Flash flooding in High Wycombe

Weather conditions before the floods

During the afternoon of 3 August 2004 a series of intense thunderstorms hit parts of London and its surroundings. This was part of a large system of storms known as a 'Mesoscale Convective System', or MCS.

The animation on the right shows the rainfall rate from this system is shown here from our operational radar network - this is a composite of 5 km, 2 km and 1 km data (the radar can 'see' at higher resolution near by).

The darker red areas on the animation are those with very heavy rainfall, between 16 and 32 mm per hour, while the pink areas show the heaviest, more than 32 mm per hour.

Heavy rainfall in London

Some of the storms were very intense and caused flooding in parts of the London area. More than 80 mm of rain was measured in parts of north-west London. This was enough to lead to flash flooding and major disruption to commuter traffic.

At High Wycombe 42.4 mm was collected by a rain gauge in 38 minutes, with peak rainfall rates in excess of 200 mm/hour. The value of 42.4 mm in an hour gives a return period of ~64 years using the Flood Estimation Handbook (FEH) method.

Given that High Wycombe was not in the area of highest accumulations observed by radar, it is likely that more extreme point values would have been obtained in parts of west London if gauge measurements had been obtained.

Our computer model simulations

A version of the Met Office's Unified Model has been run at a resolution of 1 km to see if it would have been possible to forecast this event.

This animation shows a representation of cloud-top temperature (white shows high cloud) and rainfall rate in mm/h (only rates greater than 1 mm/h are shown: blue=1-2, green=2-4, yellow=4-8, orange=8-16, red=16-32, purple=>32). The forecast was run from data at 0600 UTC. It does predict a number of very intense thunderstorms in the London area at around the right time, though the location is not perfect.

Computer simulation of total rainfall

The total rainfall accumulations over the period 12 UTC to 18 UTC on 3 August 2004 from the 1 km model run from 0600 UTC. Xs mark locations of peak accumulations from radar. The locations of these peaks is forecast to within 5-15 km, and a good indication is given of amounts (certainly within the accuracy of the radar).