The period 9th - 11th September was a very wet and disturbed period, characterized by numerous reports of flash flooding across Southern counties of England and Wales. This was courtesy of several organised bands of slow-moving torrential rain that occurred during the period.

Chart: Synoptic situation across North Atlantic and Europe at 00UTC 20050910.

http://www.wetterzentraleforum.de/archive/2005/brack/bracka20050910.gif

The synoptic situation on the 9th was meridional with an upper low located across Biscay and a shallow surface low of 1002mbs located SW of Brest. There was a small anticyclone centre 1022mbs located just NE of Shetland. The surface low extended across the Benelux with a second centre over Germany. A warm front was located across Northern England bringing a persistent band of moderate rainfall throughout the period, this front was expected to weaken and move very little during the period. The slow moving band of rain associated with the warm front caused some flooding problems at Ferrybridge in West Yorkshire leaving several homes flooded, however the intensity of rainfall was nowhere near as great as it was later across parts of the South.

Behind the warm front a weak easterly flow was advecting warm, moist air from the continent. This warm, moist air across the South was destabilised during the afternoon as lift ahead of an upper trough provided an effective forcing mechanism. This allowed a line of heavy showers and thunderstorms to break out in a line extending from SW London across to the North Norfolk coast. No exceptionally high rainfall totals were recorded, however rates in some of the storms approached 100mm/hr causing flash flooding because of the sheer volume of water deposited within minutes. The storms were also highly electrified producing some impressive lightning displays into the evening and overnight.

The areas affected by flooding included Putney, Barnes, Wimbledon and Chiswick. Worst affected Chiswick, Wandsworth and Roehampton saw as much as 30mm during the storms, which started just before 4pm. Many shops and homes in low-lying areas were flooded. The Underground 'District' line was shut in Richmond and temporary disruption was caused to trains in the area, including services from Clapham Junction to Victoria and Kensington. Several major roads such as the A4 and A3 were temporarily flooded by several feet of water for a time during the late afternoon, causing further disruption. There was also a much appreciated interruption to play at the Oval for the 5th Ashes Test.

A low-level convergence zone developed along the M4 corridor extending from South Wales, through Wiltshire and Berkshire and into North London during the afternoon. This developed in the warm E’ly flow ahead of a cold front lying just to the south, behind which a cooler S’ly flow followed.

Some potent cells developed along the zone moving westwards, these became highly electrified for a time and brought torrential falls of rain to many areas and unsurprisingly caused some localised flooding problems. There was some severe flooding on the A34 near Chiveley in West Berkshire turning the dual carriageway into a river for a time. Highest totals for the day (including highest hourly totals) are included below in Table 1. The active trough even spawned a funnel cloud that was observed near Filton, Bristol during the early afternoon period- indicative of turbulent atmospheric conditions during the day.

Table 1: Highest total and hourly rainfall obs from the 10th September 2005

During the evening, with surface heating cut off, most of the storms dissipated fairly readily. However, a 'shower train' that developed through the Thames Estuary - a narrow band of thunderstorms providing a persistent stream of torrential rain in a relatively small area continued to maintain itself through Northwest Kent and central London and Bexley managed to record 31.6mm in just 1 hour!