Monthly Archives: June 2015

Low Pressure Organizing Over the Southern Gulf of Mexico

A complex low pressure system over the southern Gulf of Mexico is showing signs of greater organization.  A reconnaissance aircraft investigated the system on Sunday afternoon and found evidence of a broad area of low pressure at the surface and winds to tropical storm force northeast of the low.  The official designation for this system is Invest 91L.  At 11:00 p.m. EDT on Sunday night the approximate center of the broad low was located at latitude 22.7°N and longitude 90.2°W which put it about 540 miles (870 km) south-southeast of Galveston, Texas and about 540 miles (870 km) east-southeast of Brownsville, Texas.  The low was moving toward the northwest at 14 m.p.h. (23 km/h).  The maximum sustained wind speed was 40 m.p.h. (65 km/h) and there were gusts to 50 m.p.h. (80 km/h).  The minimum surface pressure was 1007 mb.

Invest 91L is a complex system which includes an upper level low over the southern Gulf of Mexico and a broad area of low pressure at the surface.  Clusters of thunderstorms are forming intermittently east and north of the center of circulation.  The pressure gradient between a strong high pressure system centered over the Atlantic Ocean and the developing surface low is causing the strongest winds to be found in the northeastern part of Invest 91L.  The upper level low is causing the system is to exhibit a more subtropical mode of formation rather a typical tropical genesis.  However, more thunderstorms are beginning to develop closer to the center of circulation and the National Hurricane Center is giving Invest 91L an 80% chance of becoming a tropical cyclone.  The low is moving over water where the Sea Surface Temperature (SST) is around 27°C to 28°C, which is warm enough to support the development of a tropical cyclone.  The upper level low was generating vertical wind shear, but if the surface low develops under the center of the upper low, then the wind shear will have less of an effect.  If more thunderstorms form near the center of circulation and a warm core develops, then the low could be classified as a tropical storm on Monday.  Some intensification is possible because of the warm SSTs.

An upper level ridge centered over the southeastern U.S. is expected to steer the system toward the northwest and it could make landfall on the coast of Texas in 24 to 48 hours.  The primary risk will be locally heavy rainfall, although a minor storm surge could occur in some coastal locations.

Small Tropical Storm Carlos Moving West of Acapulco

Carlos weakened to a tropical storm as it moved west of Acapulco, Mexico on Sunday.  At 11:00 p.m. EDT on Sunday the center of Tropical Storm Carlos was located at latitude 16.7°N and longitude 101.3°W which put it about 95 miles (150 km) west of Acapulco and about 110 miles (175 km) southeast of Lazaro Cardenas.  Carlos was moving toward the northwest at 6 m.p.h. (9 km/h).  The maximum sustained wind speed was 70 m.p.h. (110 km/h) and there were wind gusts to 85 m.p.h. (135 km/h).  The minimum surface pressure was 998 mb.  Carlos is a small tropical cyclone.  The tropical storm force winds only extend out about 50 miles (80 km) from the center and it has a Hurricane Size Index (HSI) of 1.7.  The government of Mexico has issued a Hurricane Warning for the portion of the coast that extends from Punta San Telmo to Tecpan de Galeana.  A Hurricane Watch has been issued for the portion of the coast from Playa Perula to Punta San Telmo.

Carlos was nearly stationary for several days and its winds mixed cooler water to the surface.  In addition the circulation pulled in drier air from Mexico as it got closer to the coast.  The combined effects of cooler water and drier resulted in less energy to drive the circulation and Carlos weakened to a tropical storm on Sunday.  Late in the day it started to move toward the northwest and get away from the cooler water.  Carlos is moving over an area where the Sea Surface Temperature (SST) is near 30°C.  Recent satellite images indicate that stronger thunderstorms are forming near the center of circulation.  The flow around Carlos is still pulling in drier air from Mexico and there are fewer thunderstorms in the northern half of the tropical storm.  The small size of Carlos means that it can strengthen or weaken more quickly than a larger storm because there is less mass to accelerate.  The upper level winds over Carlos have lessened and there is not as much vertical wind shear.  If Carlos remains over water, it is likely to intensify back to a hurricane on Monday.

An upper level ridge over Mexico is moving slowly toward the east and it is starting to steer Carlos toward the northwest.  The ridge is expected to continue to steer the tropical storm toward the northwest or west-northwest on Monday.  As Carlos approaches the western end of the ridge is expected to turn more toward the west-northwest.  There is some uncertainty about where the turn will occur.  Guidance from some models have Carlos turning farther east and making a landfall between Manzanillo and Cabo Corientes, while some other models have the tropical storm moving farther west before turning.  Either scenario is plausible and hence, the high degree of uncertainty about the future track of Carlos.

Possible Tropical Development in Western Gulf of Mexico

An area of showers and thunderstorms near the Yucatan Peninsula has shown signs of a cyclonic circulation, although the rotation may be in the middle troposphere and not at the surface.  The formal designation of this system is Invest 91L.  At 2:00 p.m EDT on Saturday the center of Invest 91L was located at latitude 20.0°N and longitude 88.4°W which put it about 100 miles (165 km) southeast of Merida, Mexico and about 760 miles (1220 km) south-southeast of Galveston, Texas.  Invest 91L was moving toward the northwest at 5 m.p.h. (8 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. (70 km/h).  The minimum surface pressure was 1009 mb.  The National Hurricane Center is now giving Invest 91L a 50% chance of developing into a tropical depression or tropical storm in the next five days.  An additional reconnaissance plane has been scheduled to investigate it on Sunday afternoon.

The circulation of Invest 91L is not very well organized.  Almost all of the showers and thunderstorms are occurring in the eastern half of the circulation.  Visible satellite images indicate a possible center of cyclonic rotation over the Yucatan peninsula, but it is not clear if the rotation extends all of the way down to the surface.  Counterclockwise winds around an upper level low over the Gulf of Mexico are generating significant vertical wind shear over Invest 91L.  However, the upper level winds east of the upper low are diverging, which is enhancing rising motion and is supporting the development of the showers and thunderstorms in that area.  The upper level divergence is also causing the surface pressure to decrease slowly, which could lead to the formation of a surface center of low pressure.

When Invest 91L moves into the southern Gulf of Mexico, it will move over water where the Sea Surface Temperature is near 27°C to 28°C.  So, there is sufficient energy in the upper ocean to support the development of a tropical cyclone.  The key factor will be the interaction between the upper low and any surface center that forms.  If the surface center forms too close to the upper low, then vertical wind shear will likely inhibit development and the system will remain fairly weak.  If the surface low develops a little farther southeast of the upper low, then there will be less shear and more upper level divergence.  That could allow for more intensification.

Invest 91L is expected to move with the upper low as the upper low moves northward.  Most guidance from numerical models moves the system toward the north-northwest toward the Upper Texas Coast.  However, there will be above normal uncertainty in the track until a well defined center of circulation forms at the surface.

Interests in the western Gulf of Mexico should monitor the progress of Invest 91L.

Carlos Strengthens to a Hurricane

An eye developed in the center of Carlos and the National Hurricane Center upgraded it to a hurricane.  At 5:00 p.m. EDT on Saturday the center of Hurricane Carlos was located at latitude 15.2°N and longitude 100.0°W which put it about 120 miles (190 km) south of Acapulco, Mexico and about 240 miles (390 km) southeast of Lazaro Cardenas, Mexico.  Carlos was moving toward the north at 2 m.p.h. (4 km/h).  The maximum sustained wind speed was 85 m.p.h. (140 km/h) and there were gusts to 105 m.p.h. (170 km/h).  The minimum surface pressure was 986 mb.  The Hurricane Intensity Index (HII) is 12.7.  The Hurricane Size Index (HSI) is 10.3.  The Hurricane Wind Intensity Size Index (HWISI) is 23.0.  These indices suggest that Carlos is capable of producing regional serious wind damage.  The government of Mexico has issued a Hurricane Warning for the portion of the coast from Lazaro Cardenas to Tucpan de Galeana.  A Hurricane Watch is in effect for the portion of the coast from Punta San Telmo to Lazaro Cardenas.  A Tropical Storm Warning has been issued for the portion of the coast from Tecpan de Galeana to Punta Maldonado.

The circulation near the center of Carlos is more well organized than it was 12 hours ago and an eye is clearly apparent on visible satellite imagery.  The hurricane appears to still be pulling in some drier air from Mexico because there is a gap on the northern side of the eyewall.  However, the drier air is less of a factor than it was on Friday.  Carlos is over water where the Sea Surface Temperature (SST) is near 30°C.  At some point it’s lack of movement will cause its winds to mix cooler water to the surface but that has not happened yet.  An upper level ridge located to the north of Carlos is generating some easterly winds near the top of the hurricane, but upper level divergence has increased since yesterday.  The upper level divergence is pumping out mass, which is allowing the surface pressure to decrease and Carlos to strengthen.  Drier air and modest vertical wind shear will slow the intensification of Carlos, but the hurricane should get stronger in the short term while it remains over warm SSTs.

A combination of the upper level ridge north of Carlos and an upper low over the Gulf of Mexico is creating a zone of weak steering winds near Carlos, which is why it has moved so little.  Later this weekend the upper level low is forecast to move away and the ridge will move northeast of the hurricane.  The change in position will cause Carlos to move toward the northwest.  Most of the guidance from numerical models keep the center of Carlos west of the coast of Mexico, but some models are now bringing it much closer to land.  The change in the guidance prompted the issuance of new watches and warnings for portions of the Mexican coast.

Tropical Storm Carlos Stationary South of Acapulco

Tropical Storm Carlos moved very little on Friday.  At 5:00 p.m. EDT on Friday the center of Tropical Storm Carlos was located at latitude 14.7°N and longitude 100.6°W which put it about 160 miles (255 km) south-southwest of Acapulco and about 250 miles (405 km) south-southeast of Lazaro Cardenas, Mexico.  Carlos was stationary.  The maximum sustained wind speed was 60 m.p.h. (95 km/h) and there were gusts to 70 m.p.h. (115 km/h).  The minimum surface pressure was 997 mb.

The intensity of Carlos changed little on Friday and the organization of the circulation fluctuated during the day.  The core of the circulation exhibited greater organization during the afternoon when more thunderstorms developed near the center.  Carlos remains over water where the Sea Surface Temperature (SST) is near 30°C.  At times it appears as though Carlos may be pulling in some sinking drier air from Mexico, because most of the thunderstorms are developing in the southern portion of the circulation.  An upper level ridge north of Carlos is generating some northeasterly winds that are affecting the outflow on the northern side of the tropical storm.  However, an upper level low over the Gulf of Mexico may be enhancing some upper level divergence on the eastern side of the storm.  If Carlos were to remain stationary for several more days, its winds would mix up cooler water.  Carlos should not have as big an effect on the SSTs as Blanca did when it was stationary, because Carlos is weaker than Blanca was.  The intensity forecast is highly uncertain because the SSTs are favorable for intensification, but drier air and vertical wind shear would tend to inhibit strengthening.  The most likely outcome is modest intensification during the weekend.

The interaction of the upper ridge over Mexico and the upper low over the Gulf of Mexico is creating a zone of weak steering winds near Carlos, which is the reason it has been stationary.  Eventually, during the weekend, the upper ridge is expected to move eastward and begin to steer Carlos toward the northwest.  On its anticipated track the center of Carlos is expected to remain off the coast of Mexico, but it is expected to move parallel to the coast.  Any deviation of the track to the east could bring the center very near the coast of Mexico.

TD Three-E Intensifies and Is Now Tropical Storm Carlos

A tight center of circulation consolidated in Tropical Depression Three-E and it intensified on Thursday.  It is now designated as Tropical Storm Carlos.  At 11:00 p.m. EDT on Thursday the center of Tropical Storm Carlos was located at latitude 14.5°N and longitude 100.4°W which put it about 170 miles (270 km) south of Acapulco, Mexico and about 235 miles (380 km) south-southeast of Zihuatanejo.  Carlos was moving toward the north-northwest at 2 m.p.h. (4 km/h).  The maximum sustained wind speed was 60 m.p.h. (95 km/h) and there were gusts to 70 m.p.h. (115 km/h).  The minimum surface pressure was 997 mb.  The government of Mexico has issued a Tropical Storm Watch for the portion of the coast from Acapulco to Zihuatanejo.

Although a tight circulation developed at the center of Carlos, it is still not fully organized.  Thunderstorms are developing mainly to the south of the core of the circulation.  Some spiral bands are beginning to form in the outer part of Carlos.  However, there are not as many thunderstorms in the northern half of the circulation which may be an indication that some drier air from Mexico could be getting pulled into the circulation.   Carlos is moving over water where the Sea Surface Temperature is near 30°C and there is sufficient energy in the upper ocean to support intensification.  An upper level ridge over Mexico is generating northeasterly winds over the top of Carlos and the vertical wind shear is inhibiting the development of the tropical storm.  An upper level trough moving southward over the Gulf of Mexico could weaken the ridge for a day or two.  That could reduce the wind shear and allow Carlos to intensify into a hurricane.

As the upper level trough weakens the ridge, the steering currents will remain weak and Carlos could move slowly toward the coast of Mexico.  The ridge is forecast to strengthen over the weekend and if that happens it will steer Carlos toward the northwest.  On its anticipated track, Carlos will approach and then move parallel to the coast of Mexico.  Most of the guidance from numerical models keeps Carlos off the coast during the next few days, but it could get close enough to bring some wind to the coast.  That is the reason why the government of Mexico issued a Tropical Storm Watch for a portion of the coast.

Tropical Depression Three-E Forms South of Mexico

A low level circulation in a large area of thunderstorms south of Mexico showed evidence of more organization and the National Hurricane Center classified it as Tropical Depression Three-E on Wednesday.  At 11:00 p.m. EDT on Wednesday the center of Tropical Depression Three-E (TD3E) was located at latitude 12.9°N and longitude 99.7°W which put it about 275 miles (445 km) south of Acapulco, Mexico.  TD3E was moving toward the northwest at 10 m.p.h. (17 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. (70 km/h).  The minimum surface pressure was 1003 mb.

The circulation of TD3E is still organizing and there could be multiple smaller centers of circulation rotating around inside a larger, broader center of low pressure.  The tropical depression is moving over water where the Sea Surface Temperature is near 30°C, which means there is plenty of energy in the upper ocean to support intensification.  An upper level high ridge over Mexico is generating modest amounts of vertical wind shear over TD3E.  The wind shear could slow the organization and intensification of the depression, but it should not prevent it.  The rate of intensification could quicken after the circulation becomes more well organized.

The ridge over Mexico is currently steering TD3E toward the northwest.  Guidance from numerical models is inconsistent about the future track of the depression.  Some models predict the ridge over Mexico will weaken and that TD3E will move northward toward the coast.  Other models maintain the ridge and forecast the depression will move northwest and stay west of the coast of Mexico.  The current motion of TD3E may suggest that the latter scenario has a higher probability of being correct, but there is still significant uncertainty about its future track.

Tropical Cyclone Ashobaa Moving Slowly Toward Oman

Tropical Cyclone Ashobaa moved slowly westward toward Oman on Wednesday.  At 11:00 p.m. EDT on Wednesday the center of Tropical Cyclone Ashobaa was located at latitude 20.6°N and longitude 61.1°E which put it about 160 miles (260 km) east of Masirah Island, Oman.  Ashobaa was moving toward the west at 4 m.p.h. (6 km/h).  The maximum sustained wind speed was 50 m.p.h. (80 km/h) and there were gusts to 65 m.p.h. (105 km/h).  The minimum surface pressure was 992 mb.

Most of the strong thunderstorms are occurring south and west of the center Ashobaa.  An upper level ridge located over southern Asia is generating easterly winds over the top of the tropical cyclone.  Those easterly winds are producing moderate amounts of vertical wind shear and account for the asymmetric distribution of thunderstorms.  Ashobaa is moving over warm Sea Surface Temperatures and there is enough energy in the upper ocean to support intensification.  However, the vertical shear is strong enough to inhibit intensification.  Once Ashobaa moves closer to the coast of Oman, the circulation will begin to pull in drier air from land.  The tropical cyclone could start to weaken before the center moves on land.

The ridge over southern Asia strengthened on Wednesday and it pushed Ashobaa a little south of due west.  The ridge is expected to remain in place and to continue steering the tropical cyclone westward for the next several days.  On its anticipated track, Ashobaa would make landfall in Oman in about 24 to 36 hours.

Tropical Cyclone Ashobaa Moves Toward Oman

Tropical Cyclone Ashobaa turned toward the west on Tuesday and it now appears on track to make a landfall in Oman.  At 11:00 p.m. EDT on Tuesday the center of Tropical Cyclone Ashobaa was located at latitude 21.2°N and longitude 61.6°E which put it about 200 miles (325 km) east-northeast of Masirah Island, Oman.  Ashobaa was moving toward the west-northwest at 11 m.p.h. (18 km/h).  The maximum sustained wind speed was 65 m.p.h. (105 km/h) and there were gusts to 80 m.p.h. (130 km/h).  The minimum surface pressure was 985 mb.

The circulation around Tropical Cyclone Ashobaa is showing signs of greater organization.  A band of thunderstorms has wrapped around the southern portion of the center of circulation and it could represent the formation of a partial eyewall.  Ashobaa is moving over water where the Sea Surface Temperature is warm, but an upper level ridge north of the tropical cyclone is generating easterly winds.  Those winds are producing some vertical wind shear which is inhibiting intensification.  The shear decreased somewhat on Tuesday and there was a modest increase in the wind speed.  The strong thunderstorms near the center of Ashobaa are generating some upper level divergence, mainly on the western side of the circulation and the tropical cyclone could intensify further during the next 24 hours.  As the circulation nears the coast of Oman, the circulation will start to pull in drier air from land and Ashobaa will start to weaken.

An upper level ridge north Ashobaa has strengthened and it appears the ridge will steer the tropical cyclone westward to the coast of Oman.  On its anticipated track the center of Tropical Cyclone Ashobaa will approach the coast of Oman near As Suwayh in about 24 hours.

Tropical Cyclone Ashobaa Moving South of Pakistan

Tropical Cyclone Ashobaa moved northwestward across the Arabian Sea south of Pakistan on Monday.  At 11:00 p.m. EDT on Monday the center of Tropical Cyclone Ashobaa was located at latitude 20.4° and longitude 64.8°W which put it about 340 miles (550 km) south-southwest of Karachi, Pakistan.  Ashobaa was moving toward the northwest at 12 m.p.h. (19 km/h).  The maximum sustained wind speed was 50 m.p.h. (80 km/h) and there were gusts to 65 m.p.h. (105 km/h).  The minimum surface pressure was 990 mb.

More thunderstorms developed near the center of Ashobaa on Monday, but vertical wind shear is still inhibiting intensification.  Ashobaa is moving over water where the Sea Surface Temperature (SST) is warm and there is sufficient energy in the upper ocean to promote intensification.  The circulation is extracting energy from the ocean and it is generating vigorous convection west of the center of circulation.  However, an upper level ridge over southern Asia is creating moderate easterly winds over the top of Ashobaa.  The resulting vertical wind shear is the reason why the strongest storms are west of the center.  Ashobaa will spend several more days in a thermodynamic environment that favors intensification and if the upper level winds lessen, it will intensify.  Eventually, when Ashobaa gets closer to land, the circulation will start to pull in drier air, which will weaken the tropical cyclone.

A ridge over India is steering Ashobaa toward the northwest.  A trough passing north of the Arabian Sea is expected to weaken the western end of the ridge and cause the tropical cyclone to turn more toward the north for a time.  Eventually, most models are forecasting that the ridge will rebuild and steer Ashobaa toward the west-northwest.   However, some models are still predicting that Ashobaa could move northward and make landfall on the northern coast of the Arabian Sea.  The spread in the guidance from the models makes the long-term track more uncertain.