Practical and Intrinsic Predictability of a Warm‐Sector Torrential Rainfall Event in the South China Monsoon Region

Warm‐sector torrential rainfall (WR) in the South China monsoon region has long been a forecasting challenge because of the limited capability of numerical models in heavy rainfall without strong synoptic forcing. Through convection‐allowing ensemble forecasts, this study explores both the intrinsic and practical predictability of a coastal WR event on 19–20 May 2015 during SCMREX (the Southern China Monsoon Rainfall Experiment). The results show a large variability in forecast performance among different members, indicating the practical limit of predictability. In general, GOOD members tend to have a stronger low‐level southerly wind over the sea (monsoon flow) and a considerable surface cooling over the northern mountains (associated with land/mountain breeze). Further investigation via ensemble‐based sensitivity analysis shows that the occurrence of WR is closely related to the nighttime strengthened (cooling) southerly wind (temperatures) over the sea (mountains), 1–3 hr prior to the convection initiation. In contrast, spatial scaling of the initial perturbations has little impact on the forecast after 3 hr and the meso‐γ‐scale rainfall is fully decorrelated after 12 hr, suggesting an intrinsic predictability limit for lead times as short as 6–12 hr. Sensitivity experiments are conducted with the initial‐condition differences reduced by almost an order of magnitude smaller than typical ensemble perturbations, with the results demonstrating that the rainstorm might be near the point of bifurcation, where predictability is intrinsically limited. The limits of both intrinsic and practical predictability highlight the need for rapidly updated and probabilistic convection‐allowing ensemble forecasts for events of this type. Key Points Both the practical and intrinsic predictability of a coastal warm sector torrential rainfall event during the Southern China Monsoon Rainfall Experiment (SCMREX) are investegated The occurrence of warm‐sector rainfall is closely related to the nighttime strengthened (cooling) monsoon flow (temperature) over the sea (mountain) The warm‐sector rainstorm could be near the point of bifurcation, where predictability is intrinsically limited