Analysis of forecast error for a short-term extreme precipitation event in the Shaying River Basin

Using routine meteorological observations, intensive observations from the automatic weather stations, satellite images, radar products, NCEP reanalysis data, fine grid model data and regional mesoscale model data, we have conducted an examination on the reasons for the miss of forecast and the predictability of a short-term extreme precipitation event occurred in the Shaying River Basin at the night on 18 August 2017. The results indicate that this event occurs under the circulation of a high in the west and a low in the east. The stability and continuance of shear lines in the low-level and boundary layer and the establishment of the ultra-low level southwest jet provide the water vapor transport and convergence conditions for the occurrence of severe precipitation. The weak cold air spreads southwards along the south of North China and gradually intrudes into the inverted trough forming a mesoscale convergence line (convergence center) in the Shaying River Basin and maintaining such condition for a long time to trigger the release of unstable energy, which is a good indicator in predicting the occurrence of rainstorm. The two mesoscale convective cloud clusters reinforced and developed jointly into a Meso-β scale Convective System (MβCS) with a quasistatic state. It maintains for 6 hours in the lower reaches of the Shaying River Basin, which is the direct reason that leads to the enhancement of precipitation. The mesoscale convective system is triggered by the cold pool and its front surface mesoscale convergence line, and the strong echo continuously develops and maintains in the high value zone of temperature gradient in front of the cold pool, resulting in short-term extreme precipitation. The weak dynamic condition, the water vapor being mainly concentrated in the boundary layer and the extremely favorable thermal instability condition during this event, as well as the limited capture ability of the numerical model in forecasting this kind of convective rainstorm, all these factors are the causes of the miss of forecast of the rainstorm. The insufficient estimation of numerical models and operational forecasts in the precipitation intensity and its extreme, and the lack of forecasters'experience in forecasting extreme weather, are also some of the reasons for the forecast errors.