Typhoon Nepartak Threatens Taiwan
Current location: 20.1°N, 127.0°E (at 12:00 UTC on July 6, 2016)
Movement: West-northwestward at 30 km/h
Maximum sustained winds: 216 km/h (134 mph)
Minimum pressure: 900 mb
Forecast track and intensity: A continuation west-northwestward is expected through Thursday, followed by a turn northward on Friday
Typhoon and tropical storm warnings: Friday morning in Taiwan, with a possible second landfall Saturday morning in eastern China
Other hazards: Damaging winds; flooding and mudslides from excessive rainfall in excess of 100 mm; significant coastal flooding from possibly significant storm surge
Dominant construction type(s) in area: Commercial buildings are reinforced concrete and steel, although there is still a significant percentage of masonry; Residential buildings are low- to mid-rise and usually have reinforced concrete frames with brick infill walls, although some masonry buildings can also be found; Mid- and high-rise apartment buildings and three-story street houses (tow tien) are generally made of reinforce concrete or steel, and have commercial establishments on the first floor with residences above
Expected damage at forecast wind speeds: Wind-borne debris in the wind field will cause destructive nonstructural damage to roof and wall claddings of engineered structures, but some structural damage will also occur; Non-engineered structures within the path of the highest winds will experience significant structural damage
Over the weekend, Nepartak formed to the south of Guam and strengthened to typhoon status on Monday as the first named storm in the western North Pacific basin since mid-December 2015, setting a new record for the longest period (200 days) without tropical storm in the basin in 66 years.
In just 48 hours, Nepartak rapidly intensified from a tropical storm to a typhoon with maximum sustained winds of 216 km/h (134 mph)—equivalent to a Category 4 hurricane on the Saffir-Simpson scale—and a minimum central pressure of 900 mb. It is currently located at 20.1°N, 127.0°E, to the southeast of Taiwan, in an area of high ocean heat content and low vertical wind shear, which has contributed to its rapid intensification. Satellite imagery shows a clearly defined eye and good upper-level outflow. It is expected to make landfall early Friday before turning northward toward eastern China.
The Northwest Pacific Basin produces more tropical cyclones each year than anywhere else in the world. On average, this basin spawns 29 tropical cyclones each year—20 of which reach typhoon status. This high frequency can be primarily attributed to the extremely large expanse of very warm water in the basin. In Southeast Asia (i.e., Guam, Hong Kong, Macau, the Philippines, Saipan, Taiwan, and Vietnam), Taiwan experiences the third-highest number of landfalling tropical cyclones annually—two per year on average—after the Philippines (first) and Vietnam (second).
Satellite image of Typhoon Nepartak. (Source: NOAA)
Nepartak is forecast to continue intensification, potentially reaching the equivalent of a Category 5 hurricane. However, it is moving into an environment with cooler ocean temperatures than its current location, which is projected to contribute to weakening prior to landfall. Model consensus continues to point to a Taiwan landfall early Friday in Hualien or Taitung County. Nepartak is projected to bring torrential rains to Taiwan, with 100 mm of rainfall anticipated. Significant storm surge is also projected, as waves of up to 13 meters have already been reported at sea. However, inland penetration will be mitigated by the steep elevation along the island’s east coast. Flooding, landslides, wind-borne debris, coastal flooding, and damage or interruption to infrastructure (power, transportation systems, etc.) is expected.
Movement over the mountainous terrain of Taiwan will weaken the storm before it has a chance to turn northward and make a potential second landfall in eastern China on Saturday. There still exists considerable uncertainty in the location and intensity of the possible impact to China.
Track map of Typhoon Nepartak. (Source: JMA)
Typhoon warnings have been issued for counties on the eastern side of Taiwan. An emergency center is set to be operational beginning Wednesday evening according to a spokesperson from the Taiwan Central Emergency Operation Center. Food prices in Taiwan have already begun to soar as people have started their own emergency preparations.
Exposure at Risk
While high winds typically cause the most tropical cyclone damage in other parts of the world, rain-induced flooding often causes the most damage in the Northwest Pacific Basin. Because tropical cyclones typically weaken rapidly upon interaction with the central mountain range in Taiwan and the eastern coast of the island is relatively unpopulated, heavy wind losses tend to be restricted to the east coast of Taiwan, including the areas of Taitung and Hualien. Strictly enforced building codes throughout the nation have resulted in structures with generally good wind resistance.
The country faces a serious threat from flooding, which is greatest along the southwestern mountains due to orographic lifting combined with occasional South China Sea monsoon-scale influences. The heavily urbanized areas of Taipei and Kaohsiung have sustained heavy flood damage during typhoons in the past, including the Tyhpoons Nari (2001) and Morakot (2009), both of which produced unprecedented levels of flooding.
Damage to property in Taiwan is generally distributed equally between wind and flood, but storm surge also has some appreciable contributions. Storm surge is significant in the west coast of Taiwan, and could heavily affect Taipei area and Kaohsiung area through Tamsui River and Love River. The deep bathymetry of the east coast minimizes storm surge risk on that side of the island.
Residential take-up rates in Taiwan are low and commercial take-up rates vary. The population density along the eastern coast—the region most vulnerable to flood—is quite high. Such high concentrations of people increase the risk of loss of life and destruction of property. Industry impacts will depend on the direction and intensity of Typhoon Nepartak at landfall, as well as duration post-landfall.
The AIR tropical cyclone team is monitoring Typhoon Nepartak and will provide updates as warranted.