Tropical Depression 04W formed east of the Northern Marianas very late on Friday. At 11:00 p.m. EDT on Friday the center of Tropical Depression 04W was located at latitude 18.8°N and longitude 148.5°E which put it about 185 miles (300 km) east of Agrihan. It was moving toward the northwest at 11 m.p.h. (17 km/h). The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (75 km/h). The minimum surface pressure was 1003 mb.
An area of showers and thunderstorms persisted for much of the past week east of the Northern Marianas. Several smaller centers of circulation formed within the area of showers and storms, but those centers ran into strong upper level winds and weakened because strong vertical wind shear and they dissipated. Another center formed on Friday, but the upper level winds were a little weaker and the center persisted. An upper level trough south of Japan was still producing southwesterly winds which were blowing toward the top of the circulation. Those winds were producing moderate vertical wind shear, but the center of circulation persisted. Most of the thunderstorms were occurring in bands east of the center of circulation. The bands west of the center consisted primarily of low clouds and showers.
Tropical Depression will move through an area marginally favorable for intensification during the next 24 hours. it will move over water where the Sea Surface Temperatures are near 28°C. So, there will be enough energy in the upper ocean to support some intensification. The upper level trough south of Japan will continue to cause moderate vertical wind shear, but the upper level winds could be weak enough to allow for further intensification. Tropical Depression 04W could strengthen into a tropical storm on Saturday.
Tropical Depression 04W is moving near the southwestern part of a ridge over the North Pacific Ocean. The ridge is steering the depression toward the northwest and that general motion is expected to continue for another day or two. On its anticipated track Tropical Depression 04W is expected to pass near the northernmost Mariana Islands. It could pass near Asuncion Island, the Maug Islands, Supply Reef and Farallon de Pajaros. Tropical Depression 04W could bring gust winds, locally heavy rain and higher waves to those places.
Tropical Depression 04W formed west of Luzon on Saturday. At 11:00 p.m. EDT on Saturday the center of Tropical Depression 04W was located at latitude 16.6°N and longitude 116.8°E which put it about 430 miles (690 km) south-southeast of Hong Kong. The depression was moving toward the northwest at 10 m.p.h. (16 km/h). The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (75 km/h). The minimum surface pressure was 1006 mb.
An area of thunderstorms moved west-northwestward across the Philippines and a circulation slowly consolidated around a distinct center. The center of circulation became well enough defined on Saturday to cause the system to be classified as Tropical Depression 04W. The inner core of the depression is still organizing. A band of thunderstorms wrapped around the western and southern sides of the center of circulation. Additional bands of showers and thunderstorms developed in other parts of the depression. The area of showers and thunderstorms southwest of the center began to generate upper level divergence which pumped mass away from the depression.
Tropical Depression 04W will be moving through an environment that is favorable for intensification. The depression will move over water where the Sea Surface Temperature is near 30°C. The depression is moving under the eastern end of an upper level ridge. The ridge is producing northerly winds which are blowing toward the top of the circulation. The vertical wind shear may be part of the reason why there are more thunderstorms south of the center of circulation, but the shear is not strong enough to prevent intensification. Tropical Depression 04W is expected to intensify into a tropical storm during the next 24 hours.
Tropical Depression 04W is moving around the western end of a subtropical ridge which is steering it toward the northwest. A general northwesterly motion is expected for another 36 to 48 hours. A turn toward the east will occur after that time. On its anticipated track Tropical Depression 04W could be near Hong Kong in about 36 hours. Tropical Depression 04W will have strengthened into a tropical storm by that time and there is a chance it could intensify into a typhoon by the time it nears the coast of China.