Typhoon Ragasa, the world’s most powerful tropical cyclone in 2025, barrelled towards tens of millions of people in southern China on Sept 24 after killing at least 14 people in Taiwan, leaving scores missing and lashing Hong Kong with ferocious winds and heavy rain.
Many also went missing in Taiwan’s eastern Hualien county after a barrier lake overflowed and sent a wall of water into a town, Taiwan’s fire department said, with Ragasa’s outer rim having drenched the island since Sept 22.
How the barrier lake formed and grew
The lake was first discovered in July. It was formed when earlier typhoons caused landslides, blocking a valley.
Three satellite images side by side. The first one shows the valley with no lake, the second shows the valley with the lake formed on July 25 and a month later on Aug 18, the lake is much bigger.
Barrier lakes are formed when rocks, landslides or other natural blockages make a dam across a river – normally in a valley – blocking and holding back water, hindering or even stopping natural drainage. This lake is in a remote part of the mountains above the town of Guangfu, on a tributary of the Matai’an Creek.
A Reuters analysis of satellite imagery by Planet Labs shows that the lake first started forming some time between July 17 and July 25. The surface area of the lake increased to about five times its original size between July 25 and mid-September.
Size of the lake
A Reuters analysis of satellite imagery shows that the lake grew to a size of 0.92 sq km by mid-September.
A diagram of the lake showing its estimated dimensions on different dates from July to Sept.
Before the dam forming the lake overflowed this week, it stood about 120m high, with the water behind it covering an area 500m long and 1,650m wide, according to the government.
The lake has since shrunk in size by about 75 per cent. The government said the overflow released about 60 million tonnes of water from the barrier lake, which held an estimated 91 million tonnes – enough to fill about 36,000 Olympic-sized swimming pools.
The water hit like a tsunami, said Guangfu postman Hsieh Chien-tung, who was able to flee to the second floor of the post office just in time. Later, he got home to find that his car had been swept into the living room.
Typhoon Ragasa brought heavy rainfall
The mountainous region of Taiwan saw heavy rainfall, according to satellite analysis data.
This shows a map with rainfall patterns in circular motions along the typhoon path but also highlights a strong accumulation of rain along the mountains in Taiwan, where the lake burst.
The lake’s remote, hazardous location has hampered the authorities’ efforts to lower the water level and clear the rubble.
Some parts of the mountains in Hualien remain unstable after a 7.2-magnitude quake – Taiwan’s biggest earthquake in at least 25 years – hit the region in 2024.
Strongest storm of the year
Ragasa formed over the Western Pacific last week. Fuelled by warm seas and favourable atmospheric conditions, the tropical cyclone rapidly intensified to become a Category 5 super typhoon on Sept 22, with winds exceeding 260kmh.
It has since weakened, but was still powerful enough to bring down trees and power lines.
“The authorities have taken lessons from (typhoons) Hato and Mangkhut, which both caused billions of dollars in damage in 2017 and 2018,” said Mr Chim Lee, a senior energy and climate change specialist at the Economist Intelligence Unit.
How Ragasa’s windspeed compares with other typhoons
Maximum sustained wind speeds (1-minute mean)
There are two line charts. The first one shows how Ragasa has the strongest winds of 2025, compared with other category 4 and 5 cyclones. The second line chart compares it with other category 4 and 5 typhoons that made landfall in China, Ragasa is not as fast or as sustained as Meranti or Mangkhut.
After passing around 100km south of Hong Kong, Ragasa made landfall along the south Chinese coast. Guangzhou, Shenzhen, Foshan and Dongguan, the largest cities in the storm’s path, are home to around 50 million people.
More than 100 people were injured in Hong Kong, where the authorities imposed the highest typhoon signal 10 for most of Sept 24.
Before Ragasa’s arrival, the authorities handed out sandbags on Sept 22 for residents to bolster their homes against flooding in low-lying areas, while many people stockpiled daily necessities, leading to bare supermarket shelves and surging fresh vegetable prices.
In the gambling hub of Macau, next to Hong Kong, casinos were forced to shutter their gambling areas. One user on China’s Xiaohongshu app showed videos of doors being sealed at a casino resort for protection against gales and debris.
Strong winds reached up to 280kmh
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Hong Kong’s airport authority said airlines would gradually resume flights starting from 6am on Sept 25, with all three runways operating simultaneously.
More than two million people were evacuated in Guangdong, and the Chinese ministry for emergencies dispatched tens of thousands of tents, folding beds, lighting equipment and other rescue supplies, state media reported.
Storm surges in Hong Kong, southern China
In Hong Kong, where huge waves crashed over areas of the Asian financial hub’s eastern and southern shorelines, some roads and residential properties were submerged.
At the Fullerton Hotel on the island’s south, videos on social media showed seawater surging through glass doors. No injuries have been reported, the hotel said.
China’s marine authority issued its highest “red wave” warning for the first time in 2025, forecasting storm surges of up to 2.8m in parts of Guangdong province, as Ragasa headed towards the densely populated Pearl River Delta.
Sea levels rose up to 3m along the coast
A high tide at the same time also impacted water levels.
A map of China, Hong Kong, Taiwan and the Philippines. There are yellow data points ranging from white for higher levels of water, to darker yellow for lower levels of water. Most of the area around southern Hong Kong, Macau and southern China show higher levels of water.
“We live on an upper floor and saw that there wasn’t too much danger, so I brought the kids out to experience the heavy rain and wind,” a 40-year-old Shenzhen resident, surnamed Liang, said. “We walked along the open road to make sure to stay safe.”
A crowd chasing the storm under Shenzhen Bay Bridge was moved on by traffic police.
“The typhoon was really intense, but I’ve not been out long,” said an electric scooter delivery driver named Tim, who was using his vehicle to assess the damage.
A woman and her five-year-old son were swept into the ocean on Sept 23 after watching the typhoon from the Hong Kong waterfront, according to the South China Morning Post, which said they were now in intensive care at a hospital.
The typhoon injured at least 90 people, the Hong Kong hospital authority said, and the government opened 50 temporary shelters that provided refuge to 885 people.
On Sept 25, the Hong Kong Observatory maintained its second-lowest typhoon signal 3, keeping kindergartens and some schools shut as Ragasa moved away from the city and weakened into a tropical storm.
In Guangfu, Taiwan, soldiers operating from an armoured personnel carrier to keep clear of thick mud in the streets went door to door handing out water and instant noodles. Wrecked cars and scooters lay abandoned in the mud.
As heavy rain continued on and off in Hualien, police cars sounded sirens for a new flood warning in Guangfu on Sept 24, sending people scrambling for safer areas as residents and rescuers shouted: “The floodwaters are coming, run fast.”
Disaster command centre deputy chief Huang Chao-chin said with rainfall easing and much of the water from the lake already released, he did not expect a repeat of the mass flooding on Sept 23.