Abstract: The first snowstorm event in Jiangsu during 3–5 January 2018 had heavier snowfall but lower snow accumulation efficiency, whereas the second snowstorm event during 24–28 January had lighter snowfall but higher snow accumulation efficiency. This study investigated the temperature and humidity conditions in these two snowstorm events using the ERA-Interim reanalysis data and observation data from the China Meteorological Administration and explored the underlying physical processes in the framework of isentropic atmospheric mass circulation. The main findings are as follows: (1) Compared with the second snowstorm event, the early stage of the first snowstorm event was characterized by higher temperature in the entire troposphere, which was attributed to a relatively deeper and stronger poleward warm air branch of isentropic atmospheric mass circulation to the south of Jiangsu. In contrast, the stronger equatorward cold air branch of isentropic atmospheric mass circulation resulted in a temperature lower than 0°C in the second snowstorm event, promoting higher snow accumulation efficiency. (2) The deep water vapor mass inflow layer in lower isentropic layers collaborated with the wide range of ascending motions during the first snowstorm and further brought lower-level water vapor to higher layers for the formation of larger snowfall. Stronger meridional water vapor transport but weaker zonal water vapor net mass outflow in the lower isentropic layers increased near-ground specific humidity, contributing to the lower snow accumulation efficiency. However, there was a deep layer of water vapor mass outflow in the lower isentropic layers during the second snowstorm event, which contributed to the greater snow accumulation efficiency. Colder and dryer conditions resulting from the abnormal meridional cold air transport and weak water vapor transport in both meridional and zonal directions caused higher snow accumulation efficiency in the second snowstorm event. The comparison of the spatial distribution of temperature and humidity with snow accumulation efficiency further reveals that under high temperature and humidity conditions, snow accumulation efficiency is more sensitive to the local temperature and humidity changes.