Abstract: The Loess Plateau is the largest loess deposit region in the world and a typical ecologically fragile area. During the cold season, heavy snowfall is a major meteorological disaster on the Loess Plateau, posing severe threats to agricultural production, transportation, and power facilities. Based on ground station observations from the China Meteorological Administration and hourly ERA5 reanalysis data from ECMWF, this study investigates the heavy snowfall event on the Loess Plateau from December 10 to 11, 2023, through weather analysis, vorticity budget, and Lagrangian moisture budget. The main conclusions are as follows: (i) The short-wave trough in the mid-tropospheric westerlies provided the most favorable synoptic background for this snowfall event. The warm advection ahead of the trough contributed to both the maintenance of ascending motion and the pressure reduction in the lower troposphere. (ii) Two mesoscale vortices (V1 and V2) formed and merged (into mesoscale vortex V3) along a low-level shear line northeast of the Tibetan Plateau. The merger significantly intensified the snowfall, with V3 being the most critical mesoscale system in this event. Vorticity budget analysis revealed that the vertical stretching term associated with low-level convergence and horizontal vorticity advection were the primary and secondary factors, respectively, in the genesis of V3. During its development and maintenance stages, vertical stretching and vertical vorticity advection dominated, while horizontal vorticity advection also played a favorable role. In the decay stage, the vertical stretching term turned negative due to enhanced low-level divergence, leading to the vortex"s dissipation. (iii) Lagrangian moisture budget based on backward tracking indicated that the snowfall moisture primarily originated from local evaporation (contributing ~44.5%), followed by southeastern China and its vicinity (24.8%) and northwestern China and its vicinity (17.8%). Contributions from distant sources (Europe, North Africa, the Arabian Peninsula, the Indian Peninsula, etc.) were negligible (<2%). This study elucidates the mechanism by which mesoscale vortex mergers amplify snowfall intensity through enhanced low-level convergence and moisture convergence, providing a theoretical basis for cold-season disaster early warning and mitigation on the Loess Plateau.