A rare thunderstorm system that would normally move rapidly instead remained nearly stationary for hours over Chiba Prefecture last month, contributing to the record rainfall that killed at least 13 people, researchers said Thursday. A preliminary analysis by the Meteorological Research Institute — part of the Japan Meteorological Agency — said a squall-line-type precipitation system (a linear rainband) was responsible for the Aug. 13 downpours. Squall-line-type systems are characterized by highly developed thunderclouds capable of producing intense rain, hail, lightning and strong winds, and typically move quickly.
In Chiba, however, the storm remained near the Boso Peninsula, with its northern portion lingering there for more than six hours. We need to properly understand what happened.” Researchers said the storm’s movement was likely suppressed by opposing air flows. A layer of cold air generated and strengthened by rainfall pushed the storm system eastward, while warm easterly winds from the Pacific acted as a headwind — pushing against it from opposite directions.
Those forces largely balanced each other, causing the rain-producing system to stall rather than move away as squall lines normally do. The central and southern portions of the storm moved only about 20 to 30 kilometers between 6 p.m. and 9 p.m., while its northern section stayed almost stationary for over six hours. The institute said the system developed in two stages.
Daytime rainfall first created a pool of cold air over inland parts of the Kanto region. Near the Boso Peninsula, warm air flowing in from the Pacific met the cold air, forming a localized boundary known as a shear line. The warmer air was forced upward along that boundary, allowing cumulonimbus clouds to develop.
From around 5:30 p.m., rainfall from those clouds strengthened the cold-air layer and pushed the boundary outward into a bow shape, creating what meteorologists call a gust front. At the same time, a strong current known as a rear-inflow jet developed on the western side of the system, helping intensify the front and fuel further thunderstorm growth. The clouds repeatedly regenerated above the gust front and reached heights of more than 15 km.
That was almost twice the roughly 8-km height of thunderclouds observed during the 2015 Kanto-Tohoku torrential rains, according to the institute. A linear rainband developed at around 6 p.m. on Aug. 13, and a special heavy rain warning was issued roughly 90 minutes later. The city of Chiba recorded 367 millimeters of rain over 24 hours — the highest level ever observed there.
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