How rare, how risky? A return period-based early warning index for extreme precipitation

Extreme precipitation events are expected to become more frequent and more intense under climate change. Already today, the underestimation of extreme precipitation in weather forecasts can lead to an underestimation of impact and inadequate early warnings. Extreme precipitation can cause inundation, especially over less permeable urban areas, and culminate in urban flooding with potential socio-economic damage and loss of life. In 2024, the Copernicus Emergency Management Service (CEMS) monitored 30 flood events that inundated more than 460.000 hectares and affected more than 1 million people. While flood early warning systems, such as the European Flood Awareness System (EFAS), contain flash flood and extreme precipitation products, their primary focus remains on river flooding. Consequently, operational warning systems do not yet fully account for the increasing risk of impermeable urban areas to localized extreme precipitation.

 

Here, we present a novel and actionable early warning index that predicts the rarity and risk of extreme precipitation up to 5 days in advance and targets localised urban and pluvial flooding, thereby addressing a gap in existing warning systems. The index focuses on the prediction of the rarity of an event to circumvent the use of biased precipitation forecasts: how often did the predicted event intensity occur in the past? We predict the likelihood of extreme precipitation to exceed critical return period thresholds by exploiting the information embedded in existing numerical ensemble forecasts and hindcasts. The presented warning index then estimates the associated risk through the mapping of likelihood and potential impacts, incorporating a fuzzy neighbourhood approach that accounts for displacement errors in the prediction of extreme events as a function of lead time. Through this risk approach, the warning index aims to capture extreme but less probable events to improve the index’ reliability. An evaluation over 30 activations from CEMS Rapid Mapping in 2024 demonstrates that the derived risk index for extreme precipitation provides reliable and actionable information that could be used to supplement the existing warning systems. A corresponding operational system is being tested for all medium-to-large cities in Spain, Italy, France, Germany, and Mozambique, using an email notification as part of the Copernicus Evolution service implemented by the HORIZON project CENTAUR.