Friday, August 31, 2012

Hurricane Leslie (2012)

Storm Active: August 30-September 11

A tropical wave that emerged off of Africa on August 27 produced scattered showers in the Cape Verde Islands the following day. It continued westward and organized quickly, developing a large area of convection by August 29. The next day, thunderstorms concentrated near the center of circulation, and a spin in the clouds became evident. Therefore, in the afternoon of August 30, the system was designated Tropical Depression Twelve.

Later that day, Twelve's winds increased, and it was upgraded to Tropical Storm Leslie. The center generally became closer to the deep convection overnight, and strengthening began, bringing Leslie to a strong tropical storm by the morning of August 31. The cyclone began to curve towards the north as it exploited a weakness in the Bermuda High, formed by Hurricane Kirk.

Though Leslie was near hurricane strength early on September 1, upper-level conditions deteriorated later that day, and wind shear increased significantly, displacing the convection to the southeast of the center. Meanwhile, the tropical storm made its closest approach to the Windward and Leeward islands of the Caribbean, bringing high surf, especially to the northeasternmost areas.

The cyclone turned northwest overnight, and despite the center being outside the deep convection, the winds associated with the system remained strong and actually increased slightly early on September 2, again bringing Leslie close to hurricane strength.

An upper-level low to Leslie's east, over the Bahamas, continued to induce moderate shear during the day. However, the convection associated with Leslie increased in area and intensity over the following day. By that night, an extremely large blob of thunderstorms, well over 500 miles wide, was churning southeast of the circulation center, featuring extremely cold cloud tops. However, the circulation itself remained disorganized, and Leslie's intensity change little into September 3.

By the afternoon, Leslie had turned farther to the north into the weakness of the aforementioned ridge. Despite this, the forward speed of the cyclone slowed to a crawl that evening, as the ridge restrengthened.

During the night, the system became much more organized as the deep convection assumed a more symmetrical appearance and persisted closer to the center. Soon after, though, dry air invaded the circulation, and the center once again became nearly devoid of thunderstorm activity. By this time, Leslie began to produce rough surf along the U.S. East coast, elevating the rip current threat.

Over the next few days, Leslie drifted northward, and gradually atmospheric conditions improved, allowing the system to periodically develop bursts of central convection. Additionally, on September 4 and 5, the cyclone generally increased in size, with gale force winds extending over 200 miles from the center by the afternoon of September 5. Around this time, an eye briefly appeared on visible satellite imagery, allowing the cyclone to be upgraded to Hurricane Leslie.

Since Leslie was nearly stationary during the day of September 6, strengthening was inhibited by the decrease of water temperature just below the system. Due to this, Leslie could not achieve any further eyewall development, and the inner core actually degraded in organization overnight and into September 7. This caused Leslie to weaken back to a tropical storm later that morning.

The cyclone finally began to move northward at a steadier pace that afternoon, as it became entrenched in the flow of a trough to its northwest. As Leslie moved over warmer waters not affected by upwelling, it began to regain convection, particularly in the northern semicircle.

The outflow of Leslie improved further overnight, and a large curved rain band developed on September 8 that surrounded the inner core. However, the center itself had a large eye feature, too large for a compact eyewall. Thus, the maximum winds of Leslie remained below hurricane strength, and the pressure, formerly near 980 mb, stayed relatively high.

Conditions also deteriorated in Bermuda that evening, as the outer bands swept across the island. By September 9, tropical storm conditions swept across Bermuda, and the center of Leslie made its closest approach late that morning, passing well to the east. The cyclone accelerated further that afternoon, and recovered some deep convection, though winds near the center remained somewhat light.

On September 10, Leslie started to rapidly accelerate north-northeastward, and already showed signs of extratropical transition that afternoon; the already broad center expanded further, and the cyclone became very asymmetric. However, winds recorded in a band north of the center supported raising Leslie's winds to 70 mph that evening. During the morning of September 11, Leslie made landfall in Newfoundland, bringing a wide area of gale force winds to the island and surrounding areas as it became extratropical.

The powerful cyclone continued over the open waters of the north Atlantic, passing just south of island on September 12, and finally combining with another low pressure system over northern Europe on September 13.

Leslie at peak intensity, with a small eye visible.

Track of Leslie.

Tuesday, August 28, 2012

Hurricane Kirk (2012)

Storm Active: August 28-September 2

On August 23, a tropical wave, already featuring a broad circulation, emerged off of Africa. Though the circulation deepened significantly over the next two days, convection remained scant, as the bane of the 2012 season, dry air, prevented further organization. The low moved northwest into less favorable environment the following day and wind shear took its toll on the system.

There was convection associated with the system as it moved over open waters, but it was displaced to the northwest. However, as shear abated slightly on August 27, the convection became nearer to, and even covered the center at times. On August 28, the system became more organized, and took a turn back to the west-northwest. By later that day, convection had persisted to such a degree that the cyclone was classified Tropical Depression Eleven.

Deep convection generally increased with the system overnight, giving it a more symmetrical appearance by the morning of August 29. Therefore, the cyclone was upgraded to Tropical Storm Kirk. Kirk increased in organization rapidly during that day, and, being a small cyclone, was able to quickly develop strong winds in its eyewall. On August 30, the structure of the system improved further as shear decreased, and a small eye formed, meriting the upgrade of Kirk to a hurricane that morning.

The cloud tops of the eyewall continued to cool that day, and the circulation became even more defined later that day, and Kirk rapidly strengthened through the night, bringing the cyclone to its peak intensity of 105 mph winds and a pressure of 970 mb early on August 31. By this time, the small cyclone was beginning to accelerate northward, as it came under the influence of mid-latitude winds.

However, conditions surrounding Kirk started to degrade later that day, as shear increased and the hurricane moved over cooler waters. Dry air finally invaded the system that afternoon, and the eye quickly vanished, as the compact circulation lost organization. Rapid weakening ensued.

By early September 1, Kirk had weakened to a tropical storm, and was accelerating towards the northeast. The system had already begun extratropical transition by later that day, but an area of deep convection near the center allowed Kirk to remain tropical through the morning of September 2, at this time still a moderate tropical storm. However, by the afternoon, the storm had lost any vestige of central convection, and was declared extratropical. It had been absorbed by a frontal boundary by the next day.

Kirk near peak intensity. Though a small system, Kirk featured a very symmetrical eye feature.

Track of Kirk.

Thursday, August 23, 2012

Tropical Storm Joyce (2012)

Storm Active: August 22-24

On August 19, a tropical wave, already associated with a low pressure center, emerged off of the African coast. Over the next few days, the circulation gradually became more organized, as the system assumed a general west-northwest motion on August 21. Convection decreased early on August 22, but the circulation became sufficiently defined for the cyclone to be upgraded to Tropical Depression Ten.

Throughout that day, deep convection slowly increased, but the center remained to the south of the coldest cloud tops, and the cyclone remained a tropical depression into August 23.

Late that morning, Ten gained the necessary organization to be upgraded to Tropical Storm Joyce. However, the cloud cover over the center was very brief, and shear out of the southwest quickly displaced all deep convection to the north. Though isolated areas of deep convection continued to form, they were well north of the center, and the circulation itself was losing definition.

Joyce weakened to a tropical depression later that day, and on August 24 was designated a remnant low. The remnants continued northwest, producing shower activity as they tracked over open waters. However, upper-level winds were unfavorable for regeneration, and the low soon dissipated.

Joyce, being the 10th tropical storm of the 2012 season, was named on August 23, and this is tied for the second-earliest a 10th named storm has ever formed in the Atlantic with 1995, behind only 2005's Jose, which formed on July 22 if that year.

Joyce as a minimal tropical storm on August 23. Shortly afterward, shear tore the system apart.

Track of Joyce.

Tuesday, August 21, 2012

Hurricane Isaac (2012)

Storm Active: August 21-September 1

A tropical wave moved off of the African coast on August 16, quickly developing widespread shower activity. A circulation slowly became defined in association with the system as it moved west over the open Atlantic over the following few days.

The low jogged to the north on August 18, and became exposed to some dry air the next day, limiting the convection near the center. However, the circulation continued to deepen, and early on August 21 the system was organized enough to be classified Tropical Depression Nine.

Nine's center was exposed by shear several times during the day, but deep convection persisted sufficiently for it to be upgraded to Tropical Storm Isaac that evening The general satellite appearance of the cyclone improved on August 22, but dry air completely prevented consolidation of the center and the concentration of deep convection. Meanwhile, Isaac crossed over the Lesser Antilles, bringing heavy rain and tropical storm force gusts.

The system consisted of two hemispheres of convection for much of the evening of August 22 and August 23, one northeast of the circulation, and the other, also the more intense, to the southwest. These areas of heavy rain were quite broad, but the winds near the center remained very light. Early on August 23, Isaac center reformed to the south of its former position.

During the day, Puerto Rico experienced rain and scattered downpours as Isaac passed well to the south. The convection had by this time amalgamated into a single mass, but the center was still slightly displaced from the deep convection. The system's motion shifted toward the west-northwest that night, and modest strengthening began as conditions in Hispaniola steadily deteriorated, with tropical storm force winds sweeping through the island beginning on August 24. That day, Isaac generally increased in organization, as the numerous vortices associated with the low-level center finally combined, and the circulation became more defined.

The cyclone made landfall in Haiti very early on August 25, packing winds just below hurricane force. Though land disrupted the development of deep convection in association with Isaac, its circulation remained largely intact and the storm weakened only slightly that morning as it emerged back over water. However, the center experienced a westward shift a few hours later, bringing it near the coast of eastern Cuba. Despite Isaac's northwestward motion away from Hispaniola, much of the moisture associated with the system remained entangled in the mountains of the island, causing drenching rainfall to continue over most of that day.

The cyclone moved roughly parallel to the northern coast of Cuba overnight at a faster clip than before, and so had difficulty recovering convection near its center. Additionally, Isaac's circulation remained somewhat entangled with a larger low-pressure system over the northwest Caribbean, which had an additional weak center south of Isaac, near Jamaica. Not only did this system cause dry air and moderate shear to invade the southern portion of Isaac's circulation, but the two lows, under the Fujiwhara effect, began to orbit slightly about a common center, causing the tropical storm's motion to shift toward the west early on August 26.

The atmospheric conditions steadily improved during the day, and Isaac experienced some strengthening as it approached the Florida Keys. A distinct eyewall finally formed later that day, and the windfield of Isaac broadened, causing tropical storm conditions to sweep over much of southern Florida and northern Cuba during that day. Overnight, the circulation jogged to the north, and appeared to be closer to the deepest convection than previously, but the maximum winds remained nearly constant at strong tropical storm intensity. Though the winds did not change, the central pressure dropped rapidly overnight and into August 27, falling below 985 mb that afternoon as Isaac moved northwest into the central Gulf of Mexico.

No longer under the influence of the low pressure area to its north or the ridge to its northwest, Isaac slowed considerably later that day, though maintaining a general northwestward motion. The outer circulation of the cyclone was impressive, with prominent outflow and powerful rain bands, one of which swept across the northern Gulf that morning. However, the internal structure of Isaac never fully coped with the dry air invading the circulation. A broad eye occasionally formed, but the eyewall was never closed completely. Despite this, the winds increased just enough during the late morning hours of August 28 for Isaac to be upgraded to a hurricane.

Isaac strengthened slightly even as it encountered land; the system reached its peak intensity of 80 mph winds and a pressure of 968 mb that night. At around 7:45 pm, Isaac made landfall in southeastern Louisiana. However, the cyclone barely moved overnight, and in fact briefly emerged over water early on August 29 after wobbling westward. By the morning, the hurricane had resumed a slow northwest motion, bringing it over Louisiana again, slightly farther westward than before.

Isaac weakened very slowly over the following day, losing hurricane status that afternoon. The cyclone moved slowly northwest, its lack of forward speed causing flooding rains throughout Louisiana, Mississippi, and Alabama, with localized amounts over 20 inches. The system maintained tropical storm strength through the morning of August 30, at which point it was moving through northern Louisiana.

It weakened to a tropical depression that afternoon, and became asymmetric, as is typical of systems tracking inland. Most of the thunderstorm activity remained on the north side of the center, though more scattered rain bands to the south and east still persisted. The center moved over Arkansas later that day, and crossed in Missouri on August 31. It took a turn to the east later that day, and dissipated as it combined with a front on September 1.

The remnants of Isaac and the tropical moisture it carried northward caused shower activity throughout the Ohio River Valley and the northeast during the following few days.

Isaac's legacy throughout the Caribbean and U.S. was heavy rainfall. Particularly near its final landfall in Louisiana, the cyclone moved very slowly, and therefore had the opportunity to dump flooding rains over a huge swath of the southern U.S. However, it also provided needed rain to states farther north.

Isaac as a minimal hurricane. Even at peak strength, it had a poorly defined inner core.

Track of Isaac.

Thursday, August 16, 2012

Hurricane Gordon (2012)

Storm Active: August 15-20

A strong tropical wave and associated low pressure system emerged off of Africa on August 9 a produced an organized area of showers and thunderstorms. However, it tracked west-northwest, and took a path slightly north of the Cape Verde Islands. Soon after, unfavorable conditions caused convection associated with the system to diminish.

However, the low continued to move across the open Atlantic, and emerged into a area of lower wind shear on August 14. Later that day, significant shower activity developed, and the circulation became better organized. On August 15, the satellite presentation was sufficiently impressive to merit the classification of the system as Tropical Depression Eight, despite surface pressures remaining somewhat high in the area.

The cyclone was at that time well east of Bermuda, and it moved north as it exploited a weakness in the Bermuda high ridge. By early on August 16, convection had markedly increased, and the system was upgraded to Tropical Storm Gordon. Deep convection continued to concentrate about the center during that day, and Gordon strengthened as it curved to the east under the influence of a subtropical ridge.

By late that night, Gordon had reached an intensity of 70 mph winds and a pressure of 995 mb. However, under the influence of moderate shear, it weakened slightly that night.

Gordon reasserted itself early on August 18, and a prominent eye feature appeared, resulting in the upgrade of Gordon to a hurricane. The cyclone continued to strengthen throughout the day as it moved west, becoming a Category 2 hurricane that evening as it approached the Azores, and reaching its peak intensity of 110 mph winds and a pressure of 965 mb late that night.

By the morning of August 19, Gordon began to interact with a trough to its northwest, and took a turn to the north, adopting a east-northeast motion. In addition, an increase in shear and a decrease in sea surface temperatures caused Gordon to steadily weaken that day. Conditions in the Azores began to deteriorate during the evening.

Early on August 20, Gordon made landfall in Santa Maria Island of the Azores as a minimal hurricane, where winds gusts in excess of 80 mph were recorded. The convection in the southern portion of the system had degraded significantly by this time, and the cyclone was in the midst of extratropical transition. By later in the morning, Gordon had weakened to a tropical storm and lifted out of the Azores. The system was declared extratropical that afternoon, and dissipated a few days later. Minor damage and no injuries were reported in the Azores.

Gordon at peak intensity moving rapidly east over the central north Atlantic.

Track of Gordon.

Friday, August 10, 2012

Tropical Storm Helene (2012)

Storm Active: August 9-11, 17-18

On August 5, a tropical wave and associated low pressure system moved off of Africa. It exhibited small areas of thunderstorm activity and increased in organization over the following days as it moved west over the tropical Atlantic.

During the afternoon of August 9, convection became concentrated at the center of the system, and it became Tropical Depression Seven. From its genesis, the system interacted with dry air, and this inhibited development of shower activity north of the center later that day and into August 10. The persistent ridge over the north Atlantic caused Seven to track westward rapidly.

By August 11, the system was moving so rapidly that an organized center of circulation no longer existed in conjunction with the system. It was thus downgraded to a tropical wave. The wave continued to move into the Caribbean, producing some heavy rain around the Windward Islands. Convection increased as the wave approached central America, but land prevented redevelopment.

By August 14, the remnants of Seven had moved over land, but the system took a turn to the northwest, ultimately allowing it to emerge into the Bay of Campeche on August 16. Convection increased in association with the system, and a center of circulation became defined on August 17, causing the system to be upgraded to Tropical Storm Helene, already at its peak intensity of 45 mph winds and a minimum pressure of 1004 mb.

The system moved northwest towards the coast of Mexico, and weakened upon interaction with land. Convection remained scant, and the center was hard to identify on satellite imagery during the morning of August 18. The cyclone made landfall during the late morning hours in Mexico, bringing heavy rainfall to some inland areas.

The cyclone weakened to a tropical depression that evening, and dissipated completely late that night, its circulation destroyed by the mountains of Mexico. Overall, the impact of the system was heavy rain, particularly in Trinidad and Tobago (as a tropical wave), and the coast of the Bay of Campeche.

Helene at peak intensity after reforming in the Bay of Campeche.

Track of Helene, including its path through the Caribbean as a non-tropical system (non-tropical points indicated by triangles).

Saturday, August 4, 2012

Tropical Storm Florence (2012)

Storm Active: August 3-6

On August 3, a low pressure center center formed in association with a tropical wave near the Cape Verde Islands. The system was already producing a wide area of thunderstorm activity, and it organized rapidly, perhaps fueled by a local surface water temperature anomaly. In any case, the circulation was already very well-defined by the evening of August 3, and it was therefore classified Tropical Depression Six.

At that time, Six was southwest of the Cape Verde Islands. It adopted a west-northwest motion, and, following a more northerly path, encountered some dry air on August 4. However, the satellite presentation indicated that the system's winds had reached tropical storm force and the depression was named Tropical Storm Florence.

A well-defined center of circulation as well as an eyewall developed that day, and Florence strengthened into August 5, reaching its peak intensity of 60 mph winds and a pressure of 1000 mb. Soon after, however, dry air infiltrated the system, and convection decreased markedly. By that evening, the center was exposed, and the circulation already showed signs of elongation. Florence weakened quickly as a ridge to its north turned the cyclone westward. By the morning of August 6, there was no convection within 50 miles of Florence's center. It was downgraded to a tropical depression and a remnant low shortly after.

The remnants of Florence tracked across the Central Atlantic over the next few days, producing small areas of convection, despite the stable air environment. It degenerated further into an open wave on August 8. By August 12, the system had dissipated completely. Florence, becoming a tropical storm on August 4, was tied second-earliest formation of a sixth named storm in the Atlantic with 1936, behind only the most active season on record, 2005.

Tropical Storm Florence shortly after developing into a tropical storm.

Track of Florence over the Eastern Atlantic.

Thursday, August 2, 2012

Hurricane Ernesto (2012)

Storm Active: August 1-10

On July 29, a weak low pressure center was identified southwest of the Cape Verde Islands. Some isolated showers and thunderstorms were associated with the system, and it was monitored for development. The system began to organize over the following days, though its circulation was inhibited somewhat by its proximity to the equator. The wave remained embedded in the ITCZ until July 31. However, on that date, the center shifted northward, allowing the low to organize further.

On August 1, there was enough convection accompanying the center to declare the system Tropical Depression Five. El NinĂ³-related shear from the northwest affected the system immediately, pushing dry air into the northern portion of the system. Meanwhile, strong steering currents were pushing Five westward at a fast clip, causing it to approach the Windward Islands on August 2. The circulation became more organized that day, and the confirmation of tropical storm force winds east of the center of circulation by aircraft reconnaissance resulted in the tropical depression being upgraded to Tropical Storm Ernesto.

Ernesto's initial winds were 50 mph, but its convection diminished overnight, causing it to weaken slightly as it crossed the Windward Islands early on August 3. The center reformed farther to the south later that morning, and the system began to reorganize as it entered the Caribbean. Ernesto's intensity fluctuated under marginally favorable ambient conditions, but the outflow and satellite presentation consistently improved overnight and into August 4.

As the storm moved through the central Caribbean that day, it decelerated slightly, though still bearing mostly west. Its outer bands skirted Puerto Rico and the southern Dominican Republic that evening. Over the next day, Ernesto lost some organization, its center became less well-defined, possibly due to dry air invading the cyclone. Ernesto's rapid motion also contributed to its disorganization.

On August 5, the weakened Ernesto lost any vestige of organized central convection, but still maintained low-end tropical storm intensity. It changed direction, now heading due west towards Central America.

That night, however, Ernesto continued its erratic behavior, and rapidly organized. Bursts of convection soon covered the center, and the cyclone's forward motion showed. During the morning of August 6, Ernesto quickly strengthened, and the beginnings of an eye became apparent on satellite imagery. Additionally, the system took a turn to the west-northwest.

That night, Ernesto's outer bands swept over Honduras, causing locally heavy rainfall and tropical storm force wind gusts. Dry air was evident in the interior of the northwest quadrant overnight, but the system's outflow continued to improved and the central pressure decreased into August 7.

By later that day, Ernesto finally became sufficiently organized to be upgraded to a hurricane. It continued to strengthen until that evening, reaching its peak intensity of 85 mph winds and a pressure of 980 mb just as it made landfall in the Yucatan Peninsula just north of the Mexico-Belize border at about 11:15 p.m. EDT, or 10:15 local time.

Over land, Ernesto caused localized flash flooding and heavy winds throughout the Yucatan and the neighboring parts of Central America. The system was downgraded to a tropical storm early on August 8, and continued to weaken through the morning, as it moved west toward the Bay of Campeche.

Late in the afternoon, Ernesto's center emerged over water, and the cyclone immediately began to restrengthen, despite its proximity to the Mexican coast. Its organized circulation allowed moisture to be drawn into the center, and the winds increased rapidly, bringing the system to near hurricane strength early on August 9, as torrential downpours continued over land. Under the influence of a ridge to its north, Ernesto turned west-southwest and made its final landfall in Mexico around noontime that day.

The cyclone maintained its structural integrity quite well over land, and so caused rain and tropical storm force winds through the remainder of the day and into early August 10. It was downgraded to a depression and dissipated later that morning. The remnants of Ernesto moved into the Eastern Pacific basin soon after, where they contributed to the development of Tropical Storm Hector.

Ernesto as a hurricane before landfall in Mexico.

Track of Ernesto.