Steven Stills

Analysis · Climate & water

Heatwaves and Hydrophobia

With four heatwaves before August, is the threat of rain going to turn a drought into a flooding emergency?

If you ever wondered why forecasting departments always have progressive views on climate change, it's because they've been on the front line of data like this for years. We rebuild the seasonal normals every year, and every year they've moved. See the chart below:1 the numbers increase the shorter the history we observe, and the last 12 months beat the 30-year normal in every single month. July was 4.8 °C over.

30-yr normal 10-yr 5-yr Last 12 months 0 5 10 15 20 25 °C JFM AMJ JAS OND
Monthly mean temperature against three normals, London.1

By late July this year the UK was in its fourth heatwave. It's the first year on record to hit 35 °C in three separate months, with a provisional 37.3 °C hottest June day and amber heat-health alerts across most of England.2 So what do we have next? Heat dries the ground out, and dry ground sheds rain instead of absorbing it. The Environment Agency's new national assessment puts 6.3 million properties in England at flood risk. The biggest element isn't rivers or the coast, it's surface-water flooding from summer downpours, up 43% on the previous mapping.3

Heat-health alert level by region, 3 to 5 August.9

Surface-water risk isn't spread evenly. It concentrates in cities, where the ground is sealed and the drains can't take intense rain. In London, 12.6% of all properties sit in areas at high or medium surface-water risk, twice the national rate of 6.3%.10 Only London and the East of England beat that rate, and both are on amber heat this week. Below them the two maps stop matching. The North West and the North East both carry more surface-water risk than the South East, which is on amber while they aren't. Heat and water rank the country differently.

Properties at high or medium surface-water flood risk, by region.10

Relief isn't coming where it's needed. Over the fourteen days from 2 August the average daily chance of rain is 46% in the North West and 20% in London and the South East, against an England mean of 30%.11 The two regions carrying the most surface-water risk are the two driest in the forecast, and more than half of England is already in drought.12 The rain is landing on the wrong half of the country. For now, I don't know enough about water infrastructure to know if that's okay yet.

Mean daily chance of rain by region, 2 to 15 August.11

And then, what can we do to fix it, in a country currently curtailing renewable generation because the grid isn't fit to use it? Britain switched off a record ~10 TWh of wind in 2025, enough to power every home in London, and paid close to £1.5 billion for it once the replacement gas is counted.4

I'm keeping my eyes on some initiatives that could prevent the worst of this through system reform, to see if any have impact. The Eastern Green Links are two subsea HVDC cables between Scotland and England, both due around 2029 under the accelerated transmission programme.5 Ofgem reordered the grid queue in April 2025 so projects that are ready and needed connect first, in line with Clean Power 2030.6 The REMA decision last July dropped zonal pricing for reformed national pricing, so we get locational signals without a seven-year market rebuild.7 And NESO's Local Constraint Market now pays Scottish consumers to turn demand up when the grid is constrained, instead of paying wind farms to turn off.8

I'll chime in where I can help.

Notes & sources

  1. Daily 2 m mean temperature for the London grid point (51.49, −0.16), ERA5 reanalysis via the Open-Meteo historical weather API, aggregated to monthly means. Windows: 1991–2020, 2016–2025, 2021–2025, and Aug 2025–Jul 2026. ↩
  2. Fourth heatwave of 2026 from late July; first year on record with 35 °C in three separate months; provisional 37.3 °C at Santon Downham on 26 June; amber UKHSA alerts. Wikipedia, 2026 United Kingdom heatwaves; ITV News, 26 July 2026. ↩
  3. The Environment Agency's updated National Flood Risk Assessment (NaFRA2): 6.3 million properties in England at flood risk; surface-water properties at risk up 43% under the new mapping. Environment Agency, on NaFRA2; Environment Agency, July 2026. ↩
  4. A record ~10 TWh of wind curtailed in 2025 (Montel analysis), with combined curtailment and replacement-generation costs around £1.46 billion; NESO projects balancing costs of £4–8 billion a year by 2030 if transmission is late. The Telegraph via Yahoo, on 2025 curtailment; NESO, 2025 Annual Balancing Costs Report. ↩
  5. Eastern Green Link 1 (Torness–Hawthorn Pit) and 2 (Peterhead–Drax) both due for delivery around 2029. RenewableUK, an overview of constraint payments. ↩
  6. Ofgem approved the TMO4+ connections reform package on 15 April 2025; Gate 2 offers give "ready and needed" projects confirmed dates and queue positions. Freeths, on the connections reform package. ↩
  7. The REMA Summer Update, 10 July 2025: zonal pricing ruled out, reformed national pricing adopted, partly on a seven-year implementation estimate. Norton Rose Fulbright, REMA Summer Update. ↩
  8. NESO's Local Constraint Market pays Scottish consumers (around 20p/kWh in the trial) to increase consumption during constraint events. Modo Energy, on GB constraints. ↩
  9. Amber alerts for the East Midlands, East of England, South East and London, and yellow alerts for the North West, Yorkshire and The Humber, West Midlands and South West, from 9am on 3 August to 9am on 5 August 2026. No alert for the North East. UKHSA issues these alerts for the nine English regions only, so Scotland, Wales and Northern Ireland are drawn as not assessed. UKHSA, updates heat health alerts across England. ↩
  10. Environment Agency, National assessment of flood and coastal erosion risk in England 2024, Table 13: properties in areas at high or medium risk of surface water flooding, as a percentage of all properties in each region. London 12.6%, East of England 7.6%, North West 5.8%, North East 5.1%, East Midlands 5.0%, West Midlands 4.9%, South East 4.7%, South West 4.6%, Yorkshire and The Humber 4.3%, England 6.3%. GOV.UK, the 2024 national assessment. ↩
  11. Daily maximum precipitation probability, 2 to 15 August 2026, from the Open-Meteo forecast API, run 2 August 2026. Three representative points per region, averaged over the points and the fourteen days. North West 46%, North East 40%, Yorkshire and The Humber 34%, West Midlands 28%, South West 28%, East Midlands 27%, East of England 26%, London 20%, South East 20%; England mean 30%. Forecast totals over the same window run from 38.8 mm in the North West to 3.3 mm in the South East. ↩
  12. More than half of England formally in drought as of late July 2026. BBC News, "More than half of England in drought, Environment Agency declares", 29 July 2026. ↩

Map boundaries are the ONS International Territorial Level 1 regions, January 2025, ultra generalised. Contains OS data © Crown copyright and database right 2025; contains National Statistics data © Crown copyright and database right 2025. ONS Open Geography Portal.

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