Abstract:The structure characteristics and thermodynamic mechanism of the extreme snowstorm in the Northeast and North China during 6-8 November 2021 are analyzed with multiple observations and the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data. The results show that this process was successively affected by the upper transversal trough, the westerly trough in Hetao Plain and the upper cold vortex at 500 hPa. The high-level jet over the influence system was gradually strengthened, featuring an “S” curve. Meanwhile, the southern low-level jet was formed, strengthened, and coupled with the high-level jet in Northeast China. The influence systems of this process emanated distinct phased characteristics with significant differences in the structure characteristics and water vapor transport. The cold surface area formed by the return-flow cold front was relatively shallow, and the warm moisture flow was rising obliquely along it. The cold front was relatively steep with a westward tilting updraft, while the ground cyclone front was deep and vertical, making the air flow ascend vertically. With the enhancement of baroclinic forcing, the 850 hPa shear line turned to north-south distribution from quasi-east-west direction and then evolved into a low vortex shear structure. Correspondingly, the horizontal vorticity changed from weak to strong, and the overlying vertical distribution of positive vorticity gradually strengthened, featured with a weak slantwise updraft evolving into a strong vertical ascending zone and a secondary circulation sinking branch forming on the east of the system. Frontogenesis fostered this extreme snowstorm. The pseudo-equivalent potential temperature front area gradually strengthened in the three stages of snowfall, and the vertical front area developed from an inclined state into a nearly vertical structure. The snowfall area was consistent with the evolution of the front area and the snowfall intensity was proportional to the magnitude of frontogenesis function. The diagnosis of the moist potential vorticity shows that all the snowfall areas in the three stages occur in the configuration region where the barotropic (baroclinic) term is greater (less) than 0. Conditional symmetric instability is the main dynamic mechanism of this extreme snowstorm.