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Tropical Cyclone Donna Turns South As Eye Forms

An eye formed at the center of Tropical Cyclone Donna as it turned southward on Saturday.  At 11:00 p.m. EDT on Saturday the center of Tropical Cyclone Donna was located at latitude 14.1°S and longitude 164.1°E which put it about 390 miles (625 km) northwest of Port Vila, Vanuatu.  Donna was moving toward the south at 5 m.p.h. (8 km/h).  The maximum sustained wind speed was 105 m.p.h. (170 km/h) and there were wind gusts to 120 m.p.h. (205 km/h).  The minimum surface pressure was 965 mb.

A primary rainband finally wrapped around the center of circulation on Saturday and a circular eye appeared on visible satellite imagery.  Additional bands of showers and thunderstorms revolved around the core of Tropical Cyclone Donna.  Thunderstorms in the core of the tropical cyclone generated strong upper level divergence which pumped out mass.  Winds to hurricane/typhoon force extended out about 30 miles (50 km) from the center of circulation.  Winds to tropical storm force extended out about 185 miles (295 km) from the center.

The Hurricane Intensity Index (HII) for Tropical Cyclone Donna was 17.8.  The Hurricane Size Index (HSI) was 14.4 and the Hurricane Wind Intensity Size Index (HWISI) was 32.2.

Tropical Cyclone Donna will move through an environment favorable for intensification for another 24 to 36 hours.  Donna will move over water where the Sea Surface Temperature is near 29°C.  Donna is moving near the western end of a upper level ridge and there are light northerly winds blowing toward the top of the circulation.  There is little vertical wind shear.  Tropical Cyclone Donna is likely to intensify during the next 24 hours.  The formation of an eye and a more organized inner core could allow Tropical Cyclone Donna to convert energy more efficiently and a period of rapid intensification is possible.  Tropical Cyclone Donna is likely to intensify into the equivalent of a major hurricane during the next day or so.

Tropical Cyclone Donna moved slowly westward as it neared the western end of a subtropical ridge.  Donna appears to have reached the end of the ridge and it has turned southward.  The tropical cyclone is likely to move south for another 24 hours.  A upper level trough will approach Donna from the west in about a day and northwesterly winds in the leading side of the trough are likely to push Donna southeast at a faster speed.  The westward movement of Tropical Cyclone Donna during the past several days reduced the potential threat to Vanuatu.  However, that same motion increased the potential threat to New Caledonia.

Rainbands in the eastern half of Tropical Cyclone Donna are likely to bring gusty winds and locally heavy rain to Vanuatu, but the core and strongest winds are likely to stay west of those islands.  On its anticipated track Tropical Cyclone Donna could approach New Caledonia in 36 to 48 hours.  The core of Tropical Cyclone Donna could pass over the Iles Loyaute.

Stronger Tropical Cyclone Donna Moves North of Vanuatu

Tropical Cyclone Donna strengthened on Thursday as it moved north of Vanuatu.  The center of Donna passed near the Banks Islands and the Torres Islands and it is currently located northwest of Espiritu Santo.  At 11:00 p.m. EDT on Thursday the center of Tropical Cyclone Donna was located at latitude 13.2°S and longitude 166.0°E which put it about 350 miles (560 km) north-northwest of Port Vila, Vanuatu.  Donna was moving toward the west-southwest at 7 m.p.h. (11 km/h).  The maximum sustained wind speed was 100 m.p.h. (160 km/h) and there were wind gusts to 120 m.p.h. (195 km/h).  The minimum surface pressure was 967 mb.

A partial eyewall continued to wrap around the center of circulation, but a well defined eye did not form.  A band of thunderstorms extended about two thirds of the way around the center.  Other bands of showers and thunderstorms were revolving around the core of the circulation.  Thunderstorms near the core of the circulation generated upper level divergence that pumped out mass to the southeast of Tropical Cyclone Donna.

Winds to hurricane/typhoon force extend out about 40 miles (65 km) from the center of circulation and winds to tropical storm force extend out about 140 miles (225 km) from the center.  The Hurricane Intensity Index (HII) is 16.5.  The Hurricane Size Index (HSI) is 13.4 and the Hurricane Wind Intensity Size Index (HWISI) is 29.9.

Tropical Cyclone Donna will move through an area that will be somewhat favorable for intensification.  Donna will move over water where the Sea Surface Temperature is near 30°C.  An upper level ridge east of Donna is producing northerly winds which are blowing toward the top of the tropical cyclone.  The upper level winds are generating moderate vertical wind shear and the shear seems to have slowed the intensification of Donna.  The strength of the upper level winds could decrease and intensification could resume on Friday.  Tropical Cyclone Donna could intensify into the equivalent of a major hurricane during the next 24 to 48 hours.

A subtropical ridge to the south of Donna is steering the tropical cyclone toward the west-southwest and that general motion is expected to continue for about another 24 hours.  Tropical Cyclone Donna will reach the western end of the ridge in about a day or so and it will turn toward the south.  On its anticipated track the center of Tropical Cyclone Donna will pass west of Espiritu Santo.  Guidance from the numerical models suggests the center of Donna could pass between Vanuatu and New Caledonia, but different models are predicting different tracks and so there is more uncertainty in the track at that time.

Tropical Cyclone Donna Moves Toward Vanuatu and Strengthens

Tropical Cyclone Donna moved toward Vanuatu and strengthened into the equivalent of a hurricane/typhoon on Wednesday.  At 8:00 p.m. EDT on Wednesday the center of Tropical Cyclone Donna was located at latitude 12.4°S and longitude 168.2°E which put it about 270 miles (435 km) north-northeast of Port Vila, Vanuatu.  Donna was moving toward the west at 11 m.p.h. (17 km/h).  The maximum sustained wind speed was 85 m.p.h. (135 km/h) and there were wind gusts to 100 m.p.h. (160 km/h).  The minimum surface pressure was 976 mb.

The circulation of Tropical Cyclone Donna organized further on Wednesday.  An eyelike feature appeared on some microwave satellite images.  An eyewall wrapped about two thirds of the way around the western side of the center of circulation.  A primary rainband curled around the southern and western portions of the core of Tropical Cyclone Donna.  Additional bands of showers and thunderstorms formed and revolved around the core of Donna.  Thunderstorms around the core generated strong upper level divergence which pumped out mass and allowed the pressure to decrease quickly.  A stronger pressure gradient force accelerated the air and resulted in faster wind speeds.

Tropical Cyclone Donna will continue to move through a favorable environment during the next several days.  Donna will move over water where the Sea Surface Temperature is near 30°C.  It is currently in an area where the upper level winds are weak and there is little vertical wind shear.  The combination of warm water and little shear will produce further strengthening and Tropical Cyclone Donna could continue to intensify rapidly.  Donna could intensify into the equivalent of a major hurricane within 24 to 48 hours.

Donna is moving around the northern side of a subtropical ridge which is steering the tropical cyclone toward the west.  A general westerly motion is expected to continue for another 24 hours.  After that time Tropical Cyclone Donna will approach the western end of the ridge and it will turn more toward the south.  Guidance from numerical models differs on the timing and location of the turn toward the south.  On its anticipated track the center of Tropical Cyclone Donna could approach some of the islands of northern Vanuatu in about 24 hours.  It could be a significant tropical cyclone at that time.

Tropical Cyclone 18P Develops Northeast of Vanuatu

Tropical Cyclone 18P developed quickly northeast of Vanuatu on Tuesday.  At 8:00 p.m. EDT on Tuesday the center of Tropical Cyclone 18P was located at latitude 12.7°S and longitude 171.7°E which put it about 695 miles (1120 km) north-northeast of Port Vila, Vanuatu.  The tropical cyclone was moving toward the east at 4 m.p.h. (6 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 992 mb.

The circulation of Tropical Cyclone 18P organized quickly on Tuesday.  More thunderstorms formed near the center of circulation and a  primary rainband wrapped around the eastern side of the center.  Additional bands of showers and thunderstorms developed in the outer regions of the circulation.  Thunderstorms near the core of the circulation generated upper level divergence which pumped out mass and allowed the surface pressure to decrease.

Tropical Cyclone 18P will be moving through an environment that will be very favorable for intensification.  It will move over water where the Sea Surface Temperature is near 30°C.  Tropical Cyclone 18P is centered underneath an upper level ridge and the upper level winds are light.  There is little vertical wind shear.  Tropical Cyclone 18P is likely to intensify rapidly and it could become the equivalent of a hurricane/typhoon within 24 hours.  Once an eye and eyewall form, Tropical Cyclone 18P could intensify very rapidly and it could become the equivalent of a major hurricane within 48 to 72 hours.

Tropical Cyclone 18P is moving through an area where the winds are the steering level are weak.  It has been moving slowly toward the east.  A subtropical ridge to the south of Tropical Cyclone 18P is expected to strengthen and it will turn the tropical cyclone toward the west for about 48 to 72 hours.  When Tropical Cyclone 18P nears the western end of the ridge, it will turn toward the south.  On its anticipated track, Tropical Cyclone 18P could approach Vanuatu in about three days and it could be a significant tropical cyclone at that time.

Tropical Cyclone Frances Intensifies Into Equivalent of a Hurricane

Tropical Cyclone Frances intensified into the equivalent of a hurricane/typhoon on Friday as it moved northwest of Australia.  At 5:00 p.m. EDT on Friday the center of Tropical Cyclone Frances was located at latitude 12.6°S and longitude 124.1°E which put it about 200 miles (325 km) north of Kuri Bay, Australia.  Frances was moving toward the west-southwest at 9 m.p.h. (14 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 981 mb.

Tropical Cyclone Frances intensified into the equivalent of a hurricane/typhoon when it moved through a favorable environment.  Frances moved over water where the Sea Surface Temperature (SST) was near 30°C and the upper level winds were weak.  A primary rainband wrapped almost entirely around the center of circulation and an eyelike feature seemed to be indicated on some satellite imagery.  Thunderstorms around the eye generated upper level divergence which pumped out mass and the circulation assumed a more symmetrical circulation.  Other bands of showers and thunderstorms formed in the outer portions of the circulation.

Tropical Cyclone Frances may have reached its peak intensity.  Frances will continue to move over water where the SST is near 30°C.  However, it is about to move near the western end of an upper level ridge where there are stronger northerly winds.  Those stronger winds will create much more vertical wind shear and Tropical Cyclone Frances is likely to weaken during the next several days.

A subtropical ridge to the east of Frances is steering the tropical cyclone toward the west-southwest and that general motion is forecast to continue for the next several days.  On its anticipated track Tropical Cyclone Frances poses no threat to Western Australia, although it could cause increased wave action along the coast.

Tropical Cyclone Frances Develops North of Western Australia

An area of low pressure northwest of Western Australia developed into Tropical Cyclone Frances on Thursday.  At 10:00 a.m. EDT on Thursday the center of Tropical Cyclone Frances was located at latitude 11.0°S and longitude 128.3°E which put it about 255 miles (410 km) north-northeast of Kalumburu, Australia.  Frances was moving toward the west-southwest at 12 m.p.h. (19 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. (110 km/h).  The minimum surface pressure was 997 mb.

The organization of Tropical Cyclone Frances improved significantly during the past 24 hours.  A well organized center of circulation developed at the surface.  A primary rainband wrapped about two-thirds of the way around the southern and western sides of the center.  Additional bands of showers and thunderstorms formed in the outer portions of the circulation.  Thunderstorms near the core of the circulation generated upper level divergence which was pumping mass away to the south of the tropical cyclone.  Frances is a fairly small tropical cyclone and winds to tropical storm force only extend out about 100 miles (160 km) from the center.

Tropical Cyclone Frances will move through an environment that will be favorable for intensification during the next 24 to 36 hours.  Frances will move over water where the Sea Surface Temperature is near 30°C while is moves across the Timor Sea.  An upper level ridge east of Tropical Cyclone Frances is producing northeasterly winds which are blowing toward the top of the circulation.  Vertical wind shear may be the reason that the primary rainband wrapped around the southern and western sides of the center.  However, the vertical shear does not appear to be strong enough to significantly affect the upper level divergence.  Frances is likely to intensify during the next day or so.  Eventually, Tropical Cyclone Frances will move into an area where there are strong upper level northwesterly winds.  A significant increase in vertical wind shear should weaken Frances when that occurs.

A subtropical ridge to the east of Frances is steering the tropical cyclone toward the west-southwest.  A general motion toward the west-southwest is expected to continue during the next few days.  On its anticipated track the center of Tropical Cyclone Frances is forecast to stay north of the coast of Western Australia.  However, any southward deviation of the track could bring stronger winds closer to the coast.

TD 03W Intensifies Into Tropical Storm Muifa

Tropical Depression 03W intensified into Tropical Storm Muifa on Tuesday as it moved slowly northwest of Yap.  At 11:00 p.m. EDT on Tuesday the center of Tropical Storm Muifa was located at latitude 13.3°N and longitude 134.6°E which put it about 335 miles (540 km) northwest of Yap.  Muifa was moving toward the west-northwest at 4 m.p.h. (6 km/h).  The maximum sustained wind speed was 45 m.p.h. (75 km/h) and there were wind gusts to 60 m.p.h. (95 km/h).  The minimum surface pressure was 1000 mb.

The circulation of Tropical Cyclone Muifa is not well organized.  There is a well defined low level center.  However, almost all of the showers and thunderstorms are in a cluster east of the low level center.  There are almost no showers and thunderstorms in the western half of the circulation.  Few well defined rainbands are evident in the eastern half of the circulation.  The cluster of thunderstorms east of the center of circulation is generating some upper level divergence which appears to be pumping mass out to the northwest of the tropical storm.

Tropical Storm Muifa will be moving through an environment that will be marginally favorable for intensification.  Muifa will move over water where the Sea Surface Temperature will be near 29.5°C.  A subtropical ridge east of Muifa is producing easterly winds which are blowing toward the tropical storm.  Tropical Storm Muifa is currently south of the upper level westerly winds in the middle latitudes.  Muifa is currently in a region where the upper level winds are weaker and the vertical wind shear is moderate.  Areas of stronger vertical wind shear surround the tropical storm.  Tropical Storm Muifa could intensify a little more during the next 24 hours before it reaches an area where the shear is stronger.

Tropical Storm Muifa is moving around the western end of the subtropical ridge to its east and the steering winds are weak.  As a result Muifa is moving slowly toward the west-northwest.  A gradual turn toward the north is forecast as Tropical Storm Muifa moves around the end of the ridge.  Muifa will reach an area of westerly winds as it moves farther north and the tropical storm is forecast to recurve toward the northeast.

Tropical Depression 03W Forms Northwest of Yap

Tropical Depression 03W formed northwest of Yap on Monday.  At 11:00 p.m. EDT on Monday the center of Tropical Depression 03W was located at latitude 12.5°N and longitude 135.2°E which put it about 260 miles (420 km) northwest of Yap.  The depression was moving toward the west 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 1003 mb.

A cluster of thunderstorms developed southeast of the Marianas during the weekend.  The cluster moved slowly toward the west-northwest and the circulation gradually exhibited signs of greater organization.  Satellite imagery indicated that a center of circulation formed in the lower levels of the circulation and the system was designated as Tropical Depression 03W on Monday.

The circulation of Tropical Depression 03W is still organizing.  Although there is a distinct low level center, the distribution of showers and thunderstorms is asymmetrical.  Many of the showers and thunderstorms are forming east of the center of circulation.  There are few showers and thunderstorms in the western half of the circulation.  Thunderstorms just to the east of the center of the circulation are generating some upper level divergence that is pumping out mass to the west and north of the center of circulation.

Tropical Depression 03W will be moving through an environment that will be somewhat favorable for intensification.  It will move over water where the Sea Surface Temperature will be near 30°C.  A ridge of high pressure east of the depression is producing easterly flow that is blowing toward the depression.  The flow may be pushing the lower part of the circulation to the west of the middle and upper portions.  Vertical wind shear could be an inhibiting factor.  Some intensification is forecast during the next day or two and the depression could intensify into a tropical storm.

The ridge of the depression is steering it toward the west.  The depression is expected to turn toward the northwest when it reaches the western end of the depression in a day or so.

TD 01 Intensifies Into Rare April Tropical Storm Arlene

Tropical Depression 01 intensified into Tropical Storm Arlene on Thursday afternoon.  Tropical storms rarely form over the Atlantic Ocean in April and Arlene is only the second storm known to do so during the satellite era.  Of course, it would have been much harder to detect tropical storms like Arlene prior to the use of geostationary satellites for meteorological monitoring.  At 5:00 p.m. EDT on Thursday the center of Tropical Storm Arlene was located at latitude 37.7°N and longitude 42.0°W which put it about 815 miles (1315 km) west of the Azores.  Tropical Storm Arlene was moving toward the west-northwest at 25 m.p.h. (41 km/h).  The maximum sustained wind speed was 45 m.p.h. (75 km/h) and there were wind gusts to 60 m.p.h. (95 km/h).  The minimum surface pressure was 936 mb.

A circular ring of showers and thunderstorms developed around the center of circulation of Subtropical Depression 01.  Additional narrow rainbands organized around the core of the circulation and the convection assumed a more circularly symmetrical shape.  In addition data from satellites indicated that a weak warm core had formed at the top of the circulation.  A more circular shape and a warm core are characteristics of a tropical cyclone and the National Hurricane Center changed the classification of Subtropical Depression 01 to Tropical Depression 01 in the 11:00 a.m. EDT advisory on Thursday.  Showers and thunderstorms continued to develop around the center of circulation and Tropical Depression 01 was upgraded to Tropical Storm Arlene in the 5:00 p.m. EDT advisory on Thursday.

Tropical Storm Arlene is in an environment that would not normally be considered favorable for a purely tropical weather system.  It is moving over water where the Sea Surface Temperature (SST) is near 19°C.  However, an upper low just to the west of Arlene contains colder air.  Colder air in the upper levels is providing sufficient instability to generate showers and thunderstorms even though the SST is relatively cool.  The cooler SST does mean that the showers and thunderstorms are not as tall as they would be if the water was warmer.  The upper low west of the system is producing southerly winds which are blowing over the top of Tropical Storm Arlene.  However, since the thunderstorms are not as tall, some of the stronger upper level winds are blowing over the top of Arlene’s circulation and the vertical wind shear is not having as much of an effect as might be expected.

The environment is marginal for further intensification, but the circulation looks fairly intact at the current time.  The cool SST is limiting the amount of energy that the tropical depression can extract from the ocean.  However, since the vertical wind shear is not having as much of a negative impact on the depression, some intensification may be possible during the next 24 hours.  Eventually, the wind shear is forecast to increase and Tropical Storm Arlene is forecast to weaken.

Tropical Storm Arlene is caught in the circulation of a large low pressure system to its west.  Arlene is forecast to make a counterclockwise loop as it moves around the circulation of the larger low pressure system.  It is possible that the larger low could absorb the circulation of Tropical Storm Arlene.

Subtropical Depression 01 Transitions to Tropical Depression 01

A low pressure system designated as Subtropical Depression 01 made a tropical transition to Tropical Depression 01 on Thursday morning.  At 11:00 a.m. EDT the center of Tropical Depression 01 was located at latitude 36.1°N and longitude 40.0°W which put it about 730 miles (1170 km) west of the Azores.  The tropical depression was moving toward the northwest at 14 m.p.h. (22 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 996 mb.

A circular ring of showers and thunderstorms developed around the center of circulation of Subtropical Depression 01.  Additional narrow rainbands organized around the core of the circulation and the convection assumed a more circularly symmetrical shape.  In addition data from satellites indicated that a weak warm core had formed at the top of the circulation.  A more circular shape and a warm core are characteristics of a tropical cyclone and the National Hurricane Center changed the classification of Subtropical Depression 01 to Tropical Depression 01 in the 11:00 a.m. EDT advisory on Thursday.

Tropical Depression 01 is in an environment that would not normally be considered favorable for a purely tropical weather system.  It is moving over water where the Sea Surface Temperature (SST) is near 19°C.  However, an upper low just to the west of the depression contains colder air.  Colder air in the upper levels is providing sufficient instability to generate showers and thunderstorms even though the SST is relatively cool.  The cooler SST does mean that the showers and thunderstorms are not as tall as they would be if the water was warmer.  The upper low west of the system is producing southerly winds which are blowing over the top of the depression.  However, since the thunderstorms are not as tall, some of the stronger upper level winds are blowing over the top of the depression’s circulation and the vertical wind shear is not having as much of an effect as might be expected.

The environment is marginal for further intensification, but the circulation looks fairly intact at the current time.  The cool SST is limiting the amount of energy that the tropical depression can extract from the ocean.  However, since the vertical wind shear is not having as much of a negative impact on the depression, some intensification may be possible during the next 24 hours.  If the depression intensifies into a tropical storm, then it would be named Arlene.  Eventually, the wind shear is forecast to increase and the depression is forecast to weaken.

Tropical Depression 01 is caught in the circulation of a large low pressure system to its west.  Tropical Depression is forecast to make a slow counterclockwise loop as it moves around the circulation of the larger low pressure system.  It is possible that the larger low could absorb the circulation of Tropical Depression 01.