Derecho-like event over France: Observation and Evaluation of the Forecast using an Automatic Verification Tool

Thunderstorms are known to have an significant impact on air traffic, but it highly depends on their organization. Clusters of thunderstorms for example, can enhance this impact, but well organised widespread convection can hardly reduce the air traffic, due to its high vertical extension, long life time and severity. Forecasting the type of storm is thus a real challenge in order to alert the Network Managers within the Air Traffic Control centres.

Mesoscale Convective Systems (MCS) such as derecho events are part of this organised convection. During the 25th of June 2025, a derecho-like event has triggered from the southwestern part of France and spread to the German borders, generating violent wind gusts between 110 and 135 km/h, damages and a fatality. The synoptic meteorological situation presented a highly wind sheared and unstable environment and a large scale lift ahead of a cold front, which tends to favor supercells or bow echoes organizations.

In order to assess the human-made operational forecast, we have developped an automatic tool based on differents observations data (radars and lightning). It provides a bulletin that can be directly compared to the human forecast, one can thus evaluate objectively its quality. Over the last 2024 convective season, the algorithm showed generally good results, but it needs to implement other observation data to improve convection detection (radars from other countries to cover mountains regions or satellite data to cover sea areas). Other types of skill scores can also be implemented.