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June 15th 2009 Thunderstorms and Flooding
Thunderstorms developed widely across Eastern England during the late afternoon and evening of the 15th June bringing localised torrential rain and flooding.
The 13th and 14th June were uneventful, but pleasantly warm days across southeast England. Across the English Channel the weather was thundery with some heavy downpours. At Dunkirk, nearly 34 millimetres of rain fell on the 14th, with 23 millimetres falling along the coast at Le Touquet. It was hot over much of France, too, with the temperature reaching 29 Celsius as far north as Paris.

The frontal boundary responsible for the heavy rain brought some bursts of rain to Kent early on the 15th, and a veil of thick high cloud across Southeast England and East Anglia through the morning of the 15th. During the early afternoon, the high cloud layer dissipated as it moved away eastwards, and as temperatures rose, convective cloud rapidly built. 

A pool of cold air aloft was forecast to cross southeast Britain late on the 15th, an aspect well-handled by the computer models,  and which prompted accurate warnings of heavy rain that were issued by the Met. Office several days in advance. On the early morning of the 15th, cumulonimbus clouds built to the south of Ireland with sea temperatures of only 15 Celsius, and later in the morning a few heavy showers developed in western Britain. A thunderstorm developed around 11 am in the Shrewsbury (Salop) area, and further storms developed in the Midlands around lunchtime.

The first thunderstorm in the London area appears to have occurred in eastern suburbs in the early afternoon, but the slack gradient, the formation of a 'heat low' over Greater London, sea-breeze formation and the continuing de-stabilisation of the upper atmosphere were the perfect ingredients for the more active storms to follow. 

The temperature peaked at 24.7 Celsius at 1530 hours in south London. This was rather earlier than on a normal day of heating and can be traced back to the weak southwesterly wind  that  had developed. This cooler air led to a cutting off of convection over southern counties as seen in the satellite images below.

 
© Copyright EUMETSAT/Met Office
 
© Copyright EUMETSAT/Met Office

The satellite infra-red images above  are for 1600 and 1900 hours respectively.  The whitening (cooling) of the cloud tops between 1600 and 1900 hours is very noticeable across  north  London and East Anglia. Whereas slight drying and subsidence of the air to the rear of the frontal system on the near continent, combined with a cooling sea breeze from the south coast, greatly limited convection south of the Capital. By way of contrast, after a long land track the southwest wind was relatively warm by the time it reached London, and this,  combined with added moisture from the easterly sea breeze through the Thames estuary,  produced a potent brewing area for vigorous thunderstorm development over Essex. Norfolk and Suffolk also had a battering from the storms, but some of the worst flooding occurred in Essex .

The photograph below, taken in south London at 1930 hours shows a mighty cumulonimbus, over and to the northeast of, London.



The photograph below was taken by Steve Mahon in Chingford (Essex) at 1940 hours.



More images of the flooding in Chingford.
   
 Flooded river Ching Storm aftermath

Photographs kindly supplied by Steve Mahon(http://www.wix.com/stevemahon/home).

Despite the intensity of the storms, in south London they rumbled on from 1720 to 2050, there doesn't appear to be any high rainfall totals where regularly read rain-gauges are positioned. Coleshill, near Birmingham and Cranwell (Lincolnshire) had over 20 millimetres, Wattisham (Suffolk) had 8 millimetres, but in the London area amounts at accessible recording centres were small. Nearly 5 millimetres was recorded in Morden (Surrey), including 3 millimetres in 10 minutes around 1900 hours.  Otherwise most London sites had 1 millimetre or less. Hopefully, more interesting totals will come to light at a later date.  


 

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