This study examined a midlatitude low pressure system that deepened to 974 hPa over the Sea of Japan on 31 August 2016 using the Japanese 55-year reanalysis (JRA-55) dataset. The low appears to have developed by absorbing Typhoon Lionrock (2016). This unusual development of the low occurred in a relatively weak baroclinic environment in association with high potential vorticity air that moved southeastward and downward along a slantwise isentropic surface in the upper troposphere. Middle and lower tropospheric warming also contributed to the deepening of the surface low. In the last stage of its development, the upper-tropospheric trough became coupled with Typhoon Lionrock. Lionrock also contributed to the deepening of the low at an earlier stage by inducing moist air to flow in the lower troposphere between Lionrock and a high pressure system located to its north. The consequent latent heat release over the Sea of Japan led to intensification of the upper-tropospheric ridge and increased vorticity advection. These are also considered to have contributed to the deepening of the low.