How rare, how risky? Actionable warnings for extreme precipitation despitebiased forecasts

The underestimation of extreme precipitation in forecasts can lead to severe socioeconomic damage and loss of life. Especially in densely populated urban areas with low permeability, intense precipitation can cause local inundation, trigger pluvial flooding, and threaten lives. These events are becoming more frequent and intense due to climate change, posing growing challenges to society. While weather centres routinely forecast precipitation, these forecasts often carry biases and tend to severely underestimate extremes. As a result, weather predictions fail to contextualise precipitation volumes, often leading to an underestimation of impact. Hydrological centres, meanwhile, focus on river discharge and fluvial flooding, but
offer little coverage for urban areas away from major rivers — thus leaving a critical gap in current early warning systems.

To bridge this gap, we propose a novel, impact-based approach to weather forecasting that enables early warnings of pluvial flooding. The project focuses on two core objectives:

(i) to quantify the rarity of extreme precipitation events by predicting return periods to support impact-focused interpretation, and
(ii) to use return period forecasts to develop a risk-based early warning index with three actionable levels — prepare, watch, and act — to guide timely
responses to pluvial flood threats.

Our innovation taps into an overlooked opportunity in meteorology: using readily available forecast data to produce clear, contextual insights that
enhance weather forecast interpretation, without the need for new highresolution models or simulations. By focusing on rarity and risk, the project
bridges the gap between hazard prediction and impact communication, empowering communities to take proactive measures against pluvial flooding and contribute to climate change adaptation.