This publication, developed under APEC Project PPSTI 02 2023, presents a comprehensive framework aimed at enhancing sustainable and resilient water management among APEC developing economies through the utilization of open environmental data. Derived from an international workshop held in Bangkok, the publication outlines methods for predicting seasonal rainfall, emphasizing region-specific adaptations and localized feature engineering techniques. It identifies crucial meteorological variables and observational data sources, providing detailed guidelines for data integration, predictive modeling, and the development of decision-support systems. The document also highlights best practices for capacity building and stakeholder engagement, facilitating informed decision-making in water resource management, disaster mitigation, and climate adaptation. This strategic framework equips economies with scalable tools and actionable insights, promoting effective management strategies tailored to unique geographical and climatic challenges.