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Low Pressure Develops East of the Bahamas

An area of low pressure developed east of the Bahamas and the system was designated as Invest 90L on Sunday afternoon.  At 5:00 p.m. EDT on Sunday the center of Invest 90L was located at latitude 24.0°N and longitude 71.0°W which put it about 260 miles (420 km) east of the Bahamas.  Invest 90L was moving toward the northwest at 11 m.p.h. (17 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 1009 mb.

The circulation of Invest 90L consists of a large asymmetrical low pressure system.  Most of the showers and thunderstorms are occurring northeast of the center of circulation.  Those showers and thunderstorms are being generated by convergence of winds from a large high pressure system over the north Atlantic Ocean into the area of low pressure.  A swirl of low clouds has emerged on the southwestern side of the low, but it is unclear if this is the actual center of circulation or is just a transient mesoscale feature.  A strong pressure gradient between the high pressure system and the low is producing an area of winds to tropical storm force northeast of the center of the low.  The winds are weaker in other parts of the circulation.

Invest 90L is in an environment that is mostly unfavorable for tropical cyclones.  It is moving over water where the Sea Surface Temperature is near 25°C.  So, there is less energy in the upper ocean to support tropical development.  An upper level trough over the southeastern U.S. is producing strong southwesterly winds which are blowing across the top of Invest 90L.  Those winds are producing strong vertical wind shear.  Marginal Sea Surface Temperatures and strong vertical wind shear make the classical development of a tropical cyclone unlikely.  However, the temperature in the upper troposphere is also cold and there may be enough instability in the atmosphere to produce thunderstorms.  If the wind shear decrease, then more thunderstorms could develop closer to the center of circulation and a subtropical cyclone could form.

The high over the north Atlantic is blocking northward movement of Invest 90L and the high is steering it toward the northwest.  A general northwesterly motion is expected to continue for another day or so.  Eventually the high will move off to the east and Invest 90L will start to move toward the northeast.  On its anticipated track Invest 90L will stay east of the U.S. and the Bahamas.  Invest 90L could pass close to Bermuda and it has the potential to bring gusty winds.

Hurricane Joaquin Moving Slowly Away from the Bahamas

Hurricane Joaquin turned northward on Friday and it is starting to move slowly away from the Central Bahamas.  However, it is still producing strong winds, heavy rain and storm surges on San Salvador and nearby islands.  At 5:00 p.m. EDT on Friday the center of Hurricane Joaquin was located at latitude 24.1°N and longitude 74.7°W which put it about 15 miles (25 km) west-northwest of San Salvador in the Central Bahamas and about 745 miles (1200 km) south of Cape Hatteras, North Carolina.  Joaquin was moving toward the north at 7 m.p.h. (11 km/h).  The maximum sustained wind speed was 125 m.p.h. (205 km/h)  which made Joaquin a Category 3 hurricane on the Saffir-Simpson Scale.  There were wind gusts to 155 m.p.h. (250 km/h) and the minimum surface pressure was 942 mb.  The Hurricane Intensity Index (HII) was 23.6.  The Hurricane Size Index (HSI) was 20.3 and the Hurricane Wind Intensity Size Index (HWISI) was 43.9.

A Hurricane Warning is in effect for the Central Bahamas including Cat Island, the Exumas, Rum Cay, Long Island and San Salvador.  A Hurricane Warning is in effect for the Northwestern Bahamas including the Abacos, Berry Island, Eleuthera, Grand Bahama Island and New Providence.  A Hurricane Watch has been issued for Bimini and Andros Island.  A Tropical Storm Warning is in effect for Andros Island and the Southeastern Bahamas including the Acklins, Crooked Island, Long Cay, the Inaguas, Mayaguana, the Ragged Islands and the Turks and Caicos.  A Tropical Storm Watch has been issued for Bermuda.

Hurricane Joaquin did not change a lot on Friday, although there were some indications that vertical wind shear may be starting to affect it.  Microwave satellite imagery and aerial reconnaissance indicate that there is still and eye, although the eyewall is thinner on the north side.  Most of the stronger thunderstorms are on the southern side of the circulation.  An upper level trough over the eastern U.S. may be creating stronger winds that are hitting Joaquin from the southwest.  Those southwesterly winds may be limiting upper level divergence on that side of the hurricane.  The hurricane is still over water where the Sea Surface Temperature is near 30°C.  So, vertical wind shear is the only factor inhibiting intensification.  The trough is expected to produce more vertical wind shear on Saturday, which could start to weaken Joaquin.

The upper level trough is starting to steer Joaquin slowly toward the north.  The trough is expected to steer Joaquin toward the northeast at a faster rate during the next several days.  Guidance from all of the numerical models has come into agreement that Joaquin will stay east of the U.S.  Conditions in the Central Bahamas should improve on Saturday as Hurricane Joaquin moves farther away.  The possibility that Joaquin could be near to Bermuda on Sunday prompted the issuance of a Tropical Storm Watch for that island.

Category 4 Hurricane Joaquin Bringing Strong Winds and Floods to the Bahamas

Hurricane Joaquin intensified to Category 4 on the Saffir-Simpson Scale on Thursday and it brought strong winds and floods to some of the islands of the Bahamas.  At 11:00 p.m. EDT on Thursday the center of Hurricane Joaquin was located at latitude 22.9°N and longitude 74.6°W which put it about 20 miles (32 km) east-southeast of Clarence Town on Long Island in the Central Bahamas and about 810 miles (1310 km) south of Cape Hatteras, North Carolina.  Joaquin was moving toward the west at 3 m.p.h. (5 km/h).  The maximum sustained wind speed was 130 m.p.h. (210 km/h) and there were wind gusts to 160 m.p.h. (260 km/h).  The minimum surface pressure was 935 mb.  The Hurricane Intensity Index (HII) was 25.1.  The Hurricane Size Index (HSI) was 21.4 and the Hurricane Wind Intensity Size Index (HWISI) was 46.5.

A Hurricane Warning has been issued for the Central Bahamas including Cat Island, Rum Cay, the Exumas, Long Island and San Salvador.   A Hurricane Warning has been issued for the Northwestern Bahamas including the Abacos, Berry Islands, Eleuthera, Grand Bahama Island and New Providence.  A Hurricane Watch has been issued for Bimini and Andros Island.  A Tropical Storm Warning has been issued for Andros Island and the Southeastern Bahamas including the Acklins, Crooked Island, Long Cay, the Ragged Islands, the Inaguas, Mayaguana and the Turks and Caicos.  A Tropical Storm Warning has been issued for the Cuban provinces of Camaguey, Los Tunas, Holguin and Guantanamo.

The eye in Hurricane Joaquin contracted on Thursday and it has only been visible intermittently on conventional satellite imagery.  The intensity of the hurricane appears to have stabilized in recent hours.  While the intensity of Joaquin has been constant, the circulation has increased in sized and hurricane force winds now extend out about 50 miles (80 km) from the center.  Joaquin is still generating upper level divergence which seems to be pumping out as much mass as is converging in the lower levels of the circulation.  As a result, the pressure has also remained fairly steady during the past few hours.  There is not much vertical wind shear and the overall environment would seems to support further intensification.  However, the slow movement of Joaquin may be causing it to stir cooler water to the surface, which will reduce the energy available to the hurricane.

Hurricane Joaquin has moved little during the past six hours.  An narrow ridge is blocking its motion and the ridge is preventing it from moving toward the north.  A large upper level trough over the eastern U.S. is expected to weaken the ridge on Friday.  The upper level trough is expected to steer Joaquin toward the north on Friday.  Most of the reliable numerical models predict that the trough will carry Joaquin toward the northeast during the weekend and that it will stay east of the U.S.  A couple of other models still suggest a landfall could occur, but they are outliers at the current time.  Until Hurricane Joaquin makes the turn toward the north and northeast, it could still pose a potential risk to the U.S., but that possibility is much less than it was 24 hours ago.  Joaquin will continue to bring high winds and floods to parts of the Bahamas on Friday.

Joaquin Almost a Hurricane, Watch Issued for Central Bahamas

Tropical Storm Joaquin is on the verge of reaching hurricane intensity.  At 11:00 p.m. EDT on Tuesday the center of Tropical Storm Joaquin was located at latitude 25.8°N and longitude 71.7°W which put it about 360 miles (580 km) east of the Northwestern Bahamas and about 680 miles (1100 km) south-southeast of Cape Hatteras, North Carolina.  Joaquin was moving toward the west-southwest at 5 m.p.h. (8 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 988 mb.  The government of the Bahamas has issued a Hurricane Watch for the Central Bahamas.

The organization of the circulation of Tropical Storm Joaquin increased on Tuesday in spite of moderate amounts of vertical wind shear.  More thunderstorms developed near the core of circulation and a partial eyewall formed around the southern and eastern parts of the center.  Although it is more organized, the circulation is still asymmetrical and the stronger winds were found in the eastern side of the tropical storm.

A thin upper level ridge is east of Florida and clockwise flow around the ridge is producing northerly winds over the top of Joaquin.  Those winds produced moderate vertical wind shear on Tuesday but the shear appears to be decreasing with time.  Joaquin is over water where the Sea Surface Temperature is near 30°C.  As the upper level winds diminish, the environment favors intensification and Joaquin is likely to become a hurricane on Wednesday.  Joaquin will continue to be in an environment that favors intensification during the next several days.

A ridge of high pressure north of Joaquin is blocking it from moving north and the ridge is forcing the tropical storm to move toward the west-southwest.  That general motion is expected to continue for another 24 to 48 hours and it could bring Joaquin near the Central and Northwestern Bahamas.  When Joaquin reaches the western end of the ridge it will turn toward the north.  An upper level trough over the eastern U.S. will cause Joaquin to move more quickly toward the north at the end of the week.  On its anticipated track, Joaquin could be approaching the Mid-Atlantic Coast of the U.S. by the weekend.

TD11 Becomes Tropical Storm Joaquin

The circulation in Tropical Depression 11 exhibited more organization on Monday night and the National Hurricane Center upgraded it to Tropical Storm Joaquin.  At 11:00 p.m. EDT on Monday the center of Tropical Storm Joaquin was located at latitude 26.7°N and longitude 70.4°W which put it about 400 miles (640 km) northeast of the Central Bahamas and about 670 miles (1080 km) south-southeast of Cape Hatteras, North Carolina.  Joaquin was moving toward the southwest at 5 m.p.h. (8 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.

Tropical Storm Joaquin formed when an upper level low sat in place over warm Sea Surface Temperatures for a few days.  Transfers of momentum slowly increased the rotation in the lower atmosphere until a distinct center of circulation developed.  However, northwesterly winds in the upper levels created vertical wind shear that inhibited the development of the system.  Eventually, the wind shear decreased enough to allow more thunderstorms to develop near the center of circulation.  Condensation in those storms created a warm core in the middle and upper atmosphere and Joaquin began to intensify.  Now, upper level divergence is beginning to develop and the environment is becoming more favorable for intensification.

A ridge north of Joaquin is steering it slowly toward the west and that steering pattern is expected to continue for the next day or two.  An upper level trough approaching from the west is expected to turn Joaquin toward the north.  Guidance from numerical models is divergent.  Some models bring Joaquin to the Mid-Atlantic Coast while other models forecast Joaquin to move toward Long Island.  The future track of Joaquin will depend on how far west it moves before it turns northward and how strong it gets.

Tropical Storm Erika Prompts Warnings for the Caribbean

The imminent approach of Tropical Storm Erika prompted the issuance of watches and warnings for locations in the northeastern Caribbean Sea.  At 11:00 p.m. EDT on Tuesday the center of Tropical Storm Erika was located at latitude 16.0°N and longitude 54.4°W which put it about 495 miles (800 km) east of Antigua and about 1780 miles (2870 km) east-southeast of Miami, Florida.  Erika was moving toward the west at 18 m.p.h. (30 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 1006 mb.  Tropical Storm Warnings have been issued for Anguilla, Saba, St. Eustatius, and St. Maarten.  Tropical Storm Watches have been issued for Puerto Rico, the U.S. Virgin Islands, the British Virgin Islands, Montserrat, Antigua, Barbuda, St. Kitts, Nevis, Guadeloupe, St. Martin and St. Barthelemy.

For much of Monday the circulation around Tropical Storm Erika consisted of a large swirl of low level clouds and a few thunderstorms well to the southeast of the center.  The minimum surface pressure rose several millibars which was indicative of a weakening storm.  In the past several hours satellite imagery suggests that a few new thunderstorms could be forming closer to the center of circulation.  Erika is moving over water where the Sea Surface Temperature (SST) is between 28°C and 29°C.  So there is plenty of energy in the upper ocean.  The circulation could be pulling in some drier air from farther north of the storm.  There is also some vertical wind shear which may also be inhibiting intensification.    The combination of positive environmental factors like SST and negative environmental factors like drier air and wind shear make the intensity forecast challenging.  Guidance from numerical models is divergent.  Some models predict intensification while others predict that Erika will dissipate like Danny did.  If more thunderstorms continue to develop around the center of circulation, then intensification would be more likely.  On the other hand, if the recently formed thunderstorms dissipate in a few hours, the Erika could weaken to a tropical depression.

A subtropical ridge is steering Erika a little north of due west and that general motion is expected to continue during the next several days.  On its anticipated track Erika would approach the northern Leeward Islands in about 24 hours and it could be near Puerto Rico in less than two days.

Danny Dissipates But Erika Forms Behind It

Vertical wind shear weakened Tropical Storm Danny as it entered the eastern Caribbean Sea and its circulation degenerated into a weak area of low pressure.  However, a new tropical storm formed about a thousand miles east of the remnants of Danny and the National Hurricane Center designated it as Tropical Storm Erika.  At 11:00 p.m. EDT on Monday the center of Tropical Storm Erika was located at latitude 14.4°N and longitude 47.7°W which put it about 955 miles (1535 km) east of the Leeward Islands and about 2240 miles (3600 km) east-southeast of Miami, Florida.  Erika was moving toward the west at 20 m.p.h. (32 km/h).  The maximum sustained wind speed was 45 m.p.h. (70 km/h) and there were gusts to 60 m.p.h. (95 km/h).  The minimum surface pressure was 1003 mb.

The circulation around Erika is a little larger than the circulation around Danny was.  There are several bands of thunderstorms near the center of circulation and the thunderstorms are generating upper level divergence in all directions.  Erika is moving over water where the Sea Surface Temperature is near 27.5°C and it will gradually move over warmer SSTs as it moves west.  Erika is moving at 20 m.p.h. and that is about the upper limit for storms that intensify over the tropical Atlantic Ocean.  Currently the low level winds and upper level winds are blowing at nearly the same speed.  So, there is not much vertical wind shear.  However, if Erika were to move toward the west faster, then vertical wind shear could be more significant.  As long as Erika does not move too fast, the environment is favorable for intensification and Danny intensified rapidly over the same region a few days ago.  When Erika enters the Caribbean Sea in two or three days, it could encounter the same upper level trough that sheared Danny apart.

A subtropical ridge is steering Erika quickly toward the west and that same general steering motion is expected to continue for the next two or three days.  Erika could be approaching the northern Leeward Islands in about 48 hours.

Danny Becomes a Major Hurricane

Hurricane Danny intensified quickly and a NOAA plane investigating it on Friday afternoon found that Danny had maximum sustained winds of 115 m.p.h.  That made Danny a Category 3 hurricane on the Saffir-Simpson Scale which also made it a Major Hurricane.  At 5:00 p.m. EDT on Friday the center of Hurricane Danny was located at latitude 14.5°N and longitude 49.1°W which put it about 860 miles (1390 km) east of the Leeward Islands and about 2140 miles (3450 km) east-southeast of Miami, Florida.  Danny was moving toward the west-northwest at 10 m.p.h. (16 km/h).  The maximum sustained wind speed was 115 m.p.h. (165 km/h) and there were wind gusts to 135 m.p.h. (195 km/h).  The minimum surface pressure was 974 mb.  Danny had a Hurricane Intensity Index (HII) of 20.6, a Hurricane Size Index (HSI) of 6.0, and a Hurricane Wind Intensity Size Index (HWISI) of 26.6

Danny remains a very small hurricane.  It is about two thirds of the size that Hurricane Charley was when it hit southwest Florida in 2004.  Danny may be starting to weaken.  The small eye is no longer visible on satellite images.  Southwesterly winds in the upper levels may be generating some vertical wind shear and those winds may also be inhibiting upper level divergence on the western side of Danny.  The small size of Danny means that it could weaken almost as fast as it intensified, if the shear increases.  Danny will move over warmer water when it passes west of longitude 55°W, which could slow the rate of weakening in about 36 hours.

Danny is being steered toward the west-northwest by a subtropical ridge located north of the hurricane.  The ridge is expected to strengthen in two or three days and steer Danny more toward the west after that time.  On its anticipated track Danny could be approaching the northern Leeward Islands in about two and a half days and it could be near Puerto Rico in about three and a half days.

Tiny Danny Quickly Intensifies Into a Hurricane

Tiny Tropical Storm Danny intensified quickly during the past 12 hours and the National Hurricane Center upgraded it to hurricane status in its 11:00 a.m. EDT advisory on Thursday.  At 11:00 a.m. EDT on Thursday the center of Hurricane Danny was located at latitude 12.5°N and longitude 44.8°W which put it about 1090 miles (1755 km) east of the Windward Islands and about 2480 miles (4000 km) east-southeast of Miami.  Danny was moving toward the west-northwest at 12 m.p.h. (19 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 992 mb.

The core of Hurricane Danny organized quickly on Thursday.  An eye formed in the center of the circulation and a ring of thunderstorms developed around the eye.  The circulation around Danny is very small.  Hurricane force winds only extend out about 12 miles (19 km) from the center and tropical storm force winds only extend 60 miles (95 km) from the center.  The Hurricane Size Index (HSI) for Danny is only 4.2.

The environment around Danny remains complex.  It is over water where the Sea Surface Temperature (SST) is near 28°C and the upper level winds are very light.  Upper level divergence is being produced by the thunderstorms around the eye.  On the other hand, slightly cooler SSTs and drier air are just to the north of Danny.  In addition, small hurricanes like Danny can be strongly affected by changes in their environment and they can intensify or weaken very quickly.  The environment would seem to support further intensification, but if Danny moves a little farther north, it could move into a more hostile environment.

Danny is being steered toward the west-northwest by the subtropical ridge to its north and that general steering pattern is expected to continue for another two or three days.  Late in the weekend the subtropical ridge is forecast to strengthen and steer Danny more toward the west.  On its anticipated track Danny could be near the Leeward Islands in about four days and near Puerto Rico in about five days.

Tropical Storm Danny Heading West

Tropical Storm Danny has moved steadily westward during the past 24 hours.  At 11:00 a.m. EDT on Wednesday the center of Tropical Storm Danny was located at latitude 11.2°N and longitude 41.1°W which put it about 1385 miles (2235 km) east of the Lesser Antilles and about 2740 miles (4410 km) east-southeast of Miami, Florida.  Danny was moving toward the west 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 60 m.p.h. (95 km/h).  The minimum surface pressure was 1000 mb.

The organization of the circulation of Danny has varied during the past day.  It has a visible tight core at the center, but earlier today there was no convection around the core.  Recent satellite images show new thunderstorms developing near the core.  The environment around Danny is complex and it contains both positive and negative factors.  Danny is currently over water where the Sea Surface Temperature (SST) is near 28°C.  However, there is slightly cooler water just to the north of the tropical storm.  Satellite imagery indicates that there could also be drier air north of the circulation and some of that drier air could be pulled into Danny.  The upper level winds are light and upper level divergence is occurring.  However, a surge in the low level trade winds is about 250 miles (400 km) east of Danny.   If the surge in the trade winds reaches the core of Danny, it could push the lower part of the circulation out ahead (to the west) of the upper part of the circulation.  In that case Danny would weaken.

The complexity of the environment around Tropical Storm Danny makes the intensity forecast challenging.  If the trade wind surge does not reach the core of Danny and it stays over warm SSTs, then gradual intensification is possible.  On the other hand, if Danny moves more northward over cooler SSTs and into drier air, or if the trade wind surge creates more vertical wind shear, then Danny could weaken.

A subtropical ridge to the north of Danny is steering Danny toward the west and a generally west or west-northwest motion is forecast for the next few days.  If Danny is a little stronger, it could move a little farther north, and if it is weaker, it could stay farther south.  On its anticipated track, Danny could approach the Lesser Antilles in four or five days.  Interests in those areas should monitor Danny for future developments.