Thursday Morning July 12th thousands of people woke up to water outside there houses as record breaking rainfall hit areas of central and Northern Kyushu especially hard. Some parts of Kumamoto Prefecture received up to 108mm of rain for a single hour, and this occurring for several hours added to totals of 508mm in a single 24hr period yet most of the rain fell in a 3-4 hour period. On top of that for the 72hrs from Thursday through Saturday ( July 12th – 14th) 813mm fell in this same location. The phrase JMA has coined for this roughly translated means “unprecedented” or “never before seen” in these parts of Kyushu. What makes this storm even worst is the fact of its nature as part of the rainy season boundary. Therefore people knew rainfall was going to happen and even here at westernpacificweather on Wednesday we forecasted heavy and flooding rainfall for Thursday due to instability rising in the front. Yet this staggering amount of rain exceeded many’s expectations. Unlike a typhoon where days of advanced warning is usually issued for the approach of a storm resulting in more preparations.
These factors combined to homes being washed away, towns having landslides crashing down on them, over 20 deaths with numerous still missing and have forced thousands on thousands to evacuate there homes. On Saturday July 14th there was reports of at least 850 homes damaged or destroyed due to landslides or flash flooding.
What was the weather factors that led to this though? The graphic to the left depicts the stationary boundary over Japan while warm moist air surge out of the east china sea and the Pacific Ocean. This surge of moisture led to development of thunderstorms that crashed in to the mountainous terrain of western Japan. As this happen two things happened, the forced uplift (adiabatic lifting) caused the storms to become stronger. At the same time the mountains squeezed the storms like sponges resulting in rapid short time heavy rain that added up to the staggering record breaking amounts. The graphic to the below shows where some of these totals really added up to. Most of them along the southern and western slopes of the mountains in this region.
The rainy season in Japan will always be a way of life for many residents. Floods like this will continue to occur yet as the human population continues to explode we become more vulnerable to natural weather events. As always though here at western pacific weather we will continue to strive to bring some heads up and forewarning when pending disasters approach.
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Also you can go back and look at our previous articles on this event for more information.