Record rain in Chiba caused by unusually slow storm system, experts say

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.

“I was honestly surprised,” Toru Adachi, a senior researcher at the institute’s typhoon and severe weather research department, said at a news conference Thursday.

“It was shocking that a phenomenon occurred for which we have not yet accumulated sufficient knowledge, and that it led to a disaster like this. 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.

Toru Adachi, a senior researcher at the Meteorological Research Institute, explains the results of an analysis on the linear rainband that caused the heavy downpours in Chiba last month. | JIJI

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.

Researchers also detected a counterclockwise vortex about 2 to 5 km in diameter inside thunderclouds in the northern part of the system, where some of the heaviest rainfall occurred.

The vortex, found at an altitude of roughly 1 to 3 km, may have locally strengthened upward air currents and accelerated the formation of precipitation particles, contributing to the extreme rainfall, the institute said.

The institute said the findings remain preliminary and that researchers plan further analysis to determine more precisely how the opposing air masses allowed the storm to remain in place for so long. The research could eventually help improve monitoring and forecasting of similar extreme rainfall events, the institute said.