The season of heavy rain and typhoons has come around again. With rainfall in wide areas predicted through the early morning hours of June 5, the Kanto region around Tokyo seems to be on the verge of rainy season.

We would be better advised to prepare ourselves for the danger posed by massive amounts of rainfall.

Rain causes disasters almost every year. One major threat is “senjo kosuitai,” or moving or stationary bands of torrential rain caused by rows of dense, towering vertical rain clouds stretching dozens to hundreds of kilometers. Such training rainstorms tend to dump huge amounts of precipitation in wide areas over many hours.

The Japan Meteorological Agency will launch on June 17 a new early warning program to alert the public to the imminent danger of torrential rain due to senjo kosuitai. The warning will come in the form of “information about exceptionally heavy rain.”

The agency will issue the alert as soon as it detects signs of this meteorological phenomenon occurring. While it serves the public interest by publishing the latest information about potential weather-related danger, the program still needs improvements to ensure safe evacuation.

Criteria for issuing the alert include the emergence of linear bands of areas registering 100 millimeters or more of rainfall in three hours and encompassing 500 square kilometers or larger.

The agency will indicate areas in danger with a narrow ellipse and warn that the risk of disaster is growing rapidly in the areas from “extremely heavy rain due to senjo kosuitai.”

But how many people can really understand the seriousness of the situation when they are alerted to “exceptionally heavy rain?”

The agency should hold a news conference to explain what local residents should do to protect themselves when it issues the alert.

Since it will be information about the occurrence of the phenomenon, rather than a forecast, the agency should be specific about the risks for affected areas, which could already be in a dangerous situation.

If a band of torrential rain forms around the upper reaches of a river, for example, areas around the lower reaches are also likely to be threatened before long as the river swells. It is vital to ensure that people understand that areas immediately outside the ellipse should also be on the alert.

In 2019, a new disaster alert system based on a five-level warning scale was introduced. The senjo kosuitai alert is expected to be issued in situations corresponding to the second highest Level 4 on the scale or more serious. The Level-4 warning urges people to escape from “high-risk places.”

The agency should work with the land ministry to also announce indicators of water levels in rivers and landslide risks in affected areas to help local residents accurately grasp the gravity of the situation.

It is a tough challenge to predict the occurrence of the dangerous weather phenomenon. The agency was criticized for failing to do enough to alert local residents to the imminent danger when Kumamoto Prefecture and surrounding areas were hit by devastating rain last summer. More than 60 people died in the rain disaster.

The agency will use observation ships to measure water vapor in the air above the sea to beef up its ability to monitor and predict weather patterns with an eye to starting offering in fiscal 2022 forecasts of senjo kosuitai, such as the probabilities of the formation of such dangerous rainbands, at least half a day in advance.

The wide variety of meteorological information provided could confuse the public. There have been calls for consolidating and integrating weather information. While it is important to supply new types of information, the agency should also step up efforts to better communicate risks to the public in simple and easy-to-understand language.

It should also do more to evaluate, through post-disaster analyses, the actual usefulness of various pieces of information provided to the public for evacuation during weather-related emergencies.

Some changes have been made recently to the government’s guidelines for warning people about weather-related danger. “Evacuation recommendations” by municipal governments have been abolished and integrated into “evacuation instructions,” for example.

The revised guidelines advise people to consider taking refuge in the homes of relatives and friends instead of evacuation centers under certain circumstances.

Individual citizens need to familiarize themselves with local evacuation routes and make other efforts as well so that they can take proper actions during crises.

--The Asahi Shimbun, June 4