Predictability of meso‐γ‐scale, localized, extreme heavy rainfall during the warm season in Japan using high‐resolution precipitation nowcasts

Abstract

The development and verification of a forecast method for localized, meso‐γ‐scale (2–20 km), extreme heavy rainfall ( MγExHR ) is important, because it can cause urban flash flooding and inundation with accompanying damage and potential loss of life. Although previous studies have examined the predictability of precipitation at a very short range (≤1 h) using extrapolation‐based nowcasts, they did not specifically focus on MγExHR . In this study, we examine the predictability of 23 selected events of MγExHR (1 h rainfall accumulation ≥50 mm) that occurred during the warm season of 2014 in Japan using High‐Resolution Precipitation Nowcasts ( HRPNs ) provided by the Japan Meteorological Agency, which are extrapolation‐based nowcasts. Traditional grid‐scale verification using the equitable threat score shows that the HRPNs usefully predict the heavy rainfall areas of ≥20 mm h −1 for at least 12 min at the grid scale of 1 km. Neighbourhood verification using fractions skill scores shows that HRPNs usefully predict the areas of ≥20 mm h −1 up to 29 min by tolerating 11 km displacement errors. After 30 min, a useful forecast cannot be obtained, even if the 11 km displacement errors are tolerated for the ≥20 mm h −1 areas. This result suggests that a numerical weather prediction ( NWP ) model, whose accuracy is useful after ∼30 min, is necessary to seamlessly provide useful forecasts for heavy rainfall areas of ≥20 mm h −1 for MγExHR with ∼10 km displacement errors, by blending the extrapolation‐based nowcast with NWP .