Typhoon Koinu intensifies as it approaches southern Taiwan, southeastern China

In the West Pacific basin, a robust typhoon is tracking northwestward across the northwestern Philippine Sea, threatening locations across Taiwan, the northernmost tip of the Philippines and southeastern China with the impacts of heavy rain and gusty winds.

The center of Typhoon Koinu, known as Jenny in the Philippines, was churning a few hundred miles off of the southernmost coast of Taiwan Tuesday. It was packing maximum sustained winds equivalent to that of a Category 3 hurricane (111-129 mph, or 178-208 km/h) on the Saffir-Simpson Hurricane Wind Scale.

The Central Weather Administration in Taiwan issued sea and land typhoon warnings Monday and Tuesday, alerting residents of the heavy rain and wind expected from Koinu. Beaches were closed beginning Tuesday in Kenting National Park, located in southern Taiwan, in preparation for the incoming storm conditions.

Additionally, the Philippine Atmospheric, Geophysical and Astronomical Services Administration issued tropical cyclone warnings for several northern provinces in the Luzon region.

The life of Koinu began last Wednesday, Sept. 27, when a tropical zone of low pressure formed near Guam and the Northern Mariana Islands. By Friday, the feature organized into a tropical depression over the central Philippine Sea.

Over the course of the weekend, Koinu ramped up to tropical storm strength (maximum sustained wind speed of 39 to 73 mph, or 63-118 km/h) by Saturday and reached Category 1 hurricane status (maximum sustained wind speed of 75-95 mph, or 120-152 km/h) by Sunday.

Between Sunday and Monday, the cyclone met the qualifications for rapid intensification as it trekked westward across the Philippine Sea, with equivalent strengthening from a Category 1 to a Category 3 hurricane. Rapid intensification is defined as a 35 mph (55 km/h) increase in the maximum sustained winds of a tropical cyclone over the course of 24 hours.

Another recent cyclone in the West Pacific to undergo rapid intensification was Typhoon Haikui, named Hanna by the Philippines, which moved across the basin from Aug. 27 to Sept. 6. The storm had a similar path to the projection of Koinu. It moved westward across the northern Philippine Sea and slammed into southern Taiwan and southwestern China. Haikui underwent rapid intensification less than 24 hours before making landfall, while Koinu is expected to gradually lose wind intensity as it nears Taiwan and southeastern China.

Typhoon Haikui was the first storm to make landfall in mainland Taiwan in four years, after Bailu in 2019.

Over the upcoming week, Koinu is forecast to slowly track across the northwestern Philippine Sea, approaching southern Taiwan and eventually locations in far southeastern China.

"Koinu is expected to turn more toward the west Tuesday night or Wednesday, local time, and is expected to pass over or near the Hengchun Peninsula of Taiwan late Wednesday night or Thursday morning, local time," AccuWeather Lead International Forecasters and Senior Meteorologist Jason Nicholls said.

AccuWeather meteorologists say that the storm may be equivalent to a Category 2 hurricane as it passes over southern Taiwan. However, bands of heavy rain and gusty winds had already arrived or were closely approaching parts of Batanes, Babuyan Islands, Taiwan, and northern Luzon, the largest and most populated island of the Philippines, by Tuesday night, local time.

• Have the app? Unlock AccuWeather Alerts&trade with Premium+

A general 1-2 inches (25-50 mm) of rain will spread across the northern Luzon region of the Philippines, western and northern Taiwan and parts of southeastern China. Higher rainfall amounts ranging from 2-4 inches (50-100 mm) can occur along the upslope regions in eastern Taiwan and along the coast of southeastern China, while 4-8 inches (100-200 mm) can spread along the far southeastern and east-central coast of Taiwan as the typhoon passes along the southern half of the country.

Rainfall totals can range between 8-12 inches (200-300 mm) across the far southeastern tip of Taiwan with the AccuWeather Local StormMax&trade of 18 inches (450 mm). Heavy rain to this degree can bring impacts such as mudslides, road washouts, flash flooding and structural damage.

As the wind field from the storm expands, wind gusts between 40-60 mph (60-100 km/h) can spread from northern Taiwan to the northern Philippines and southeastern China, with gusts to 60-80 mph (100-130 km/h) possible across southern and eastern areas of Taiwan.

AccuWeather RealVue&trade Satellite view of Koinu on Tuesday, Oct. 3 swirling around the northwestern Philippine Sea.

"Wind gusts to 120 mph (180 km/h) can occur in far southern Taiwan and Batanes with the AccuWeather Local StormMax&trade of 155 mph (250 km/h)," Nicholls said.

The strongest wind gusts associated with Koinu can occur near the storm's center as it passes near southeastern Taiwan, AccuWeather forecasters say.

"Wind gusts of this magnitude can cause downed trees, power outages and structural damage. Due to the combination of rain and wind, we are forecasting a 2 on the AccuWeather RealImpact&trade Scale for Tropical Cyclones in Taiwan and the Batanes," stated Nicholls.

Impacts from Koinu may be felt across southeastern China as early as Friday, local time, as it gradually tracks to the west into the upper half of the South China Sea. Moisture and wind from Koinu are expected to reach Hong Kong later this week into the weekend as the feature slowly tracks westward.

Looking ahead, AccuWeather forecasters warn that conditions across the Philippine Sea can remain conducive for additional tropical development this weekend into early next week.

Chances remain low for tropical formation at this time, but any feature that forms is likely to track west-northwest or northwest. Interests from the Philippines to Taiwan and the Ryukyu Islands of Japan should remain alert for possible impacts later next week as a result.

The annual West Pacific Typhoon season takes place from May through October. Cyclone activity can still occur across the Pacific basin from November to April, although tropical development is less likely outside of these core months.