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Record July Rainfall Causes Widespread Flash Flooding Across London

Reviewing July’s month of floods

July 2021 saw record rains with large parts of the UK including London experienced extreme flash flooding. The 12th July was the wettest day on record in Hampstead with around 42mm of rainfall in just one day. The floods had widespread impacts, ranging from the closure of several stations to significant residential and commercial buildings. Severe damage to people’s livelihoods has been felt when the flood water caused damage to businesses, homes and public infrastructure. For insurers, the cost was significant with LV and Allianz alone seeing £22m in claims from the July floods and storms[1].

However, although these incidents have been branded as ‘freak’ flood events by major news outlets[2], surface water floods like these are more common and expected to increase fivefold by 2080[3]. As flooding increases in intensity and frequency, being prepared is more important now than ever. It is estimated that over 3 million households are at risk of surface water flooding[4], but the losses (like those felt in the London flash floods) are avoidable with appropraite mitigation strategies. This includes having a reliable early warning system that notifies you when you are likely to be impacted by flooding several hours, or days in advance so you can prepare accordingly.

Minimising flood impacts

At Previsico, our mission is to minimise the impact and cost of flooding globally. We have developed our own live hydrodynamic flood modelling solution over the last two decades. The techonology takes advantage of the best available rainfall nowcasts and forecasts to generate event specific predictions at a 25m resolution. Using this approach, floods that have previously caught people by surprise can be predicted with enough time to take mitigating action. This includes July’s floods.

During the floods, we gathered ~250 observations of floods, satellite images as well as data from our own sensors to assess what types of flooding were most prominent in these areas and to carry out an accuracy assessment of our forecasts. We continuouly assess the accuracy of our forecast assessments by comparing it to posts on social media and by news outlets on some of the worst affected areas.

Real-time surface water forecasting

These highlight how we successfully predicted the vast majority of locations affected by surface water flash floods across London on the 12th July, with a large degree of accuracy. The flooding seen in these instances was surface water flooding as a consequence of the heavy rainfall. Being able to have an early warning for surface water flooding is something that will be more and more important in coming years with a rapidly changing climate and urbanisation of land. Businesses, government and insurers all use Previsico’s flood forecasting solutions to mitigate losses with significant savings evidenced in our Lloyd’s Lab Report.

Want to learn more?

To hear more about how our technology is being used by businesses, government and insurance and to view a demo, please email Avi at or Request Demo.