Ten-day rainfall summary of the event from 28 June to 8 July (in Japanese)
Rainfall accumulations reached to 1,852.5 mm in Umaji, Kōchi
On 28 June 2018, a seasonal Meiyu front extending west from a non-tropical low near Hokkaido became stationary over Japan. Multiple rounds of heavy rain occurred in the subsequent days, primarily in northern Kyushu. On 3 July Typhoon Prapiroon brought heavy rains and winds to southwestern Japan. The surge of moisture brought north by the typhoon interacted with and enhanced precipitation along the front in Kyushu, Shikoku, and western and central Honshu. Enhanced rainfall extended as far west as Okinawa Prefecture. Large swathes of these areas saw 10-day rainfall accumulations in excess of 400 mm (16 in). Deadly floods began on 5 July, primarily in Kansai region which was struck by a deadly earthquake three weeks prior. Accumulations peaked at 1,852.5 mm (72.93 in) in Shikoku.
Multiple areas saw their greatest one-hour and three-day rainfall totals on record. Some areas were hit by more than 1,000 mm (39 in) of rain, prompting the Japan Meteorological Agency (JMA) to issue emergency heavy rain warnings[note 1] for eight prefectures: Okayama, Hiroshima, Tottori, Fukuoka, Saga, Nagasaki, Hyogo, and Kyoto. This marked the largest issuance of these warnings since their implementation. An official at the JMA described the event as "heavy rain at a level we've never experienced".
The torrential rain triggered landslides and flash flooding, with water levels reaching 5 m (16 ft) in the worst hit areas. Motoyama, Kōchi, saw 584 mm (23.0 in) of rain between 6 and 7 July. One town in Kōchi measured 263 mm (10.4 in) of rain in two hours. Mount Ontake observed its greatest three-day rainfall on record at 655.5 mm (25.81 in). Although the Yura River remained within its banks in northern Kyoto Prefecture, an embankment built after Typhoon Tokage in 2004 prevented runoff from flowing into the river. This inadvertently led to flooding in Maizuru after the flood gate was closed.
As the rain lessened on July 9, high temperatures reaching 30 °C (86 °F), coupled with some 11,200 households without electricity, raised concerns over heatstroke and unsafe drinking water.
Hiroshima prefecture alone had 1,243 mudslides in 2018, which is more than the entire nation's total in an average year. Ehime had 419 in 2018, results were not broken down by month, but its inferred that most of these were during this major event.
Throughout the affected areas at least 225 people died in various rain-related incidents, primarily due to mudslides, landslides and vehicles being swept away by the flood waters. Many of the dead had ignored evacuation orders, and chose to stay in their homes despite repeated warnings. Police received numerous reports across the country of people trapped in homes buried by landslides, of people being swept away by swollen rivers, and from people trapped in cars. At least ten people were buried inside their homes in Higashihiroshima; rescuers were able to confirm seven survived but remained trapped as of 7 July.
By 7 July no bullet trains were running west of Shin-Osaka Station and the West Japan Railway Company officials were uncertain when the trains would be running again. The widespread cancellation of trains stranded numerous travelers; some bullet trains were utilized as temporary hotels. Some automakers (Mitsubishi Motors & Mazda Motor) halted production as the rain and flooding disrupted the companies' supply chains and risked the safety of workers. Other companies such as Daihatsu and Panasonic suspended operations at plants until debris was cleared and the water receded from the factories. The Asahi Aluminium Industrial Company plant in Okayama exploded on July 6, after workers had evacuated during the flooding.
Delivery companies Sagawa Express Co. and Yamato Transport Co, with cargo service Japan Freight Railway Co. reported that some of their shipments into and out of the affected areas have been either reduced or suspended. Regional supermarkets have also been affected, with outlets closed or hosting shortened service hours due to delivery delays and/or product shortages.
Nationwide sustained tremendous damage; losses reached an estimated ¥1.09 trillion (US$9.86 billion). Damage to agriculture, forestry, and fishery industries reached ¥629 billion (US$5.69 billion). Losses to public infrastructure, including levees, railways, and roads, amounted to ¥465 billion (US$4.21 billion).