Tokyo Faces a 500mm Rainfall Nightmare — Experts Warn of Rain Like We’ve Never Experienced Before

Sea Surface Temperatures Three Degrees Above Normal — An Alarming Situation

On October 21, the Japan Meteorological Agency announced that a linear rainband had formed over the southern Izu Islands, including Hachijojima, warning of the potential for torrential rainfall exceeding 80 millimeters per hour. Hachijojima had already been struck directly by Typhoon No. 22 on October 9, recording 489 millimeters of rain within 48 hours, and was later hit again by Typhoon No. 23.

In the past, typhoons approaching or making landfall in Japan typically formed over tropical seas near the Philippines, then moved toward Okinawa and onward to Kyushu or Shikoku. However, this year’s Typhoon No. 19 formed near Japan, and Typhoon No. 22 developed off the Ogasawara Islands.

“Typhoons usually move along the edge of the Pacific High, which is why they tend to follow a route from the Philippines to Okinawa and then toward Kyushu or Shikoku,” explains Associate Professor Yoshiaki Miyamoto of Keio University, who studies the mechanisms behind typhoons and heavy rainfall.

“But when they form near Japan, they aren’t caught in that large circulation pattern. Instead, they meander — and some may even make landfall in the Kanto region.”

Why, then, are typhoons now forming so close to Japan?

“A typhoon is essentially a massive cluster of cumulonimbus clouds that draws up water vapor from the ocean and transforms it into storm clouds,” Miyamoto says. “In the past, sea surface temperatures near Japan weren’t high enough to supply that much water vapor, so the environment wasn’t conducive to typhoon formation.

This year, however, sea temperatures in nearby waters are as much as three degrees Celsius higher than average in some areas. That’s enormous — the ocean is storing a tremendous amount of heat. Because of that, large amounts of water vapor are being generated near Japan, making it possible for typhoons to form right off our coasts.”

Normally, sea surface temperatures around Japan range from 25 to 26°C, while typhoons tend to form when the water temperature exceeds 27°C. This year, however, broad stretches of ocean south of Japan have reached 30°C or higher.

According to Miyamoto, one factor behind the abnormally high temperatures is the Kuroshio Current.

“The tropical regions where the Kuroshio originates have warmed, causing the current itself to carry more heat northward. As a result, sea surface temperatures around Japan have risen significantly.”

There is a possibility that the Kanto region could experience up to 500 millimeters of rainfall

What’s truly frightening is that when a typhoon forms near Japan, it can make landfall without weakening.

In the past, when sea surface temperatures around Japan were lower, typhoons that developed far away — such as over the seas near the Philippines — would usually lose strength by the time they approached Japan. But now, when a typhoon forms close to Japan, there is a serious risk that it will hit land at full strength.

In fact, Typhoon No. 22 brought maximum wind gusts of 54.7 meters per second and over 500 millimeters of total rainfall, causing severe damage on Hachijojima Island.

Heavy rains exceeding 500 millimeters have occurred several times in the past. For example, Typhoon No. 8, which formed in July, brought 580 millimeters of rainfall to Minamidaitō Village and 531 millimeters to Kitadaitō Village in Okinawa. In 2023, Typhoon No. 2 also caused more than 500 millimeters of rain in Hamamatsu, Shizuoka Prefecture.

Could a similar typhoon strike the Tokyo metropolitan area directly?

“Even if a typhoon doesn’t dump 500 millimeters of rain directly on the capital, it’s entirely possible for mountainous areas to receive that much. When that happens, even if rainfall in the city itself isn’t extreme, floodwaters can surge downstream from the upper reaches of rivers, causing them to overflow.

If a typhoon forms near Japan and brings 500 millimeters of rain to mountain regions, rivers such as the Arakawa or Tamagawa could overflow. Water always flows downward — so being underground in such conditions is extremely dangerous.”

There is even a possibility that a typhoon with a central pressure of 920 hPa and wind speeds of 70 to 80 meters per second could make landfall in Japan

The danger lies not only in the rainfall.

“Given current air and sea temperature conditions around Japan, a typhoon could theoretically intensify to around 920 hPa with wind speeds of 70 to 80 meters per second. In other words, we could face a storm more powerful than anything Japan has ever experienced. That’s how unusually high sea surface temperatures have become near Japan.”

During Typhoon No. 22, which struck Hachijojima, the maximum instantaneous wind speed reached 54.7 m/s. When Typhoon No. 21 hit the Kansai region in 2018, its 58.1 m/s winds caused a tanker to crash into the bridge connecting Kansai International Airport, knocked down numerous utility poles, and uprooted more than 5,000 trees in parks.

If wind speeds were to reach 70 to 80 m/s, the level of destruction would be unimaginable.

“We honestly don’t know what would happen—it’s difficult even to predict. But as sea surface temperatures around Japan continue to rise, the number of typhoons forming near Japan is likely to increase.”

It’s becoming clear that both where typhoons form and when the typhoon season occurs may change in the coming years.

Profile — Yoshinobu Miyamoto
Associate Professor, Faculty of Environment and Information Studies, Keio University; Researcher at the Typhoon Prediction Research Laboratory, Typhoon Science and Technology Center.
He conducts fundamental research on the mechanisms of typhoons and torrential rainfall, as well as the environmental impacts of global warming and air pollution, exploring how meteorology can be applied to architecture, industry, disaster prevention, urban development, and public policy.