Development of Forecast and Alert Logic for Extended-Range Heatwave Early Warning

Heatwaves are among the most impactful climate-related hazards in Europe, with increasing frequency, duration, and severity under climate change. Currently, heatwave warnings across Europe are typically issued 2–7 days in advance. Extending these lead times could substantially enhance preparedness by enabling earlier adaptive actions and more effective resource allocation.

On sub-seasonal time scales, extended-range weather forecasts (approximately 2 weeks to 1 month) have been shown to exhibit higher skill for warm extremes than for average temperature conditions over Europe. In particular, the persistence of prolonged heat waves seems to have a higher-than-average level of predictability even at a 3-week lead time. Recent verification studies have demonstrated statistically significant probabilistic skill of ECMWF extended-range ensemble reforecasts for European heatwaves defined using 5-day mean temperatures. Together, these findings indicate that extended-range ensemble forecasts can provide early warning information not only on the likelihood of heatwave occurrence, but also on the potential persistence and severity of extreme heat events.

Building on this demonstrated predictability, we take the next step towards practical early warning applications by developing a forecast and alert logic for extended-range heatwave prediction. We post-process ECMWF extended-range ensemble forecasts to produce probabilistic heatwave forecasts using a previously developed and verified methodology based on 5-day mean temperature thresholds. Building on this framework, we develop a rule-based alert logic that translates probabilistic forecasts into actionable early warning information. The alert logic combines forecasted heatwave probabilities with indicators of forecast reliability and flow-dependent predictability, including ensemble spread, the heatwave life cycle state at forecast initialization, and North Atlantic sea surface temperature anomalies at the time of forecast initialization. These components are used to define multiple alert levels, ranging from no-action conditions to high-impact heatwave risk.

The proposed framework provides a practical pathway from extended-range probabilistic forecasts to impact-oriented early warning.