Social and ubiquitous computing opens up many opportunities to engage citizens in activities that benefit their communities. Technology is ready and available, but there are still open issues concerning how to engage people in activities that are not extrinsically rewarding or whose impact is not immediately perceived. In this paper, we explore the role that situated motivational affordances can play in encouraging citizens in one of such activities, early warning. With this purpose, we designed and implemented a gamified app, IWarn that was iteratively designed following an action-research process to align the needs and capabilities of two types of stakeholders: emergency managers and citizens. The situated motivational affordances framework was used to lead the evaluation considering the motivational affordances enabled by the app and the situation in which it was used. The IWarn app was evaluated in an in-the-wild deployment where 4 emergency workers and 17 citizens took part in a real exercise for one week. Our results suggest that the gamified elements helped to improve intrinsic and extrinsic motivation and user engagement. This work contributes to the social computing domain by illustrating a use case where carefully designed gamification can help in engaging citizens in participatory processes