A synoptic discussion about the thunderstorms of the 13th & 14th September 2006

The situation at 12Z on the 13th September was complex over the Eastern Atlantic and Western Europe. The upper level chart at 300mb (shown in Fig 1, right) shows that a maturing low was coming east and throwing its occlusion east into the UK, whilst at the same time in the far western Atlantic Tropical Storm (TS)Florence had moved across SE Newfoundland, Warm advection outflow from TS Florence was throwing a ridge into the western central Atlantic, this effectively veering the transatlantic jet and forcing a sharpening of the downstream trough to the west of the UK. The net effect was that the trough was forced to sharpen as a strong (>130Kt) jet ran SE into its base. The resultant trough extension stretched south-east wards across Iberia and effectively created a mechanism for partial trough disruption to occur over the British Isles and Northern France.

By 12Z on the 13th a moist plume of 850mb Theta E/W which had been swept north ahead of the occlusion had got about as far east as it was going to get, running into the high pressure block over Europe. Upper air ascents from NE French (Trappes as an example) were very unstable but too dry for much activity to break out. Residual moisture lay from N France across SE England to off the Yorkshire coast, where some light rain was being reported.

Aloft however there was enough energy left in the trough for a pulse of vorticity to run north east on its ascending flank, whilst at the same time the backing flow over France sent the plume of high Theta -W/E air back west across central and eastern parts of England. Models were in good agreement that the low level plume would engage the higher level vorticity maximum running NE and create an active pre-frontal trough or low pressure area across central and eastern England. Conditions appeared suitable for the destabilisation of the plume with the potential existing for some strong (mostly elevated) thunderstorms to break out as the evening progressed. However where and when this was going to occur was complex and required several seperate features to come together, critically dependent on the interaction of the vorticity moving NE, the shape of the upper trough moving east and the amount of backing which took place ahead of it.

12Z model guidance indicated rainfall totals in the order of 25-35mm were quite possible in a few spots; areas just west of the meridian appearing most at risk. After the event this proved reasonably accurate, with overnight falls of over 25mm occuring in the SW Home counties and a report of 22mm in one hour in Kent

During the afternoon and evening of the 13th, satellite images (Figs 2/3 on the right) along with observations over France suggested that a small area of low pressure had developed in west central France and was moving north, in response to increased low level baroclinicity and an injection of vorticty advection from the upper trough which was moving north into the Bay of Biscay. This combination forced ascent on its forward flank and outbreaks of increasingly heavy rain developed across NW France whilst thunderstorms developed over north central France and then the English Channel as a result of an elevated moist layer (EML) at around 900mbs, which forced ascent and the residual moist plume created the required ingredients for the storms to develop. Similar elevated storms were also located further south west over the Bay of Biscay. 

The radar image at 2215Z of the evening storms is shown (Fig 4 right) with some active bands of thunderstorms moving NE across Kent, Sussex, Surrey and across London into Essex and then north east into E Anglia. There were local reports of flooding as well as homes being hit by lightning as the storms were quite electically active. Further west heavy rain was moving north across CS England and into the Thames Valley.

Table 1. 48 hour rainfall totals (above 30.0mm) 06Z 13 Sept- 06Z 15th September 2006 Courtesy of 'Weather Online'

By Thursday 14th, at 00Z, the upper air 300mb chart shows how the situation was developing (Fig. 5 right). The moist plume was lying down the eastern side of the UK whilst the small low that had initially moved this plume up from France was moving north across the Midlands. 

The day started wet across central southern England with a small area of very heavy rain moving north through the Portsmouth area, during the rush hour, which was responsible for bringing flash flooding to schools in the region and created local transport problems.

As the day went on the focus shifted to the north east Midlands & eastern England. There was still some high Theta-E/W air over this region and the high resolution visible imagery available, indicated a clearance moving into the region which allowed insolation to heat surface temperatures and create a more favourable environment for storm formation. The 11Z Nottingham upper air ascent (modified version shown as Fig 6, right) showed a fairly moist but not especially unstable profile; however modifying to allow for the warmer and slightly less saturated conditions that were occuring over Lincolnshire (where temps of 23C, and dewpoints as high as 19C existed) showed that over 300j/kg of CAPE were available, along with an increasing amount of the low (0-1Km) helicity. These conditions suggested the possibility of tornadoes occuring during the afternoon over parts of north and east England.

As the early afteroon progressed water vapour (WV) imagery showed a dry intrusion, which was associated with a shortwave upper trough moving north through the Midlands towards Northern England, and it was this, combined with peak insolation and the higher areas of CAPE over the East Midlands and Eastern England, which allowed thunderstorms to break out. In addition, the small low pressure area, which by mid afternoon was located near Leeds, was responsible for a localised increase in lowest level helicity. This backed the flow on the right side and coupled the flow aloft, increasing the 0-1km s/rh.

The 15Z surface streamlines & mean sea level pressure analyses are shown to the right  (Figs 7,8). The confluence over Eastern England is clearly visible. Local radar over Northern England (Fig 9, right) shows the intense cells which had developed around Leeds and subsequent reports indicate that a tornado was associated with this cell, which brought some notable damage to parts of the city. 

During the afternoon a line of storms moved east with reports of strong straightline gusts, along with other reports of possible tornadic activity in Derby, Harrogate and possibly Barnsley. These cells were more violent than the ones that developed the night before as they developed from the boundary layer and were not elevated, so whilst they did not contain the electrical activity of the previous nights storms they did contain strong gusts and aforementioned tornadoes.

Summary

The storms of the 13th and 14th resulted from a strong upper trough moving east and becoming slow moving against a strong block over Europe, a plume of high 850mb moisture / temp moved north from Iberia and was destabilised by the trough. In addition a small low developed and moved north which created the local environment for tornadic development. The outbreak can be related indirectly to Hurricane Florence, as outflow from Florence was contributing to the strong jet and sharp trough that were moving towards the UK, both of which were important factors in the subsequent evolution. With the effects of Gordon and possibly Helene later in the month (see related articles elsewhere) September thus proved an unusually active month for the UK in terms of ex- tropical activity, despite the rather low level of general activity during the early -mid season.     

Paul Blight 2006 (C)