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An aerial view shows flooded Kuma River caused by a heavy rain at a residential area in Hitoyoshi, Kumamoto prefecture, southern Japan, in this photo taken by Kyodo July 4, 2020. Mandatory credit Kyodo/via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. MANDATORY CREDIT. JAPAN OUT. NO COMMERCIAL OR EDITORIAL SALES IN JAPAN.

Extreme weather: Analysis of the recent flooding in Japan

Southern Japan, like many parts of the world, has experienced extreme weather events that exceeded the capacity of dams resulting in large-scale flooding, evacuation, loss of life and costly restoration of infrastructure.

aWhere’s 1.9 million weather stations across the earth can be leveraged to get ahead of these crises and support climate-resilient planning and infrastructure investments. The Kuma River dam is just one example of how infrastructure developed in the 1950’s is not designed for weather events of 2020.

How wet was it?

aWhere analysts reviewed the rainfall using our premium observed weather data and found that over 500 mm of rain fell over a 7-day period. The map below highlights the flooded areas in Southern Japan.

More rain to come

Zooming into some of the hardest hit areas, we are able to understand how much rain fell at a local level. Unfortunately, the rains are not over yet as we integrate observed rainfall with the 15-day forecast for the area. The blue bars in the charts below are the weekly rainfall totals and the orange line is the long-term normal (calculated from 2006-2020). An enormous amount of rain fell over a short 4-day period (one location received 582 mm).  This led to the devastating and deadly floods that overwhelmed drainage systems, watersheds and dams.

The warmer atmosphere has led to extreme rainfall events that are overwhelming infrastructure that was built to specifications based on rainfall patterns of 20 to 70 years ago. The dramatic changes in weather patterns of the past decade point to the urgent need to integrate modern weather analytics into the design of future bridges, dams, roads and cities.  The trends in this area point to increasing and highly variable rainfall due to climate change. The charts below illustrate the annual rainfall trends for the period from Jun 7 – July 6 since 2006. Given these trends, engineering requirements would be well served by taking into account both the trend and increased variability in rainfall as bridges, dams and waterways are constructed to deliver economic resilience to climate change for the next 50 years.


Accurate observed weather data is critical in taking proactive policy and management steps to reduce risks of flooding. aWhere’s historical data can help decision makers identify areas at risk and pragmatically act.  Contact us for more information.