Material from: Linus
1. Overview
The May 2026 heatwave over Western Europe was an exceptionally early and intense warm event that affected large parts of the continent from around 22–28 May. It was notable for the widespread breaking of monthly and all-time seasonal temperature records across countries including the United Kingdom, France, Spain, Portugal, Ireland, and Germany.
Many regions experienced temperatures 10–15 °C above climatological averages for late May, with widespread daytime maxima exceeding 30 °C and reaching 35 °C in some places. In the UK, temperatures reached 34.8 °C and then 35.1°C at Kew Gardens, establishing new national May records by a more than 2 degree margin. France recorded its hottest May day on record, with values up to around 36–38 °C locally, while Portugal observed even higher extremes, peaking at 40.3 °C. Synoptically, the event was driven by a persistent strong upper-level ridge extending from North Africa into Western and Central Europe.
Beyond meteorological extremes, the heatwave had significant societal impacts. Health alerts were issued across multiple countries, with reported heat-related fatalities in France and increased risks associated with outdoor activities and poor nighttime cooling, including “tropical nights” in parts of the UK. Infrastructure and public services were also affected. Several fatalities were reported, mainly from people drowning.
https://en.wikipedia.org/wiki/2026_European_heatwaves
2. Analyses and Observations
2.1 Event Definition
In this report we will focus on the 2-metre temperature for a 0.5x0.5 degree box over Western London (centred on London-Heathrow, 51.5N, 0.5W), both for 3-day average temperature on 24-26 May and on the instantaneous temperature on 25 May 12UTC.
2.2 Analyses
Link to Copernicus Weather Replay: https://weather-replay.climate.copernicus.eu/?dt=494328&z=4.2&a=46.0&o=7.0&A=51.6&O=-0.2&l=100100000110000
The plots below show z500 and T850 from 19 to 25 May 00UTC, every 24 hour.
2.3 Observations
2.4 Climatological perspective
3. Forecasts
3.1 Data assimilation and observation usage
3.2 Single Forecasts for the event based on 00UTC forecasts (defined in Section 2.1)
The plots in this section are for forecasts valid 25 May 12UTC.
Observations and analysis for the event
Control forecast (IFS 9-km resolution)
DestinE (IFS 4.4km resolution)
AIFS-single (AIFSv2.0 ~0.25 resolution)
AIFS-ENS control (~0.25 degree resolution)
3.3 Ensemble distribution
EFI (based on IFS 9-km ensemble)
The plots below show the 3-day EFI valid 24-26 May.
Forecast Evolution plot
The plot below shows the evolution of the forecasts for 2-metre temperature on 25 May 12UTC around London-Heathrow.
The plot below shows the evolution of the forecasts for 2-metre temperature on 24-26 May around London-Heathrow.
3.4 Sub-seasonal forecasts
The plots below show the weekly anomaly for 2-metre temperature from the ensemble mean valid 25-31 May from different initial dates (all Mondays).
The plots below show the weekly anomaly for z500 from the ensemble mean valid 25-31 May from different initial dates (all Mondays).
3.5 Comparison with other centres
4. Hazard Forecasts
5. Dedicated Experiments
6. Event Summary
6.1 Good and bad aspects of the forecasts for the event
- 3-day event over London exceeded 4 standard deviations (99.997 percentile)
- Now 7 out of 12 monthly records for UK has been broken in the past 15 years
- The heatwave had effects on air pollution such as ozone
- No predictability in the sub-seasonal range
- Signal started to appear ~12 days before the event and gradually increased
- Signal in AIFS ensemble ahead of IFS 5-8 days before the event
- Underestimation of the maximum temperature in IFS