Synoptic and Mesoscale Atmospheric Conditions Driving the Precipitation System in the August 2026 Chiba Heavy Rainfall Event

Abstract

This study investigates the synoptic and mesoscale atmospheric conditions of the precipitation system causing the August 2026 Chiba Heavy Rainfall Event in the Kanto Plain, Japan (13–14 August 2026). Synoptically, peripheral flow from a high-pressure system northeast of Japan advected a deep moist layer, which backward trajectories traced to tropical regions far east. Additionally, an approaching cutoff low from the southeast and an associated shortwave trough maintained remarkably low temperatures (> 2σ below climatology) at 500–300 hPa, inducing strong atmospheric instability. Mesoscalely, cold-air damming (CAD) existed prior to the heavy rainfall due to the high-pressure ridge. Convergence between CAD-induced low-level cold northeasterlies and southeasterly warm-moist air formed a coastal front (CF) along the Chiba coast. Diabatic cooling from CF-initiated precipitation markedly intensified the CAD and northeasterlies, accentuating the CF. Consequently, convective cells repeatedly generated on the CF, organizing and sustaining the precipitation system just west of the CF. A mesoscale low formed on the CF enhanced upwind water vapor flux and vertical shear. As the mesoscale low developed and moved, the environment degraded, leading to system decay. These findings demonstrate that the extreme heavy rainfall resulted from combined upper-level cooling, deep moisture advection, and intense CAD-induced coastal frontogenesis.