Tropical Storm Iselle weakened on Tuesday as it moved over the Eastern North Pacific Ocean west of Baja California. At 11:00 p.m. EDT on Tuesday the center of Tropical Storm Iselle was located at latitude 21.3°N and longitude 122.5°W which put the center about 815 miles (1310 km) west of the southern tip of Baja California. Iselle was moving toward the west-northwest at 9 m.p.h. (15 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 996 mb.
Tropical Storm Iselle weakened on Tuesday. An upper level trough west of the West Coast of the U.S. was producing southwesterly winds that were blowing toward the top of Iselle’s circulation. Those winds were causing moderate vertical wind shear. The vertical wind shear was causing Tropical Storm Iselle to weaken.
The vertical wind shear was also causing the distribution of thunderstorms in Tropical Storm Iselle to be asymmetrical. Thunderstorms were occurring in bands in the eastern side of Iselle’s circulation. Bands in the western side of Tropical Storm Iselle consisted primarily of showers and lower clouds. Storms near the center of Iselle generated upper level divergence that pumped mass away to the northeast of the tropical storm. The removal of mass in the upper levels was less than the convergence of mass in the lower levels of the atmosphere. So, the surface pressure increased on Tuesday.
The effects of the vertical wind shear also caused the distribution of wind speeds in Tropical Storm Iselle to be asymmetrical. Winds to tropical storm force extended out 230 miles (370 km) in the eastern side of Iselle’s circulation. Winds to tropical storm force extended out 140 miles (220 km) in the western side of Tropical Storm Iselle.
Tropical Storm Iselle will move through an environment that will be unfavorable for intensification during the next 24 hours. Iselle will move over water where the Sea Surface Temperatures are near 26°C. It will continue to move under the southern part of an upper level trough that is west of the West Coast of the U.S. The upper level trough will continue to produce southwesterly winds that will blow toward the top of Iselle’s circulation. Those winds will continue to cause moderate vertical wind shear. In addition, Iselle will move into a mass of drier air that is west of Baja California. Tropical Storm Iselle will continue to weaken on Wednesday because of the effects of the vertical wind shear and the drier air.
Tropical Storm Iselle will move around the southwestern part of a high pressure system that is over the Eastern North Pacific Ocean. The high pressure system will steer Iselle toward the west-northwest during the next 24 hours. On its anticipate track, Tropical Storm Iselle will move farther away from Baja California.
Elsewhere, Tropical Storm Lala continued to weaken slowly far to the northwest of Hawaii. At 11:00 p.m. EDT on Tuesday the center of Tropical Storm Lala was located at latitude 35.2°N and longitude 177.6°W which put the center about 470 miles (760 km) north of Kure Atoll. Lala was moving toward the west-northwest at 6 m.p.h. (10 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 995 mb.
