EGUsphere EGUsphere EGUsphere EGUsphere Copernicus Publications Göttingen, Germany 10.5194/egusphere-2026-6476 When anticipation fails: A forensic analysis of Tropical Cyclone Ditwah reveals whole-system performance limits in a small-island flood-risk system Perera Chamal https://orcid.org/0009-0000-0185-5017 1 2 Sivapalan Murugesu https://orcid.org/0000-0003-3004-3530 3 4 Peiris Nadee 1 2 Jayawardane Paboda https://orcid.org/0009-0004-9984-391X 1 2 Wickramasinghe Romitha https://orcid.org/0000-0001-8743-3148 5 Wijayaweera Nalintha https://orcid.org/0009-0001-0681-0608 1 2 Dissanayake Chatura 1 2 Rajapakse Lalith https://orcid.org/0000-0002-7967-247X 1 2 Gunawardhana Luminda 1 2 De Silva Kasun 1 2 Wijayaratna Nimal 1 2 Illangasingha Sanjeewa https://orcid.org/0009-0008-7295-9284 6 Pattiaratchi Charitha https://orcid.org/0000-0003-2229-6183 7 8 Nakamura Shinichiro https://orcid.org/0000-0001-6809-5008 5 Blöschl Günter https://orcid.org/0000-0003-2227-8225 9 10 UNESCO Madanjeet Singh Centre for South Asia Water Management, University of Moratuwa, Moratuwa 10400, Sri Lanka Department of Civil Engineering, University of Moratuwa, Moratuwa 10400, Sri Lanka Department of Civil and Environmental Engineering, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA Department of Geography and Geographic Information Science, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA Department of Civil and Environmental Engineering, Nagoya University, Nagoya 464-8603, Japan Mahaweli Authority of Sri Lanka, Colombo 10, Sri Lanka UWA Oceans Institute, The University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia School of Engineering, The University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia Institute of Hydraulic Engineering and Water Resources Management, Technical University Vienna, Vienna, Austria Department of Civil, Chemical, Environmental and Materials Engineering, University of Bologna, Bologna, Italy 30 10 2026 2026 1 51 Copyright: © 2026 Chamal Perera et al. 2026 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-6476/ The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-6476/egusphere-2026-6476.pdf

Tropical Cyclone (TC) Ditwah, which impacted Sri Lanka in November 2025, was one of the most damaging hydrometeorological disasters in the country's recent history, affecting more than two million people through widespread flooding and landslides. Early explanations attributed the disaster to failures in forecasting, reservoir operations, or institutional response. We argue instead that TC Ditwah revealed a breakdown in whole-system performance under extreme temporal stress. Using a forensic reconstruction spanning atmospheric evolution, hydrological response, reservoir operations, warning dissemination, institutional coordination, and community experience, we examine how multiple interconnected subsystems interacted during the event. Rapid cyclogenesis unusually close to Sri Lanka compressed forecast lead times, while the island's steep, rainfall-dominated catchments translated extreme rainfall into rapid flood responses across multiple basins. Reservoir preparedness, warning escalation, institutional decision processes, and community response operated on slower timescales, reducing the time available to convert anticipation into protective action. The resulting mismatch between fast physical processes and slower institutional and societal response generated an anticipation–action gap, despite the availability of hazard information. Rather than revealing isolated component failures, TC Ditwah exposed structural limitations in the design and operation of coupled flood-risk systems. We argue that improving forecasts alone is unlikely to substantially reduce future disaster risk unless forecasting, reservoir management, emergency governance, and community response are explicitly integrated into coordinated operational frameworks. More broadly, TC Ditwah illustrates the need to complement advances in prediction with simulation-based training, pre-defined decision protocols, and whole-system emergency operating modes capable of functioning under rapidly evolving compound hazards. The lessons extend beyond Sri Lanka to flood-risk management in small-island states and other catchments where rapidly evolving hazards leave little time for coordinated action.