Tag Archives: Johnston Atoll

Tropical Storm Olana Forms East of Date Line

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Tropical Storm Olana formed over the Central North Pacific Ocean just to the east of the International Dateline on Sunday.  At 5:00 p.m. EDT on Sunday the center of Tropical Storm Olana was located at latitude 14.8°N and longitude 178.0°W which put the center about 580 miles (935 km) west-southwest of Johnston Atoll.  Olana was moving toward the east-northeast at 8 m.p.h. (13 km/h).  The maximum sustained wind speed was 40 m.p.h. (65 km/h) and there were wind gusts to 50 m.p.h. (80 km/h).  The minimum surface pressure was 1002 mb.

A low pressure system over the Central North Pacific Ocean just to the east of the International Dateline strengthened on Sunday and the U.S. National Hurricane Center designated the system as Tropical Storm Olana.

More thunderstorms were forming near the center of Tropical Storm Olana.  Thunderstorms were also occurring in bands in the eastern side of Olana’s circulation.  Bands in the western side of Tropical Storm Olana consisted primarily of showers and lower clouds.  Storms near the center of Olana generated upper level divergence that pumped mass away to the east of the tropical storm.  The removal of mass caused the surface pressure to decrease.

The circulation around Tropical Storm Olana was small.  Winds to tropical storm force extended out 45 miles (75 km) from the center of Olana’s circulation.

Tropical Storm Olana will move through an environment that will be marginally favorable for intensification during the next 24 hours.  Olana will move over water where the Sea Surface Temperatures are near 28°C.  It will move under the southeastern part of an upper level trough that is over the North Pacific Ocean.  The upper level trough will produce southwesterly winds that will blow toward the top of Olana’s circulation.  Those winds will cause moderate vertical wind shear.  The vertical wind shear will inhibit intensification, but the wind shear may not be enough to prevent intensification.  Tropical Storm Olana could intensify a little during the next 24 hours, if the upper level winds to not get any stronger.

The upper level trough will steer Tropical Storm Olana toward the east-northeast during the next 24 hours.  On its anticipated track, Tropical Storm Olana will move closer to Johnston Atoll.

Hurricane Walaka Rapidly Intensifies to Category 5

Hurricane Walaka rapidly intensified to Category 5 on the Saffir-Simpson Scale on Monday.  At 11:00 p.m. EDT on Monday the center of Hurricane Walaka was located at latitude 13.2°N and longitude 169.8°W which put it about 240 miles (390 km) south of Johnston Atoll.  Walaka was moving toward the northwest at 7 m.p.h. (11 km/h).  The maximum sustained wind speed was 160 m.p.h. (260 km/h) and there were wind gusts to 195 m.p.h. (315 km/h).  The minimum surface pressure was 920 mb.

A Hurricane Warning is in effect for Johnston Atoll.  A Hurricane Watch is in effect for the Papahanaumokuakea Marine National Monument from Nihoa to French Frigate Shoals to Maro Reef.

The circulation of Hurricane Walaka is very well organized,  There is a circular eye at the center of Walaka.  The eye is surrounded by a ring of strong thunderstorms and the strongest winds were occurring in that ring of storms.  Several bands of showers and thunderstorms were revolving around the core of Hurricane Walaka.  Storms near the core were generating strong upper level divergence which was pumping mass away from the hurricane in all directions.

Winds to hurricane force extended out about 50 miles (80 km) from the center of Hurricane Walaka.  Winds to tropical storm force extended out about 185 miles (300 km) from the center of circulation.  The Hurricane Intensity Index (HII) for Hurricane Walaka was 35.0.  The Hurricane Size Index (HSI) was 16.4 and the Hurricane Wind Intensity Size Index (HWISI) was 51.4.

Hurricane Walaka will remain in an environment very favorable for strong hurricanes for several more days.  Walaka will move over water where the Sea Surface Temperature is near 30°C.  It will move through a region where the upper level winds will be weak and there will be little vertical wind shear.  If a rainband wraps around the existing eye and eyewall, then an eyewall replacement cycle could cause Hurricane Walaka to weaken.  In several days Walaka will move into an area where the upper level winds are stronger and the vertical wind shear will increase.  Hurricane Walaka will weaken more quickly when the shear increases.

Hurricane Walaka is moving around the western end of a subtropical high pressure system.  The high pressure system will steer Walaka toward the north during the next several days.  On its anticipated track Hurricane Walaka will pass near Johnston Atoll on Tuesday.