Typhoon “Koinu” — known as Jenny in the Philippines — formed over the Pacific Ocean on September 27, 2023, as the 14th named storm of the 2023 Pacific typhoon season. The system is forecast to continue strengthening and make landfall in southern Taiwan on October 4.
Koinu underwent a period of rapid intensification by the evening of October 1 into the early morning hours, intensifying to a Category 3 typhoon whilst developing a clear eye as it moved east of Cagayan, Philippines. It then intensified further in the next few hours and reached Category 4 intensity.
Koinu is forecast to continue on a generally northwestward track under the steering influence of the deep-layer subtropical ridge (STR) centered to the east, JTWC meteorologists said at 09:00 UTC on October 2.
As evidenced by the asymmetric appearance in multispectral and microwave imagery, Koinu is experiencing increased mid-level vertical wind shear and dry air entrainment. Over the next 12 hours, these negative factors are generally negated by favorable outflow aloft, which is forecast to allow for additional intensification. By 09:00 UTC on October 3, however, after reaching a peak intensity near 205 km/h (125 mph), favorable equatorward outflow is lost. Coupled with increasing mid-level vertical wind shear and dry air entrainment, the system will begin to gradually weaken.
As the STR to the east builds westward, Koinu will be forced towards the southern tip of Taiwan as it continues to slowly weaken. After making landfall near 21:00 UTC on October 4, the mountainous eastern coast of Taiwan will quickly weaken Koinu to 150 km/h (92 mph) by 09:00 UTC on October 5. In the extended track, Koinu is forecast to slowly transit the Taiwan Strait as more cool dry air streams into the system from the north, resulting in continued weakening through 09:00 UTC on October 7 as the system approaches mainland China.
Very heavy rainfall, strong winds and storm surge are forecast over the whole of Taiwan Island on October 4 through 8 and over northern Luzon and the Batanes Islands in northern Philippines on October 2 through 6. PAGASA has issued a Tropical Cyclone Wind Signal #1 over this area.
1 Typhoon 14W (Koinu) – JTWC Warning #12 – Issued at 02/0900Z
Featured image: Typhoon “Koinu” at 06:50 UTC on October 2, 2023. Credit: JMA/Himawari-9, RAMMB/CIRA, The Watchers
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