The adoption of self-tracking services to improve health-related behavior is increasing. Although psychologists claim that thinking about the future has a motivational impact on current behavior and cognition, few studies have explored using future forecast in self-tracking services. In this paper, we explore how future forecast information can be used in the design of self-tracking services. We conducted a four-week study that qualitatively investigated 11 participants' perceptions of and practices with future forecast information. Participants used the FutureSelf app that we developed, which forecasts dieters' future weights and expected goal achievement rates based on their current behavior. The findings reveal that predicting future weight based on prior performance induced participants to imagine their future selves and reminded them of their ultimate goals. In fact, the predictions became the participants' primary source of motivation. We also suggest design implications for self-tracking services that forecast users' future statuses.