Environmental Characteristics of an Extreme Rainfall Event in Yamagata–Niigata Prefectures, Japan, on 3-4 August 2022

The environmental characteristics of a heavy rainfall event caused by a linear-stationary heavy rainfall area (LS-HRA) that occurred in Yamagata–Niigata prefectures (YNP), northeastern Japan, on 3-4 August 2022 were examined and compared with similar LS-HRAs previously occurring in those prefectures using Japanese Meteorological Agency mesoscale analysis data. During the heavy rainfall event, a low-level meso-α scale low that continued to exist over the Sea of Japan contributed to maintaining strong low-level moisture flux and its convergence around the event region. Low-level, high-equivalent potential temperature provides unstable conditions in the area. Moreover, six favorable conditions for the LS-HRA occurrence were satisfied in upwind areas over the Sea of Japan. A comparison of the environment with five previous events in YNP showed similarities and differences among the events. Common features are the presence of meso-α lows and associated intense low-level moisture flux toward YNP. Meanwhile, the thermodynamic atmospheric conditions differed among the events; the upper-level cold temperature destabilized two cases, while the other three cases, including the present event, were destabilized by the low-level high equivalent potential temperature.