Japan has entered a season that calls for precautions against heavy rains.

Everyone should check their emergency provisions and remember to be ready for early evacuation in preparation for this flood-prone time of year.

Typhoon No. 3, which rose to tropical storm status on June 6, requires caution because it could cause downpours, depending on its course, over broad areas of Japan.

Earlier this month, approaching Typhoon No. 2 and a seasonal rain front produced torrential rains across Japan, setting record 24-hour rainfalls at many locations. The downpours prompted a succession of landslides, flooded many houses and caused fatalities.

Residents are advised to pay close attention to disaster management advisories because they could face rainfall amounts that they have never experienced in their lives.

Starting this year, the Japan Meteorological Agency has been issuing imminent forecasts up to 30 minutes before the prospective formation of “linear rainbands,” or belts of well-developed cumulonimbus clouds, which are responsible for many downpour disasters.

Similar warnings were issued in six prefectures, including Kochi, Wakayama and Shizuoka, during the torrential rains of early June.

There should be as much advance warning as possible if residents are to make decisions on evacuating. 

The JMA should work to release forecasts of linear rainbands as early as possible and notify the public about the efforts so they can use the advisories for making decisions.

In using similar warnings, however, we should keep in mind that the forecast technology is still developing.

The amount of water vapor and humidity are involved in a complex manner in the formation of linear rainbands, whose mechanism has yet to be understood.

Starting from last year, the JMA has also been issuing forecast information on possible downpours from linear rainbands, half a day before their possible occurrence, by regional areas, such as Kinki and Tokai.

Linear rainbands, however, formed in only three of the 10 cases where the JMA had released similar predictions between June and October last year. There were also linear rainbands that formed without being predicted.

The agency is actually improving the linear rainband forecast system even as it is implementing the system in practice.

The users should combine the linear rainband forecasts with other data, such as river water level information released by parties including the land ministry and evacuation advisories issued by local governments, to use for perceiving any imminent dangers. 

“Inland water flooding,” or submersions and inundations caused by rainfall beyond the drainage capacity of the sewer system, has necessitated the taking of precautions during short-duration downpours of recent years.

No sufficient measures have yet been taken against the phenomenon, which has been occurring with more frequency amid urbanization.

Flooding from inland waters disrupted roads and submerged vehicles in Koshigaya, Saitama Prefecture, where a record 24-hour rainfall total was set earlier this month. More than 3,000 houses were flooded, either above or below the floorboards, in the prefecture on that occasion.

Inland water flooding also occurred across Japan during the downpours of last summer and was responsible for inundations in 15 prefectures, including Tokyo, when the country was hit by Typhoon No. 19 in 2019.

The land ministry has required 1,110 municipalities, where inland water flooding is deemed likely to occur, to work out hazard maps for a similar disaster. Only 10 percent of them, however, had drawn up such maps as of September last year.

The central government should work harder on the matter, such as by providing technical assistance to local governments and by encouraging residents to use similar hazard maps, which can be utilized as manuals for making prompt decisions.

Strong earthquakes jolted Ishikawa and Chiba prefectures in May, a reminder that we should take notice of the potential of a dual disaster as well.

Though there are diverse types of disasters, they have much in common when preparing for them, such as stocking up on food and water, checking routes to evacuation shelters and confirming methods for getting in touch with family members in the event of emergencies.

Being thoroughly prepared will help reduce the risks.

--The Asahi Shimbun, June 11