Are we going to have another 2021 summer flooding event? 

In late May 2026, Kew Gardens recorded 35.1°C, the hottest May day in UK history, breaking a record that had stood since 1922. Across England and Wales, amber heat-health alerts were issued, rail services disrupted, and Britons persevered through nights where temperatures barely dipped below 20°C. 

Then the rain came. Heavy, persistent, and arriving on ground too hard to absorb it quickly. If that sequence feels familiar, it should as it is almost exactly the pattern that preceded one of the most damaging flooding episodes London has ever seen: summer 2021. 

The question being asked by forecasters, local authorities and flood risk professionals right now is not abstract. Are we heading for a repeat? 

The rain that has followed May's record heat has yet to reach the intensity needed to trigger significant surface water flooding, but with further heatwaves forecast for the coming months, the conditions that produced 2021's disasters could yet materialise 

What happened in July 2021?

London was struck by two serious flash floods just two weeks apart. On 12 July, nearly a month's worth of rain fell across parts of the capital in hours. Kew recorded 47.8mm in a single hour, against a monthly average of 44.5mm. Hampstead saw its wettest day on record. Then on 25 July, another wave of storms hit, with some gauges recording over 54mm in just two hours and fifteen minutes. 

The impact was immediate. Thirty London Underground stations closed or partially shut. Hospital wards were evacuated, schools damaged with some not reopening for weeks. Basement flats in Kensington, Camden, Westminster, Hammersmith and Wandsworth were inundated with stormwater and sewage. Insurance claims from LV and Allianz alone exceeded £22 million. 

Previsico's modelling highlighted something important: much of the worst flooding occurred outside areas that appear as high flood risk on standard maps. The culprit was surface water - rain with nowhere to go, running off hard urban surfaces faster than ageing Victorian drainage could handle. In St James's Park, daily rainfall of 41.8mm matched the entire average July total. Previsico's instcast of the event confirmed flooding in areas where it had been reported on 12 July, indicating our surface water flood modelling would have captured this event across most affected areas, predominantly northern, western and south-western Greater London. 

Why heatwaves and flash floods go hand in hand 

During a prolonged heatwave, the ground dries and hardens. Urban surfaces e.g. concrete, tarmac, compacted soil lose much of their capacity to absorb water quickly. Meanwhile, heat builds energy in the lower atmosphere. When cooler air moves in, warm moisture-laden surface air rises rapidly, generating intense convective thunderstorms: short-lived but capable of dropping a month's rain in an hour over a localised area. 

Met Office meteorologists attributed the July 2021 storms to precisely this "convergence of air currents." The same dynamic is already flagged in 2026 forecasts that following the May heatwave, yellow thunderstorm warnings were issued, with forecasters noting that rain falling on hardened ground would increase the risk of rapid runoff and localised flooding. 

Scientists call the broader pattern climate whiplash: rapid swings from one extreme to another. Research published earlier this year confirmed such swings have become significantly more frequent in recent decades, driven in the UK largely by an increasingly erratic jet stream. The conditions of summer 2026 are not identical to 2021, but they are structurally similar. 

What has changed since 2021? 

London's boroughs and agencies have not stood still. In Hammersmith & Fulham, street-level rain gardens have been installed to intercept surface water runoff before it overwhelms drains. Camden has delivered major SuDS retrofits and removed over 1,850m² of highway paving for rain gardens and pocket parks. Westminster has allocated significant funding to gully repair and now cleans around 14,000 drains annually. 

At the strategic level, the London Surface Water Strategy (2025–30), a multi-agency blueprint involving the GLA, boroughs, Thames Water and London Councils sets a framework for scaling nature-based drainage solutions across the city. Thames Water has surveyed properties and installed flood protection at the highest-risk homes in Kensington and Chelsea. 

But is it enough? 

The honest answer is that we don't yet know. 

Improvements are real but uneven. Many critical drainage hotspots across the capital and the whole of the UK have yet to see physical intervention, and London's Victorian combined sewer network which was built for a city of three million in a different climate still underpins drainage across vast swathes of the city. 

Rainfall intensity is also increasing. The storms following the May 2026 heatwave will arrive on unusually dry, hardened ground. The London Surface Water Strategy itself acknowledges that existing infrastructure was not designed for the conditions climate change is delivering. A 2021-scale event today would still cause very significant disruption. Lead times have improved and coordination between agencies is better, but the underlying vulnerability of a dense, heavily paved city has not been eliminated but only reduced at the margins. 

What you can do now 

Know your risk: Check whether your property or asset is at risk via the Environment Agency's website. 

Sign up for flood alerts: The Environment Agency's free service provides advance warning as conditions deteriorate. Previsico's platform delivers hyper-accurate surface water forecasts up to 48 hours ahead, particularly valuable for businesses and asset managers. 

Prepare your property: Flood boards, airbrick covers and non-return valves can make a significant difference for ground floor and basement properties. Know where your stopcock is and keep documents and valuables above floor level. 

Have a flood plan. Know your evacuation route, keep an emergency contact list, and ensure your team knows the flood response procedure. 

Report blocked drains: Blocked gullies significantly worsen surface water flooding. Report any you spot to your local authority.