Document Type

Article

Publication Date

7-2026

Abstract

The interaction of tropical cyclones (TCs) with the southwest monsoon frequently causes heavy precipitation events (HPEs) and widespread flooding across the Philippines. Despite their substantial socio-economic impacts, the synoptic conditions that favor these events remain insufficiently understood, underscoring the need for systematic investigation to help improve forecasting and risk assessment. Lag composite analysis of daily rainfall records and reanalysis data from July to September for the period 1981–2023 shows that HPEs are more likely to occur when: (1) enhanced monsoon westerlies are present over the tropical Indian Ocean for at least nine days before the HPE, (2) a TC passes northeast of the Philippines, (3) the presence of a moisture conveyor belt (MCB), (4) BSISO1 is active, (5) BSISO2 is active and their associated convective anomalies are near the Philippines, (6) a TC or remnant low resides near the Vietnam-China border, and (7) the El Niño-Southern Oscillation phase is either neutral or El Niño. The accuracy of GFS forecasts for such events was also evaluated. The MCB, an integral component in the occurrence of these HPEs, was well captured spatially by the GFS forecasts; however, its magnitude was slightly underestimated. This study provides critical insights into the relevant large-scale processes that need to be represented well in models, and key environmental conditions that could be monitored in order to improve the prediction of HPEs.

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United Nations Sustainable Development Goals