From forecast to foresight

WeatherMesh

Western EuropeHeat wave · May 22–31, 2026

See the heat build across Europe

7.25days

Sustained agreement with the regional peak temperature

WeatherMesh settled within 1°C of the May 26 regional temperature peak more than a week ahead, and stayed there in every later forecast—12 hours before AIFS.

Across the UK study region, Germany, France and Spain, WeatherMesh had lower temperature error than IFS at six reported leads from one to fifteen days.

Key takeaways
  • A sustained signal for the regional peak from the May 19, 12 UTC forecast.
  • Lower error than IFS across the reported 1–15 day leads; AIFS led at six days.

Forecast differences, 7 days ahead

2 m air temperature · Valid May 26, 2026 · 12 UTC

WeatherMesh − ERA5IFS − ERA5

Drag to compare · Use arrow keys when focused

Forecast − ERA5 (°C) · −6 to +6

Blue: cooler than analysis · Red: warmer than analysis

Forecast issued May 19, 2026 · 12 UTC · 168-hour lead

Source difference maps are smoothed (σ 56 km). Colors saturate beyond ±6°C.

Study details & evidence

About this study

The heat wave's peak is taken as the combined study region's temperature at 18 UTC on May 26 (26.6°C in ERA5). From the May 19, 12 UTC run onward, about a week before the peak, every WeatherMesh forecast stayed within 1°C of it. AIFS reached that accuracy 12 hours later. IFS and GFS did not reach it in any of the runs tested.

Temperature errors cover forecasts valid at 12 UTC on May 22–31. Only initialization and valid-time pairs available for every model are used. Land points are sampled on a common 0.5° grid and weighted by latitude. The UK region also includes Ireland and a small part of northern France.

WeatherMesh, IFS and AIFS are ensemble means; GFS is a single deterministic run. The accompanying map is a separate seven-day forecast, issued May 19 at 12 UTC and valid May 26 at 12 UTC, six hours before the headline peak.

This is a single heat wave, with 7–10 matched valid times per lead time. AIFS was the stronger model at many medium-range lead times in the UK and Germany. ERA5 is a reanalysis used as the reference, not direct station observations. The midday and early-evening samples are not daily maximum temperatures.

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Temperature error (left) and forecast bias (right) across the four study regions. Negative bias means cooler than ERA5. Original study colors are retained; additional reference curves use different sample coverage.
Heat wave · Study evidence
Original study chart showing temperature RMSE and forecast bias versus lead time across the UK, Germany, France and Spain, verified against ERA5 for May 22–31, 2026.

Temperature error (left) and forecast bias (right) across the four study regions. Negative bias means cooler than ERA5. Original study colors are retained; additional reference curves use different sample coverage.

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