Subseasonal and seasonal predictions of summer days and hot days online.

Following German user needs DWD focussed on indicators defined by absolute thresholds, i.e. summer and hot days (maximum daily temperature equal to or larger than 25/ 30°C), for subseasonal and seasonal predictions of the operational DWD prediction systems and observations (for evaluation). Subseasonal predictions of hot days are not possible due to small sample sizes of weekly extremes in the past. Furthermore, a threshold has been applied to define for which time in year and region in Germany the sample sizes of heat indicators are too small for statistical skill analyses.

The calculation of heat indicators in Germany has been integrated into the pre-operational workflow. The thresholds have been tested intensively within the automatic workflow, and analyses of climate prediction skills have been performed. After finishing all essential tests, subseasonal predictions of summer days as well as seasonal predictions of summer and hot days have been operationalized in May 2025 and published on the DWD climate predictions website (www.dwd.de/climatepredictions).

The heat forecasts have been used intensively for media requests on the upcoming summer 2025. Furthermore, they have been presented to 200 participants at the DWD user workshop on climate predictions and projections in June 2025. Approximately 30 users have joined the side event on heat predictions to test the use of the indicators and give feedback. They found the indicators very helpful.

Related Activity

• Responding to the Impact of Climate Change [U-CLIMADAPT]