In July 2018, record-breaking heavy precipitation caused catastrophic disasters in west Japan. This study investigated the predictability of this precipitation event using a regional ensemble data assimilation system. A series of daily ensemble forecast experiments showed that the forecast ensemble spread during the heavy precipitation event increased in the forecasts initialized on July 1 and July 3. The first peak of the forecast ensemble spread was associated with the uncertain track forecast of Typhoon Prapiroon. Namely, about a half of the ensemble members predicted eastward recurvature of the typhoon, whereas the other members predicted that the typhoon stayed near China. The later peak was associated with the southern convective disturbances near the Ryukyu Islands. Composite analysis and ensemble-based correlations showed that more active convective activities corresponded to a stronger cyclonic circulation and pushed the main precipitation band northward. The results implied that Prapiroon and the southern convective disturbances played important roles in the July 2018 heavy rain event and largely contributed to its predictability.