Abstract: The case study examines the double-rainband torrential rainstorm that occurred in Northeast China on 27 June 2022, focusing on its formation mechanisms and forecasting errors. Large-scale circulation systems associated with this rainstorm are characterized by the northeast cold vortex, a low-level jet, a shear line, and a surface cyclone. The precipitation in the western rainband is associated with the shear line and the frontal zone located on the northwest flank of the cyclone. The convergence of water vapor at lower levels of the shear line is found to be conducive to the rainstorm in the western band. The low-level jet stream contributes to rainstorms in the eastern rainband by transporting warm, humid air. The convergence of water vapor in the boundary layer on the left flank of the low-level jet is found to be conducive to the rainstorm in the eastern band. The EOF decomposition of ensemble forecasts indicates that the dominant pattern of precipitation forecasts, which accounts for 28.4% of the variance, is linked to deviations in forecasts of two rainbands. Forecasts for eastern and western rainbands demonstrate an anti-phase relationship. There is a significant correlation between precipitation forecasts and sensitive systems, including the northeast cold vortex, low-level jet stream, and surface cyclones. For ensemble members that overestimate the intensity of these systems, the western rainband is marked by higher precipitation and exhibits a more westerly position, whereas the eastern rainband is characterized by lower precipitation and a more easterly position. EOF analysis indicates that the dominant pattern of the wind field ensemble forecast at 850 hPa is the east-west positional deviation of the low-level jet stream, which accounts for 34.0% of the variance. Ensemble members that predict a westward (or eastward) bias in the position of the low-level jet stream also tend to exhibit similar biases when forecasting positions of other sensitive systems and the water vapor center. As the water vapor convergence center is situated in the lower troposphere to the left of the low-level jet core, a westward (eastward) bias in the position of the low-level jet tends to enhance heavy precipitation in the western (eastern) rainband, thereby resulting in an anti-phase distribution of precipitation amounts in eastern and western rainbands. In 6-12 h preceding the onset of precipitation, as the low-level jet strengthens, the forecast error of the wind field at 850 hPa increases significantly. The forecast error originates in the upstream precipitation areas, resulting in substantial alterations to the dominant patterns of wind field forecast errors.