Former Typhoon Koto weakened back to a tropical storm over the South China Sea on Thursday. At 4:00 p.m. EST on Thursday the center of Tropical Storm Koto was located at latitude 12.3°N and longitude 113.5°E which put the center about 435 miles (705 km) southeast of Da Nang, Vietnam. Koto was moving toward the south-southwest at 5 m.p.h. (8 km/h). The maximum sustained wind speed was 70 m.p.h. (110 km/h) and there were wind gusts to 85 m.p.h. (135 km/h). The minimum surface pressure was 986 mb.
Former Typhoon Koto moved into a region of drier air and weakened back to a tropical storm over the South China Sea on Thursday. A high pressure system over China was pumping drier air over the South China Sea. Many of the thunderstorms in Tropical Storm Koto weakened when the drier air was pulled into Koto’s circulation. Most of the remaining thunderstorms were occurring in bands in the northwestern part of Tropical Storm Koto. Bands in the other parts of Koto’s circulation consisted primarily of showers and lower clouds.
Storms near the center of Tropical Storm Koto generated less upper level divergence. The removal of mass in the upper levels of Koto’s circulation was less than the inflow of mass in the lower levels of the atmosphere. As a result, the surface pressure was increasing.
The distribution of wind speeds in Tropical Storm Koto continued to be very asymmetrical. Winds to tropical storm force extended out 205 miles (335 km) in the northern side of Koto’s circulation. Winds to tropical storm force extended out 70 miles (110 km) in the southern side of Tropical Storm Koto.
Tropical Storm Koto will move through an environment that will be unfavorable for intensification during the next 24 hours. Koto will move over water where the Sea Surface Temperatures are near 29°C. It will move under the southern part of an upper level ridge that is over the Western North Pacific Ocean. The upper level ridge will produce southeasterly winds that will blow toward the top of Koto’s circulation. Those winds will cause moderate vertical wind shear. In addition, Tropical Storm Koto will continue to move through the region of drier air that is over the western part of the South China Sea. Moderate vertical wind shear and the effects of drier air will cause Tropical Storm Koto to continue to weaken during the next 24 hours.
Tropical Storm Koto will be in a region where the steering currents are weak during the next 24 hours. On its anticipated track, Tropical Storm Koto is likely to meander slowly over the South China Sea on Friday.