The deluge that pounded wide areas of Japan on Friday and Saturday ¡ª flooding rivers, homes and communities ¡ª was for parts of Aichi, Shizuoka, Ibaraki and Wakayama prefectures a rain event that happens ¡°once in 100 years or more,¡± according to an analysis of past precipitation data by the National Research Institute for Earth Science and Disaster Resilience.

The severe weather was caused by warm and moist air carried by Tropical Storm Mawar, which was passing south of the country, and an active rain front that covered much of Japan¡¯s mainland.

Based on past rainfall observation data, the research institute separated the rarity of heavy precipitation into six groups ¡ª once in 100 years or longer, 50 to 100 years, 30 to 50 years, 10 to 30 years, 5 to 10 years and two to five years.

shows that in the 24 hours through early Saturday, many coastal areas of the Tokai region, including Toyohashi in Aichi and Hamamatsu in Shizuoka, experienced an amount of rain that falls once in 100 years or more.

The map also shows that parts of Ibaraki were hit with downpours at levels seen only once every 50 to 100 years in the early hours of Saturday, with a narrower area struck by a deluge seen once in over 100 years.

The frequency of heavy rain was calculated by noting the amount of rainfall in a day against the number of past days that saw the same amount. For example, if the data from the past 100 years shows that there were five days with 24-hour rainfall exceeding 600 millimeters, the chance of a similar rain event happening is estimated at once in 20 years.

Severe weather events are becoming more common, with experts pointing to the effects of climate change. In May 2022, the Meteorological Research Institute released a report saying that the frequency of concentrated downpours ¡ª defined as a three-hour cumulative precipitation of 130 mm or more ¡ª has seen an increase of 2.2 times over the past 45 years. The long-term trend was confirmed through data collected from about 1,300 meteorological observation points across Japan, known as AMeDAS.

Over the weekend, the damage was most severe in Aichi and Shizuoka, where train services including the Tokaido Shinkansen were suspended for hours. Four people have been reported dead, including a man in Toyohashi who was found inside a flooded car. The number of flooded houses totaled 717 in Shizuoka and 273 in Aichi as of Monday.