Tuesday, January 1, 2019

GW170817 and Multi-messenger Astronomy

The first astronomers had only their own eyes as tools, and visible light was their only source of information. Recent instruments have broadened our sight to include all types of electromagnetic radiation, from radio waves to X-rays and gamma rays. Each part of the spectrum is suited to different types of observations and gave us incredible new insight into the cosmos. However, the second decade of the 21st century saw the advent of a fundamentally new kind of astronomy: the detection of gravitational waves.

Gravitational waves, as discussed in a previous post, are the "ripples" in spacetime that propagate in response to the acceleration of massive objects (stars, black holes, and the like). All objects with mass produce these waves, but the vast majority are far too small to detect. It was only with the advent of extremely sensitive instruments that the first detection of gravitational waves was made by LIGO (the Laser Interferometer Gravitational-Wave Observatory) in 2015. This detection, and its immediate successors, were of binary black hole merger events, in which two black holes orbiting one another spiraled inwards and finally combined into a single, larger black hole. The last moments before merging brought exceptionally colossal objects (weighing perhaps dozens of solar masses) to great accelerations, the perfect recipe for producing strong gravitational waves detectible across the cosmos. However, these cataclysmic events were quite dark: little electromagnetic radiation was emitted, and no "visual" evidence for these events accompanied the wave signal. Something quite different occurred in 2017.



On August 17, 2017 at 12:41 UTC, the LIGO detectors at Hanford, Washington and Livingston, Louisiana and the Virgo gravitational wave detector in Italy simultaneously measured an event as shown above (click to enlarge). The two LIGO frequency-time diagrams clearly show a curve that increases in frequency before disappearing at time 0. This corresponds to two inspiraling objects orbiting one another faster and faster before merging finally occurs and the signal stops. In the Virgo diagram, the same line is not very visible, but further analysis of the data nevertheless expose the same signal from the noise. The gravitational wave event, designated GW170817, was genuine.

Having three detectors at different points on the Earth measure the event allowed a better triangulation of the location of the source than had occurred previously (when LIGO and Virgo were not simultaneously active).


The above figure shows a visualization of the celestial sphere (representing all possible directions in the sky from which the signal could have come) and locations from which the signal data suggest the signal originated. The green zone is the highest probability region taking all three instruments into account. This area is still 31 square degrees, quite large by astronomical standards. Fortunately, corroboration of the event came immediately from an entirely separate source.



The above figure (click to enlarge) shows at the bottom the same gravitational wave signal from before. The rest of the data come from the Fermi Gamma-ray Space Telescope and the International Gamma Ray Astrophysics Laboratory, both satellites in Earth orbit. As their names suggest, they search the cosmos for astrophysical sources of high-energy gamma rays. In particular, they monitor the cosmos for gamma-ray bursts (GRBs), especially intense flashes of radiation that typically accompany only the most explosive events, such as supernovae. As the figure shows, less than two seconds after the gravitational wave signal stopped (indicated the merger of two orbiting objects), there was an elevated count of gamma rays in each detector across the different photon energy levels. The source of this burst is indicated by a reticle in the celestial sphere figure above, lying right within the estimated location of the merger! It appeared that this merger had an electromagnetic counterpart! Further, analysis of the gravitational waves indicated that the masses of the two objects were around 1.36-2.26 and 0.86-1.36 solar masses (these were the uncertainty ranges), respectively, not heavy enough for black holes. What was going on?



The conclusion drawn from these events was that the merger was not of black holes, but of neutron stars, compact remnants of large stars that were yet not massive enough to collapse into black holes. An artist's conception of a binary neutron star black hole merger is shown above. Following the initial identification of the event, countless telescopes around the world trained on the event the very same day after a notice was released around 13:00 UTC, hoping to observe more following the merger.



And they were not disappointed. Less than a day after the initial gamma ray burst had faded, the source began to appear at other frequencies, and remained bright for several weeks before fading. The above figure shows the Hubble image of the merger's host galaxy, NGC 4993. This galaxy is at a distance of roughly 130 million light-years, and even at this distance, the collision of the neutron stars was clearly visible against the billions of other stars. Finally, the chart below demonstrates just how well documented the event was:



Many different instruments took images in X-rays as well as ultraviolet, visible, infrared, and radio waves. The horizontal axis indicates the rough timeline of events (on a logarithmic scale) in each part of the electromagnetic spectrum, stretching from less than a day to several weeks after the merger. Several representative images of NGC 4993 and the source within are shown at bottom.

Without extensive collaboration within the astronomical community, collecting this wealth of data on this binary neutron star merger would not have been possible. This marked the first time in history that a single event was measured in both gravitational waves and electromagnetic waves, not to mention how thoroughly the merger was photographed across the spectrum. This coordinated observation is known as multi-messenger astronomy, and may have profound implications on our future understanding of the universe. Some of what we learned from the binary neutron star merger is discussed in the next post (coming January 22).

Note: Most of the figures above are taken from the open access papers detailing the discovery and analysis of the binary neutron star merger. For further reading on the event, links to these papers may be found in the sources below.

Sources: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.119.161101, https://arxiv.org/pdf/1710.05834.pdf, http://iopscience.iop.org/article/10.3847/2041-8213/aa91c9/pdf

Sunday, December 9, 2018

2018 Season Summary

The 2018 Atlantic Hurricane Season had above-average activity, with a total of

16 cyclones attaining tropical depression status,
15 cyclones attaining tropical storm status,
8 cyclones attaining hurricane status, and
2 cyclones attaining major hurricane status.

Before the beginning of the season, I predicted that there would be

18 cyclones attaining tropical depression status,
16 cyclones attaining tropical storm status,
8 cyclones attaining hurricane status, and
4 cyclones attaining major hurricane status.

The average number of named storms, hurricanes, and major hurricanes for an Atlantic hurricane season (over the 30-year period 1981-2010) are 12.1, 6.4, and 2.7, respectively. The 2018 season was somewhat above average in these categories, with the exception of the number of major hurricanes. The formation of many short-lived subtropical storms inflated the named storm total, but the Accumulated Cyclone Energy (ACE) value of 127 for the season was still above average. This value accounts for the duration and intensity of tropical cyclones as well as their number.

As usual, the ENSO oscillation was a major player in tropical cyclone activity this year. As hurricane season progressed into autumn, ocean temperatures of the equatorial Pacific trended higher than normal, signaling the advent of an El Niño event. Typically, such an event causes higher wind shear over the Atlantic and suppresses tropical cyclone activity, but it arose later in the year than anticipated, mitigating its effects.



The increase in wind shear during El Niño is most pronounced in the Caribbean Sea. Indeed, this region was a "graveyard" for tropical cyclones during 2018, as indicated by the above map of all the season's tracks. Every storm that entered the eastern Caribbean dissipated shortly thereafter due to unfavorable atmospheric winds. Ocean temperatures in the tropical Atlantic east of the Caribbean were also fairly cool for much of the season. This setup prevented long-track hurricanes from forming, with one notable exception: Hurricane Florence. Florence took a highly unusual route farther north but still pushed westward into the U.S. east coast. Overall, my predictions were slightly higher than the actual season activity, but they did correctly indicate the risk to the east coast.

The two most notable storms of the season were Hurricane Florence and Hurricane Michael. Florence made landfall in North Carolina, where it stalled and brought over 30 inches of rain to some areas. The record-breaking rainfall caused unprecedented flooding and extensive damage. Michael brought torrential rain to central America as it was forming and then went on to strengthen right up until landfall in the Florida Panhandle. With a pressure of 919 mb at landfall, Michael was at the time the 3rd most intense cyclone ever to make landfall in the United States. Some other notable facts and records from the 2018 Atlantic season include:
  • The 2018 season had seven storms that were at some point subtropical, a new record
  • On September 12, Florence, Helene, Isaac, and Joyce all coexisted in the Atlantic. This was the first time four named storms existed simultaneously since 2008
  • Hurricane Leslie took a highly unusual track over the far eastern Atlantic near the end of its lifetime. As a result, the first tropical storm warning on record was issued for Madeira Island southwest of Portugal; Leslie became post-tropical just before landfalling in Portugal itself
Overall, the 2018 season was only a little above average but nevertheless featured two devastating major hurricanes.

Sources: https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf,

Saturday, October 27, 2018

Hurricane Oscar (2018)

Storm Active: October 26-31

On October 23, an area of low pressure a few hundred miles east-northeast of the Lesser Antilles began to produce some shower and thunderstorm activity. Over the next few days, the disturbance moved slowly northward and atmospheric conditions for development improved. As westerly shear diminished, convection persisted closer to the center of the developing low. Late on October 26, the low-level center had become well-defined. However, due to its interaction with a nearby upper-level low, the system was classified Subtropical Storm Oscar. This was the seventh named storm in 2018 to be subtropical during some part of its lifetime, making 2018 the first known season for such an occurrence.

Interaction with the upper-level low caused Oscar to turn sharply westward on the 27th. Meanwhile, significant deepening took place, indicating that Oscar's maximum winds had increased. Oscar's structure evolved throughout that day until the cyclone possessed a small core of deep convection and maximum winds close to the center. As a result, it was reclassified as a tropical storm. A ridge pushed the system south of west on the 28th and favorable conditions allowed an eye feature to begin forming. Around the same time, Oscar strengthened into a hurricane. The trend of gradual intensification persisted on October 29 and the system became a category 2 that evening, reaching its peak intensity of 105 mph winds and a pressure of 970 mb. Meanwhile, a mid-latitude frontal system approaching from the west began to sheer the cyclone toward the north.

By the 30th, Oscar was picking up speed toward the north and north-northeast and began to encounter cooler waters as it passed east of Bermuda. As a result, deep convection near the center waned, the maximum winds dropped, and the system began to take on extratropical characteristics. Nevertheless, it remained a potent cyclone and brought rough surf to Bermuda that day. Around midday on October 31, Oscar transitioned to a hurricane-strength extratropical storm as it sped north-northeastward over the open Atlantic. The post-tropical low deepened over the north Atlantic during the following days, reaching a minimum pressure of 950 mb on November 2. It dissipated a few days later near Iceland.



This image shows the small hurricane Oscar at peak intensity as a category 2 hurricane.



Oscar did not directly affect any land areas during its time as a tropical or subtropical cyclone.

Tuesday, October 9, 2018

Tropical Storm Nadine (2018)

Storm Active: October 9-12

A late-season tropical wave entered the Atlantic ocean on October 6 and began to show signs of development. Waters in the eastern Atlantic were still fairly warm and shear was low, so organization proceeded fairly quickly. By October 8, convection had wrapped nearly around the disturbance, but it still lacked a closed circulation. The next day, it formed into Tropical Depression Fifteen. Within a few hours, satellite intensity estimates supported its upgrade into Tropical Storm Nadine. Nadine formed unusually late in the season for a system so far east in the tropical Atlantic.

The cyclone was fairly small, and hence prone to rapid changes in intensity. Over the next day, it took advantage of a favorable environment and strengthened quickly to its peak intensity as a strong tropical storm with 65 mph winds and a pressure of 997 mb on October 10. However, wind shear sharply increased that night and displaced all of Nadine's convection to the east of the center by October 11. As a result, the storm decayed rapidly as it moved northwest. The next day, Nadine dissipated over the central Atlantic.



Nadine was a small cyclone that quickly succumbed to unfavorable atmospheric conditions a few hours after formation.



The short-lived Nadine did not affect any land areas, but was an unusually late-season storm to form in the central tropical Atlantic.

Sunday, October 7, 2018

Hurricane Michael (2018)

Storm Active: October 7-12

During the first few days of October, a broad area of low pressure developed in the southwestern Caribbean, with associated showers and thunderstorms extending from central America all the way to Jamaica and Haiti. Such systems are common in this region in the autumn, and are known as Central American Gyres (CAGs). CAGs tend to bring heavy rainfall to a wide area of central America, which held true in this case. In addition, they can sometimes spawn tropical cyclones. Nevertheless, the large circulation of a CAG takes time to consolidate, and the system only slowly organized as it moved northwestward. By October 6, the center of the low was just north of Honduras and the region of strong upper-level winds that had been affecting the system retreated to the north. This allowed further organization, and a flare up of organized thunderstorm activity led to the classification of Tropical Depression Fourteen early on October 7.

Even immediately after formation, the storm had an impressive satellite signature, with very cold cloud tops to the east of the center of circulation. Soon after, satellite and aircraft data indicated an immense radius of gale force winds, extending over 200 miles from the center in some quadrants, and the depression was upgraded to Tropical Storm Michael. Under the influence of some westerly shear, the center of Michael underwent some reformations toward the east that day, but the large cyclone strengthened steadily into the evening as it moved slowly northward. Already, the outer rainbands were hitting the southern tip of Florida, even though the center was still just east of the Yucatan Peninsula. By the morning of October 8, Michael had strengthened into a hurricane.

During that day, the inner core gradually became more organized and the cyclone steadily intensified as it passed near the western tip of Cuba. A large eyewall struggled to surround the center throughout the day, but coverage increased during the evening. The storm became a category 2 early on October 9. The system gained some forward speed toward the north that day and the warm waters of the Gulf of Mexico supported extremely intense convection. Shear also lessened, and the eyewall became complete early that evening, bring Michael to category 3 strength. The outer bands of the storm were now crossing the Gulf coastline, but proximity to land did nothing to slow the system's intensification. A symmetric eye cleared out on satellite imagery overnight and Michael rocketed to category 4 status, deepening even when the center was within 50 miles of landfall. The powerful cyclone reached a peak intensity of 155 mph winds and a pressure of 919 mb when it slammed into the Florida Panhandle around 1:00pm local time on October 10. In terms of pressure, Michael became the third strongest hurricane ever on record to make landfall in the United States, behind only Hurricane Camille of 1969 and the Labor Day Hurricane of 1935. At the time, it also broke the top 10 overall list for strongest landfall recorded for an Atlantic hurricane. Furthermore, it was the only category 4 ever known to hit the Florida panhandle.

The storm surge that Michael brought to the coastline was unprecedented and record-braking in some areas and the wind damage was catastrophic, though the worst of it was confined to quite a small area where the eyewall made landfall. However, the rainfall was not especially severe, as the system accelerated northeastward as it moved inland and did not linger. In fact, the system entered southwestern Georgia before losing major hurricane status, thus becoming the first major hurricane to impact the state since 1898. Nevertheless, the core did rapidly deteriorate once inland, and Michael weakened to a tropical storm early on October 11 over central Georgia. Later that day, it moved through the Carolinas, bringing heavy rain and wind to regions inundated by Florence's rains the previous month. Fortunately, the storm was moving so fast that the flooding impacts were not as severe as they otherwise would have been. The cyclone emerged over the Atlantic near the border of North Carolina and Virginia and became post-tropical early on October 12. The system crossed the Atlantic and eventually brought some rain and wind to western Europe on October 15.



The above image shows Hurricane Michael making landfall in the Florida panhandle at strong category 4 intensity. This was among the strongest landfalls ever recorded for an Atlantic hurricane.



Michael's speedy development amid only marginally favorable conditions and rapid strengthening prior to landfall were very unexpected and demonstrate how far there is to go in modeling intensity changes in tropical cyclones.

Sunday, September 23, 2018

Hurricane Leslie (2018)

Storm Active: September 23-25, September 28-October 13

On September 18, an extratropical system associated with the remnants of Hurricane Florence moved away from the U.S. east coast over the tropical Atlantic. A new low formed along the frontal boundary around September 22 in the central subtropical Atlantic. Over the next day, the low developed spiral banding and lots its frontal nature. By the morning of September 23, it had transitioned into Subtropical Storm Leslie. This was the sixth subtropical storm of the 2018 season, setting a new record.

At the time of formation, Leslie was drifting westward, but steering currents were quite weak and it turned southward and ultimately eastward over the next day. The system had never had much in the way of deep convection, but what was there diminished further by September 25. Meanwhile, a new front was approaching from the west and interacting with Leslie, elongating its circulation. By late that morning, the system had become extratropical. Upon transition, it underwent a rapid burst of the strengthening and was producing hurricane force winds by the 26th. Since it was non-tropical, however, it was not designated a hurricane.

At the same time it continued to turn toward the north and then back west. Conditions were still fairly favorable for tropical cyclone development so it began to transition back the next day. On September 28, enough deep convection had reappeared near the center for Leslie to again be classified as subtropical. However, its maximum winds had subsided back to around 50 mph, so this was the initial intensity. The system moved slowly southwest over the next few days and gradually developed more banding features south and east of the circulation center. Leslie transitioned to a fully tropical storm for the first time on September 29. Sea surface temperatures increased and wind shear decreased along the storm's path, leading to some slow strengthening over the next few days as thunderstorms finally wrapped entirely around the center.

By October 2, Leslie was approaching hurricane strength and had dipped in latitude to below 30° N due to its unusual southwestward motion. A ragged eye formed that evening and the system was upgraded to a hurricane for the first time. Overnight, the cyclone became stationary around 500 miles east-southeast of Bermuda. It also peaked in intensity at maximum sustained winds of 80 mph and a central pressure of 975 mb. Due to the influence of an upper-level low pressure system to the north, Leslie began to move northward on October 3. This motion took it over cooler water, and convection waned again, with a shallow ring of convection around the center separated from the outer bands by a "moat" of dryer air. Leslie began to weaken as a result and soon was a tropical storm again.

Although the system was still quite distant from any landmasses, the large size of the circulation generated significant ocean swells that led to rough surf in Bermuda and even the east coast of North America. Leslie stalled again about 450 miles northeast of Bermuda on October 5, and began to feel the influence of the mid-latitude westerlies. The cyclone turned sharply eastward that day. Meanwhile, the structure of the storm had changed quite a bit; a central area of strong thunderstorms had replaced the large eye, and a large area of convection persisted to the north of the center. Leslie began to separate from a trough to its north and turned south of east on October 7. The storm accelerated southeastward over the next day, bringing it over warmer waters, and it began to restrengthen.

The cyclone developed a central dense overcast on October 8 and approached hurricane strength on the 9th, achieving category 1 status that evening a week after doing so the first time. Leslie turned due south for a little while on the 10th, reaching a southernmost latitude of 27.8 ° N. However, another trough moving to its north turned the system east-northeast and began to accelerate it toward the far eastern Atlantic. The inner core structure fluctuated a great deal in organization during the following day, but overall it became a bit better defined and Leslie strengthened somewhat. Late on October 11, Leslie reached its peak intensity as a top-end category 1 hurricane with 90 mph winds and a pressure of 969 mb.

The system picked up even more speed the next day and colder waters weakened the storm's convection. A tropical storm warning was issued for the island of Madeira, located southwest of Portugal. This was the first ever warning issued for the island and Leslie was the first known tropical cyclone ever to affect it in modern history. The center passed north of Madeira later on the 12th. Finally, on October 13, Leslie transitioned to an extratropical low just before making landfall in northern Portugal. This transition did not prevent the cyclone from bringing hurricane force winds gusts and heavy rain to the Iberian Peninsula. The low finally dissipated inland a few days later.



This image shows Leslie during its second and final stint as a hurricane, moving east-northeastward toward Europe.


Leslie's convoluted track included some highly unusual southward dips over the central Atlantic. Just after becoming extratropical, it moved over the Iberian Peninsula, though this is not shown above.

Saturday, September 22, 2018

Tropical Storm Kirk (2018)

Storm Active: September 22-24, 26-

On September 21, a strong tropical wave entered the Atlantic. It began to travel westward at quite a low latitude, far to the south of the Cabo Verde Islands. It organized quickly and developed a closed circulation the next day. Satellite measurements indicated gale force winds at that time, so the system was named Tropical Storm Kirk on September 22. At the time of naming, Kirk's latitude was 8.3 °N, quite close to the equator for tropical cyclone genesis in the Atlantic. In fact, no north Atlantic system on record had reached tropical storm strength so far south since 1902.

Shortly after formation, a ridge to the north of Kirk began to accelerate it westward. Due to its fast forward motion and its proximity to the equator, the system had difficulty acquiring much spin, although some evidence of curved banding started to appear on September 23. After that time though, it struggled to maintain deep convection. In addition, its forward speed continued to increase to over 20 mph. On the 24th, this caused the circulation to fall apart and Kirk dissipated.

The remnants continued quickly westward and sea surface temperatures increased while upper-level winds remained fairly favorable. As a result, the system began to reorganize. On September 26, a well-defined center reformed and Kirk regained tropical storm status. It even underwent some strengthening that day to its peak intensity of 60 mph winds as measured by hurricane hunter aircraft. However, its window of favorable conditions was short-lived. By early on September 27, it had entered the area of wind shear that dominated the east Caribbean and its immediate vicinity for most of the season. This quickly exposed the center of circulation as convection was displaced eastward. Nevertheless, the center passed over the Lesser Antilles that day and deep convection continued to flare up in the eastern semicircle, bringing heavy rains and gale force winds to the islands, some of which were still recovering from the passage of Hurricane Maria the previous year.

After entering the Caribbean, Kirk began to lose organization more quickly. Late on September 28, the system dissipated in the eastern Caribbean.

Friday, September 21, 2018

Tropical Depression Eleven (2018)

Storm Active: September 21-23

On September 18, a disturbance developed in the tropical Atlantic well east of the Windward Islands. It moved generally west-northwestward over the following view days and began to exhibit a small but organized canopy of thunderstorm activity. A weak low pressure center appeared on September 20, but wind shear increased significantly around the same time and the atmosphere was quite dry as the system approached the Caribbean. Despite unfavorable conditions, sheared convection persisted near the surface circulation center long enough the next day for the system to be classified Tropical Depression Eleven.

Before long, upper level winds out of the west increased even further, and any bursts of deep convection from Eleven were swiftly blown away. The center moved erratically the next day and began to lose definition, and by the morning of September 23, the system was downgraded to a remnant low. This low dissipated soon after.



Tropical Depression Eleven never achieved much organization during its brief lifetime.



The area of high wind shear over the Caribbean was persistent during the 2018 season, and claimed Eleven as another victim.

Thursday, September 13, 2018

Tropical Storm Joyce (2018)

Storm Active: September 12-18

On September 11, a non-tropical low formed along a frontal boundary situated over the north central Atlantic, well west of the Azores. The low drifted generally southward or southwestward over the next day. By September 12, it was producing gale force winds and displayed some organized convective banding. However, the surface low was still colocated with an upper-level low, not the high found for tropical cyclones, so it was designated Subtropical Storm Joyce. Existing contemporaneously with Florence, Helene, and Isaac, the new system was one of four named storms simultaneously occupying the Atlantic basin. This was the first time this had occurred since 2008. Also, Joyce was the fifth subtropical cyclone of the season, the first time that had happened since 1969.

Shortly after formation, the cyclone felt the influence of the much larger Tropical Storm Helene to its southeast. Steered around the periphery of its circulation, Joyce moved west-southwest and then turned south around Helene's left side as it reached the same latitude. Meanwhile, on September 13, Joyce transitioned into a tropical storm. The next day, it strengthened slightly to a moderate tropical storm and turned eastward in the wake of Helene. This intensification was short-lived, however, as increasing wind shear out of the southwest stripped away the little convection that formed in bursts near the center of circulation. On September 16, Joyce weakened to a tropical depression.

The shallow system was left behind by now ex-Helene and instead followed the boundary of a mid-level high located in the subtropical Atlantic. This caused the depression to turn south of due east by September 17 and then south by the 18th. Late on September 18, Joyce had ceased to produce deep convection and was finally downgraded to a remnant low. The low moved slowly southwestward until dissipation.



The above image shows the small Tropical Storm Joyce as well as the edge of the larger Tropical Storm Helene to the east, which greatly influenced Joyce's motion.



Joyce did not affect any landmasses during its journey through the northeast Atlantic.

Friday, September 7, 2018

Hurricane Isaac (2018)

Storm Active: September 7-15

At the beginning of September, a tropical wave moved into the Atlantic. Though it was producing some showers and thunderstorms, but the disturbance remained disorganized for several days as it traveled westward. By September 5, the low had become better defined, but convection was quite limited near the center of circulation. On September 7, the disturbance was classified Tropical Depression Nine. That day, it also became nearly stationary as steering currents collapsed and it felt the pull of newly formed Tropical Depression Eight (which would become Helene) to its east. The center was nearly exposed at first due to shear, but the system's organization increased considerably on September 8. This prompted an upgrade to Tropical Storm Isaac.

The storm was very small, with tropical storm force winds extending only a few dozen miles from the center. Such storms are subject to rapid changes in intensity, and Isaac did gain strength quickly over the next day. Meanwhile, it finally picked up some forward speed toward the west. During the evening of September 9, it was upgraded to a hurricane. There was little change to the system over the next day, despite generally quite favorable conditions. The banding structure and core of Isaac struggled to improve, even with low wind shear. Soon, the satellite presentation became more ragged in appearance and the system was downgraded back to a tropical storm.

Early on September 12, an upper-level trough north of Isaac caused a sudden increase in shear on the system, quickly stripping convection away from the center. The cyclone began to rapidly weaken as a result. Nevertheless, it caused scattered heavy rains and tropical storm force winds as it passed among the Leeward Islands and entered the Caribbean during the morning of September 13. Thunderstorm activity started to make a comeback near the circulation center later that day, but the circulation itself was ill-defined and showed signs of becoming elongated. Isaac weakened to a tropical depression on September 14. Soon after, all traces of a closed circulation vanished and the storm dissipated. The remnants of Isaac brought scattered thunderstorms to Jamaica a few days later.



This image shows the small Tropical Storm Isaac moving over the open Atlantic.



Isaac dissipated shortly after entering the eastern Caribbean. This is a quite common event and this region is often referred to as a "tropical cyclone graveyard" by meteorologists.