Weather Outside Your Window: Chanthu rapidly intensifies into 2nd super typhoon of 2021

By Mary Gilbert, AccuWeather meteorologist

Updated Sep. 9, 2021 8:31 AM EDT

AccuWeather forecasters say the West Pacific basin continues to remain active as Typhoon Chanthu exploded in intensity over the open waters of the Philippine Sea on Wednesday and became just the second super typhoon of 2021.

The Joint Typhoon Warning Center (JTWC) designated Chanthu as a super typhoon based on its one-minute average sustained wind speed. The first storm of the season to achieve super typhoon status was Super Typhoon Surigae, which roared to life in mid-April.

As of Thursday evening, local time, Chanthu was located over the open waters of the Philippine Sea, but its outer rain bands had begun to impact northern portions of the Philippines. Chanthu was tracking to the west-northwest at a speed of 12 mph (20 km/h), according to the Japanese Meteorological Agency (JMA). Chanthu’s 10-minute average sustained winds were 120 mph (193 km/h), meaning it was the equivalent of a Category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale.

Due to favorable conditions for development in the Philippine Sea, Chanthu was able to undergo rapid intensification Tuesday into Wednesday. Rapid intensification is defined as a tropical system gaining 30 knots (35 mph or 55 km/h) of sustained wind speed in 24 hours or less. Chanthu met the criteria by more than double, gaining 75 knots of wind intensity in just 24 hours, according to the JTWC.

Chanthu had developed a clear eye on satellite imagery on Wednesday, further indicating just how powerful the storm had become.

"Some additional strengthening is possible before the storm is expected to pass near Northern Luzon later Friday, local time," AccuWeather Senior Meteorologist and Lead International Forecaster Jason Nicholls said.

Despite the opportunity for some strengthening, Chanthu is forecast to retain its current designation as the equivalent of a Category 4 hurricane as it comes close to the northernmost portion of the Philippines into Saturday. However, AccuWeather forecasters caution there is a non-zero chance, given the favorable conditions for development, that Chanthu could briefly reach the equivalent of a Category 5 hurricane early in the weekend.

In the Philippines, Chanthu is known as Kiko, according to the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA).

The heaviest rainfall from Chanthu is forecast to stay offshore of the Philippines. Rainfall of 1-2 inches (25-50 mm) is possible over much of Luzon, but higher amounts are likely for northeastern portions of the island chain. Up to 4-8 inches (100-200 mm) can fall across eastern portions of Northern Luzon.

Some tropical storm-force wind gusts (gusts greater than 40 mph or 64 km/h) are possible over portions of Northern Luzon. Wind gusts over this magnitude may be enough to bring down some trees or power lines, especially in areas already oversaturated with rain.

Portions of Luzon were still dealing with impacts from Tropical Storm Conson into Wednesday night. Now the area will have to brace for additional tropical impacts just 24-36 hours after the exit of Conson.

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AccuWeather forecasters say the worst impacts from Chanthu will likely target Taiwan this weekend.

Chanthu is forecast to make landfall over central Taiwan late this weekend, likely as the equivalent of a Category 3 or 4 hurricane. Once the storm encounters the mountainous terrain of Taiwan, AccuWeather forecasters say it will likely lose wind intensity rather quickly.

Widespread rainfall amounts of 8-12 inches (200-300 mm) are likely over the eastern portion of Taiwan, with an AccuWeather Local StormMax™ of 24 inches (600 mm). Rainfall amounts in excess of 1 foot (300 mm) will likely be confined to higher-elevation areas due to the effects of upsloping.

Wind gusts will also be more robust for Taiwan than what can occur over the northern portions of the Philippines. Wind gusts of 100-120 mph (160-190 km/h) with an AccuWeather Local StormMax™ of 130 mph (210 km/h) are likely to impact the eastern portion of Taiwan. Widespread wind gusts of 80-100 mph (130-160 km/h) will overspread the rest of the country.

Wind gusts of this strength can be enough to do significant damage to well-built structures as well as take down trees and power lines.

After impacting Taiwan, Chanthu is forecast to emerge over the East China Sea early next week, likely as the equivalent of a Category 1 hurricane.

At this time, there is still a degree of uncertainty in Chanthu's track past Taiwan, forecasters say. Prolonged interaction with Taiwan could cause the storm to lose more wind intensity and take more of a northerly turn and skim the coastline of China early next week. A briefer interaction with Taiwan would allow Chanthu to continue generally westward and potentially make a landfall in China.