hail and two tornadoes rated as T2 (EF-0) and T4 (EF-2). Although tornadoes in the UK are not uncommon - we receive roughly 40 tornadoes per year on average, they tend to form in high-shear, low-convective available potential energy (CAPE) environments and as many tornadoes can be expected in autumn as summer climatologically. However, on this day in June, both relatively high speed and directional shear, as well as ample surface-based CAPE (>2000 J/kg) were available, conducive for widespread organised multicell development as well as boundary layer rooted supercells. Such severe meteorological conditions as this are rare in the UK and the severe weather that followed was equally as extraordinary. 
The 28th June was characterised by a Spanish Plume: this synoptic pattern typically provides the ingredients for severe thunderstorm development across the UK. Generally, the UK will sit in the warm sector of an approaching low pressure from the Atlantic, with warm, moist air being advected north or northwestwards from the Mediterranean and continental Europe at the surface. However, at around 900-800 hPa, hot, dry air is generally advected northeastwards towards the UK from the Iberian Plateau, thus acting as a "cap" on top of the humid airmass at the surface. Finally, the approaching Atlantic low can introduce cooler air at the mid and upper levels, providing significant instability as well as introducing greater deep level shear. 
The Spanish Plume setup for the UK on the 28th June. The area of low pressure to the south west of the UK was lifting northwards, increasing the amount of time the UK was in the warm sector. This increased the time warm and moist air could be advected from the near continent and Mediterranean.
Credit: Met Office
The UK is influenced by several Spanish Plumes every summer, which produce thunderstorms and torrential downpours, some of them severe. What set this event apart was that a low-level jet was present over the UK. Unlike the jet stream, which exists at around 10,000 metres, the low level jet occurs around 1500 metres above the surface and It forms as a result of differential heating from the surface, or from an approaching low pressure system with fronts with a typical wind speed of 50 mph. In the summer, these are relatively rare and so most Spanish Plume events occur whilst being unaded by a jet. The efffects of a low-level jet on thunderstorm are significant - it provides the low-level wind shear needed for a thunderstorm to rotate (supercell) and produce a tornado. Normal directional shear which is often present in the atmosphere, can be enough for rotating supercell thunderstorms to develop, but is not usually enough to produce tornadoes. The 28th June had the unusual combination (for the UK) of high wind shear (close to the surface and aloft) and high instability (CAPE). 
In this tephigram from Larkhill (Wiltshire) for 0900 UTC 28th June, there is over 2700 J/kg of surface-based CAPE available, which is incredibly high. The build up of instability was aided by increasing temperatures and dew point temperatures through the day. It reached 26 - 28°C widely across southern UK and the Midlands. Dew point temperatures were also above 21°C widely too. The temperatures were high, but the humidity was exceptional, and this helped thunderstorms to develop with bases very close to the surface, increasing the potential for tornadoes. 
Figure taken from (Clark and Webb 2013).
The basic ingredients for thunderstorm formation were all in place: significant moisture in the lowest levels, a cool mid and upper troposphere (indicated by almost complete dry adiabatic lapse rates from around 850 hPa to the tropopause), and a lifting mechanism, indicated by the two surface troughs ahead of the cold front in the synoptic chart above. Finally, significant vertical wind shear was in place, allowing the development of severe thunderstorms - there was around 50 knots of 0-6 km deep-layer shear, as well as over 30 knots of 0-1 km shear, allowing the development primarily of multicell thunderstorms along with a few discrete supercells, with the potential to produce tornadoes. 
Thunderstorms initiated in mid-Wales at around dawn and by the time the storms had reached Birmingham, a thunderstorm on the southeast flank of this system became surface-based and supercellular, producing a T4 (EF-2) tornado which travelled 13 km, to the northwest of Leicester at around 11:15 am. Although this is very early in the day for surface-based supercells to form, substantial surface warm air advection through the morning as well as solar insolation from before 5 am helped to overcome the cap quickly.
A unique feature of supercell thunderstorms is that they generally deviate to the right of the mean flow due to the veering wind profile (ie. clockwise) with height. As a result, supercell thunderstorms often move away from any other storms that might compete for low-level moisture, therefore prolonging their life and increasing their strength.
The village of Newbold Verdon suffered damage to properties, outbuildings, garden fencing and trees, with a roof being ripped off at an industrial estate. The tornado was estimated to have winds of up to 120 mph and caused a narrow damage path around 100 m wide. Coalville and Whitwick Weather Station recorded some incredible rainfall rates, with 39.6 mm (1.56 inches) falling in 35 minutes from this storm; 26.2 mm (1.03 inches) fell in just 10 minutes. By 12:00 pm, the storms had conglomerated to form a large MCC and moved out over the North Sea.
However, later in the day, a second cluster of thunderstorms formed over the southwest Midlands and moved northeast, following a similar track to the first storms. Two supercells, shown in the radar image below, another tornado and a damaging hailstorm that tracked through Leicestershire were all recorded from this group of storms. The most notable phenomenon here was the intensity of the hailstorm, produced as a result of very strong updrafts in the supercell in the top right of the radar image.
Radar image of the second cluster of storms to develop. The two supercells are on the bottom edge of the storms, with their distinctive "hook echo" radar signatures. 
Credit: Netweather
This storm produced a 110 km-long swathe of hail, with most of the hail > 40 mm in diameter and isolated pockets with hail > 80 mm in diameter: mainly over Burbage and surrounding rural areas. It was even reported that there were several stones that fell larger than 100 mm. Where hail fell, damage was widespread: cars had windscreens cracked or completely smashed along with severe bodywork denting. House roofing tiles were cracked or smashed, conservatories completely destroyed and drains and gutters became blocked with debris from trees stripped of leaves, leading to significant flooding in Hinckley and Burbage. It has been estimated that the storms caused up to £150 million of damage to cars,  properties and businesses within Leicestershire alone.

Contours of observed hail size and maximum reflectivity associated with the Coventry-Seaford supercell. The track of the Sleaford tornado is also shown. In the top left, the track of the mescyclone is displayed by the pink line, as well as the locations of reported severe weather.
Figure taken from (Clark and Webb 2013).
A much larger band of thunderstorms developed over north Wales and the Irish Sea between 1100-1300 UTC and eventually became a MCC as it moved through Cumbria and Northumberland towards the Newcastle area. This MCC developed no severe characteristics but was very electrically active and  produced high rainfalls totals. 62 mm (2.44 inches) fell from 4:00 - 6:20 pm at Whitley Bay, enough to cause flash flooding in Newcastle and landslides which blocked both the west and east coast mainline railways. As well as this, 23,000 properties across northeast England lost power due to lightning and flooding damage. According  to the Met Office, more than 111,000 lightning strokes were detected across the UK, with more than 1000 in one 5 minute period at the peak of activity.
Clark M. R., Webb J. D. C., 2013, A severe hailstorm across the English Midlands on 28 June 2012, Weather, 68, 284 - 291, DOI: 10.1002/wea.2162