Tropical Storm Ema formed southwest of Hawaii on Saturday. At 5:00 p.m. EDT on Saturday the center of Tropical Storm Ema was located at latitude 21.1°N and longitude 163.9°W which put it about 245 miles (395 km) southeast of French Frigate Shoals. Ema was moving toward the northwest at 13 m.p.h. (20 km/h). The maximum sustained wind speed was 50 m.p.h. (80 km/h) and there were wind gusts to 65 m.p.h. (105 km/h). The minimum surface pressure was 1003 mb.
A Tropical Storm Watch has been issued for the Papahanaumokuakea Marine National Monument from Nihoa to French Frigate Shoals to Maro Reef.
A distinct low level center of circulation formed in a small area of low pressure southwest of Hawaii on Saturday morning and the Central Pacific Hurricane Center designated the system as Tropical Storm Ema. The circulation around Tropical Storm Ema was very small. Winds to tropical storm force extended out 45 miles (75 km) from the center of circulation. Several short bands of showers and thunderstorms developed around the center of Ema. Storms near the center began to generate upper level divergence which pumped mass away from the tropical storm.
Tropical Storm Ema will move through an environment marginally favorable for intensification during the next 12 to 24 hours. Ema will move over water where the Sea Surface Temperature is near 28°C. An upper level trough west of Hawaii will produce southerly winds which will blow toward the top of the circulation. Those winds will cause moderate vertical wind shear which inhibit the intensification of Tropical Storm Ema. Ema could strengthen a little during the next 12 to 24 hours. The upper level trough will produce stronger southwesterly winds on Sunday and the wind shear will increase. Stronger wind shear will weaken Tropical Storm Ema in a day or so. The upper level winds could blow the top half of Ema north of the low level circulation and the tropical storm could weaken very quickly if that happens.
The upper level trough will steer Tropical Storm Ema toward the north during the next 12 hours or so. If the wind shear separates the upper and lower parts of the tropical storm, the low level center will be steered by winds closer to the surface. Clockwise flow around a subtropical high pressure system centered northeast of Hawaii could steer Tropical Storm Ema more toward the northwest later on Sunday.