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Tropical Cyclones Winston and Tatiana Form over SW Pacific

A pair of tropical cyclones named Winston and Tatiana formed over the southwestern Pacific Ocean on Thursday.  At 10:00 p.m. EST on Thursday the center of Tropical Cyclone Winston was located at latitude 18.4°S and longitude 171.6°E which put it about 445 miles (720 km) west of Suva, Fiji.  Winston was moving toward the south at 7 m.p.h. (11 km/h).  The maximum sustained wind speed was 75 m.p.h. (120 km/h) and there were wind gusts to 90 m.p.h. (145 km/h).  The minimum surface pressure was 974 mb.

At 10:00 p.m. EST on Thursday the center of Tropical Cyclone Tatiana was located at latitude 17.3°S and longitude 159.0°E which put it about 620 miles (1000 km) northwest of Noumea, New Caledonia.  Tatiana was moving toward the east-southeast at 6 m.p.h. (10 km/h).  The maximum sustained wind speed was 60 m.p.h. (95 km/h) and there were wind gusts to 75 m.p.h. (120 km/h).  The minimum surface pressure was 982 mb.

Tropical Cyclone Winston is the stronger and more well organized storm.  It has a well developed eye surrounded by a ring of strong thunderstorms.  Winston is in a very favorable environment.  It is moving over water where the Sea Surface Temperatures are near 31°C.  There is an upper level ridge over Winston and so the upper level winds are light and divergence is occurring in all directions.  Winston is likely to continue to intensify rapidly on Friday and it could become the equivalent of a major hurricane.

Tropical Cyclone Tatiana is smaller and not as well organized.  Tatiana is moving over water where the Sea Surface Temperatures are near 30°C.  However the western end of the same upper level ridge that is over Winston is generating brisk northerly winds over the top of Tropical Cyclone Tatiana.  As a result, vertical wind shear is inhibiting upper level divergence to the north of the center of Tatiana and it is also tilting the circulation toward the south with height.  Wind shear is likely to prevent Tatiana from intensifying significantly.

A subtropical ridge is steering both Winston and Tatiana toward the south.  Tropical Cyclone Winston will pass west of Fiji, but it could come close enough to the extreme southeastern islands of Vanuatu to bring wind and rain to some of those islands.  Tropical Cyclone Tatiana is likely to pass west of New Caledonia, but it could bring rain and higher surf to the west coast of that island.

Tropical Cyclone Raquel Reorganizing Near Solomon Islands

Strong upper level winds blew away the upper potion of Tropical Cyclone Raquel on Wednesday.  However, the upper level winds lessened on Thursday and thunderstorms began to reform on the western side of the circulation.  At 5:00 p.m. EDT on Thursday the center of Tropical Cyclone Raquel was located at latitude 6.4°S and longitude 160.5°E which put it about 220 miles (360 km) north of Honiara in the Solomon Islands.  Raquel was moving toward the south-southeast at 3 m.p.h. (5 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 994 mb.

Tropical Cyclone Raquel is over water where the Sea Surface Temperatures are very warm.  However, an upper level ridge east of Raquel generated strong winds over the top of the tropical cyclone on Wednesday and blew away the upper parts of the circulation.  The upper level winds weakened on Thursday and new thunderstorms began to develop west of the center of circulation.  Upper level divergence is also occurring in the western half of the circulation and Raquel looks like a tropical cyclone again.  The upper level winds are still strong enough to inhibit intensification, but Raquel could strengthen further in the short term.

When the top parts of the circulation were blown away on Wednesday, the lower level circulation was steered by winds in the lower troposphere and it moved toward the northeast.  As the thunderstorms redevelop, the height of the circulation is increasing and Raquel is more likely to be steered by winds in the middle troposphere.  Those winds are expected to steer Raquel back toward the southwest.  On its anticipated track Raquel could again approach Santa Isabel Island on Friday.  It could bring winds and locally heavy rain to portions of the Central Solomon Islands.

Tropical Cyclone Raquel Forms Near the Solomon Islands

A distinct low level center of circulation was detected inside a large area of thunderstorms north of the Solomon Islands and the system was designated as Tropical Cyclone Raquel (25P).  At 11:00 p.m. EDT on Tuesday the center of Tropical Cyclone Raquel was located at latitude 6.5°S and longitude 158.9°E which put it about 210 miles (335 km) north of Honiara in the Solomon Islands.  Raquel was moving toward the southwest at 11 m.p.h. (18 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 989 mb.

Raquel is over water where the Sea Surface Temperature is quite warm and there is plenty of energy in the upper ocean to support intensification.  However, and upper level ridge east of Raquel is northeasterly winds over the top of the circulation and it is creating moderate amounts of vertical wind shear.  The wind shear is causing many of the stronger thunderstorms to occur on the western side of the circulation.  The wind shear may be tilting the vertical structure of the circulation toward the southwest and it is limiting the potential for intensification.

As Raquel moves around the western end of the upper level ridge, it is expected to be steered toward the south on Wednesday.  On its anticipated track Raquel is expected to move across Santa Isabel on Wednesday.  It could be near Guadacanal in 36 to 48 hours.  Raquel is likely to bring some wind and locally heavy rain to parts of the central Solomon Islands during the next two days.

Tropical Cyclone Nathan Forms Over the Coral Sea

Another tropical cyclone formed in an elongated trough of low pressure over the southwestern Pacific Ocean.  At 10:00 p.m. EDT on Tuesday the center of Tropical Storm Nathan was located at latitude 13.8°S and longitude 146.8°E which put it about 150 miles northeast of Cooktown, Australia and about 250 miles south of Port Moresby, Papua New Guinea.  It was moving toward the west-southwest at 9 m.p.h.  The maximum sustained wind speed was 60 m.p.h. and there were wind gusts to 80 m.p.h.  The minimum surface pressure was 989 mb.

Nathan is over warm Sea Surface Temperatures, but it is being partly influenced by some of the upper level outflow from large Tropical Cyclone Pam which is located farther east.  The wind shear is reducing the rate of intensification, but the environment is favorable enough that continued intensification is likely during the next day or two.

The track forecast for Nathan has a high degree of uncertainty.  It is currently being steered to the west-southwest by a subtropical ridge that is north of the circulation center.  That ridge is predicted to weaken and westerly winds are expected to turn the tropical cyclone back toward the east.  However, the timing of that turn is important.  The center of Nathan could come very close to the coast of Queensland.  If the turn does not occur, Nathan could bring high winds and heavy rain to portions of northern Queensland.

 

Tropical Cyclone Ola Intensifying Northwest of New Caledonia

Tropical Cyclone Ola intensified rapidly on Saturday as it moved over the Coral Sea northwest of New Caledonia.  At 8:00 p.m. EST on Saturday the center of Ola was located at latitude 19.9°S and longitude 162.9°E which put it about 260 miles northwest of Noumea, New Caledonia.  Ola was moving toward the south-southeast at 8 m.p.h.  The maximum sustained wind speed was 80 m.p.h. and there were gusts to 95 m.p.h.  The minimum surface pressure was 970 mb.

Ola is moving over warm Sea Surface Temperatures (SSTs) and the environmental winds are generating upper level divergence to the southeast of the circulation.  The divergence pumped out mass and surface pressure decreased rapidly.  As a result, Ola intensified into the equivalent of a hurricane on Saturday.  Ola will remain over warm SSTs, but it will move into an area of stronger upper level winds when it moves farther southward.  It could intensify more during the next 24 hours, but it is likely to begin to weaken in 36 to 48 hours because of increasing vertical wind shear.

Counterclockwise flow around the western end of a subtropical ridge is steering Ola in a general southward direction.  That motion is expected to continue for another 24 hours.  A ridge of high pressure is expected to build south of Ola in a day or so.  The ridge could block Ola from moving any farther toward the south and turn it toward the southwest.  On the expected track, Ola would stay west of New Caledonia.  It is expected to weaken before it reaches Australia.