Flood risk communication in disaster-prone communities often relies on traditional tools (e.g., paper and browser-based hazard/flood maps) that struggle to engage community stakeholders and reflect intuitive flood situations. In this paper, we applied the transitional mixed reality (MR) interface concept from pioneering work and extended it for flood risk communication scenarios through co-design with community stakeholders to help vulnerable residents understand flood risk and facilitate preparedness. Starting with an initial transitional MR prototype, we conducted three iterative workshops - each dedicated to device usability, visualization techniques, and interaction methods. We collaborated with diverse community stakeholders in flood-prone areas, collecting feedback to refine the system according to community needs. Our preliminary evaluation indicates that this co-designed system significantly improves user understanding and engagement compared to traditional tools, though some older residents faced usability challenges. We detailed this iterative co-design process, critical insights and design implications, offering our work as a practical case of mixed reality application in strengthening flood risk communication. We also discuss the system's potential to support community-driven collaboration in flood preparedness.