Thursday, September 29, 2011

Hurricane Philippe (2011)

Storm Active: September 24-October 8

A well-defined low pressure area formed west of the African nation of Guinea on September 23. The low deepened rapidly later that day, already having developed large rain bands spread out evenly around the circulation. Convection decreased that night, but became very concentrated near the center, and the system became closed early the next morning. It was then declared Tropical Depression Seventeen. Slow organization followed later in the day as Seventeen moved west-northwest, and the system soon intensified into Tropical Storm Philippe.

A weakness in the ridge to Philippe's north allowed it to turn northwestward and decelerate into the day of September 25, when it reached an intensity of 60 mph winds and a pressure of 997 mb. The tropical storm maintained this strength into September 26, by which time it began to encounter higher wind shear and cooler waters. A gradual weakening ensued, as the circulation became exposed on September 27. That afternoon, Philippe's forward motion slowed to 5 mph and its shallow circulation was steered back to the west-northwest with a restrengthening of the subtropical ridge to its north.

That night, despite the fact that Philippe was a minimal tropical storm with little convection near the center, the outflow remained fairly robust, and thunderstorm activity reappeared just north of the center on September 28. The cyclone even strengthened a little that day. On September 29, the center reformed to the north of its previous position, and once again was completely enveloped in the convection associated with Philippe. The exact center position of Philippe remained difficult to identify overnight, but the circulation became more well-defined on September 30, and the cyclone strengthened further.

As the storm moved farther to the northwest, it encountered very strong shear associated with the outflow of Ophelia, but the tropical storm once again demonstrated resilience to strong upper-level winds, and maintained its intensity, deep convection even increasing near the center of circulation overnight. An analysis of the windfield of Philippe revealed it to be stronger than expected, with winds of 65 mph. Late that night, more intensification occurred, bringing Philippe to the verge of hurricane strength early on October 2.

However, the unfavorable conditions near the cyclone finally began to take their toll that morning, exposing the center and weakening the system. As before, however, Philippe recovered its convection, as Ophelia moved north, and conditions became less hostile. The next day, winds were found to be stronger than previously estimated, and the cyclone was a strong tropical storm yet again. Meanwhile, the center reformed farther to the south than before, and Philippe moved west-southwestward during the day of October 3.

After navigating around the periphery of the ridge steering it, the tropical storm began to turn north on October 4. Recurvature occurred quickly during the following days, and shear gradually decreased. On October 5, the center remained displaced from the deepest convection, but, on October 6, strengthening finally began, bringing Philippe to hurricane strength after over 12 days as a tropical cyclone. An eye feature formed later that evening, particularly visible on infrared imagery, and the hurricane reached its peak intensity of 90 mph winds and a pressure of 976 mb that evening.

Upper-atmospheric conditions rapidly deteriorated on October 7, causing weakening as the cyclone accelerated to the east-northeast. The center quickly became exposed that afternoon, and Philippe's brief period as a hurricane was over. A front encouraged extratropical transition on October 8, and absorbtion completed in that afternoon. Philippe was the longest lived tropical cyclone in the Atlantic since Bertha of 2008, lasting over two weeks. Despite this, it affected no land.

Hurricane Philippe over the open Atlantic.

Track of Philippe.

Friday, September 23, 2011

Hurricane Ophelia (2011)

Storm Active: September 20-October 3

On September 17, a broad area of showers and thunderstorms formed in association with a large low pressure system in the eastern Atlantic. Three low pressure centers existed in close proximity near the area during the day of September 18, oriented west to east near the 10ºN parallel. Over time, the central low became stronger and dissipated the others, and convection increased. The circulation also increased in definition over the following days, but numerous reformations of the center kept the circulation open. However, late on September 20, thunderstorms became concentrated near a single circulation center, and the low was upgraded directly to Tropical Storm Ophelia.

As the tropical storm moved west over the central Atlantic, shear steadily increased, but despite unfavorable conditions, winds in Ophelia's rain bands increased significantly, and strengthening ensued. On September 21, convection was displaced to the east of the center, but notwithstanding any satellite data to the contrary, winds were reported that suggested a strong tropical storm intensity. By the morning of September 22, Ophelia had reached an intensity of 65 mph winds and a pressure of 994 mb.

Wind shear began to finally take its toll on the system later that day, and the cyclone weakened. By the morning of September 23, Ophelia, had weakened to a minimal tropical storm, and all remaining convection was pushed northeast into a rain band that extended several hundred miles. Meanwhile, Ophelia took a turn to the west-northwest. Later on September 23, shear temporarily relaxed, and shower activity re-ignited near the center of the cyclone, causing it to unexpectedly strengthen that evening.

However, this increase in intensity did not persist, as strong upper-level winds resumed early on September 24. Ophelia weakened once again during that day, and, as a result, turned back to the west. As the system's circulation continued to become shallower, a due west motion was assumed, and by the morning of September 25, Ophelia was a minimal tropical storm. A large area of convection was over 150 miles east of the center during the day, with the center itself bare but for a few showers to the northeast. The Lesser Antilles, to Ophelia's southwest, experienced some gusty winds during the afternoon, as the center of the circulation became more elongated. Subsequently, the system was observed to lack tropical characteristics, and was downgraded to a remnant low.

The exposed center quickly dissipated, but a new swirl quickly became evident in the thunderstorm activity to the east. During the day of September 26, upper-level winds relaxed, and the circulation of the newly formed low became much better defined that evening. As the system continued to organize overnight, it drifted eastward and southward, stalling close to the Leeward Islands. On September 27, moderate rainfall occurred over these islands, as convection increased further during the afternoon, and the low became organized enough to be redesignated as Tropical Depression Ophelia.

Shear still abounded over the region overnight, and strengthening was gradual, bringing Ophelia to tropical storm strength during the morning of September 28. By this time, the cyclone had assumed a definite north-northwestward motion, and it moved away from the Caribbean. Outflow improved throughout the same day, allowing the cyclone to hold its own despite marginally favorable conditions. Further strengthening followed that night, and the system was a powerful tropical storm by September 29. An impressive banding feature formed to the east of the circulation later that afternoon, causing Ophelia to be upgraded to Category 1 hurricane status.

During the same evening, Ophelia's eyewall solidified, and the eye itself became more consistent in its appearance on satellite imagery. Following these structural changes, the cyclone rapidly strengthened. By the morning of September 30, Ophelia was a Category 2 hurricane! However, the strengthening was by no means over, as the hurricane assumed a more rounded appearance, the outflow improving and the eye broadening further. By that afternoon, Ophelia was a Category 3 hurricane. Meanwhile, the storm turned north and started to accelerate, as a large trough exited the U.S. east coast and pushed it farther poleward. Late that night, the system finally stabilized in intensity at 120 mph winds and a pressure of 956 mb.

Early on October 1, the pressure dropped slightly to 952 mb, and the outer bands of Ophelia began to impact Bermuda. During the day, gusty winds and scattered heavy squalls affected the island, but the hurricane passed well to the east, making its closest approach late that afternoon. Just after passing Bermuda, Ophelia unexpectedly underwent rapid intensification, bringing it to its peak intensity as a Category 4 hurricane with 140 mph winds and a pressure of 940 mb. During the morning of October 2, Opheila finally began to weaken as it turned to the north-northeast, reaching a forward speed of 30 mph north of 35ºN latitude. The circulation slowly lost definition as it raced towards Newfoundland ahead of a cold front to its west.

That night, Ophelia lost most of its remaining tropical characteristics, weakening to a tropical storm during the morning of October 3. Any central convection still associated with the system vanished as Ophelia made landfall in the Avalon Peninsula, and the system became extratropical. The cyclone continued northeastward until dissipating on October 5. Minimal damage was the only effect of Ophelia, with no fatalities occurring.

Hurricane Ophelia nearing peak intensity as a Category 4 hurricane.

Track of Ophelia, including time spent as an non-tropical system before reforming.

Thursday, September 8, 2011

Hurricane Nate (2011)

Storm Active: September 7-11

After Tropical Storm Lee became extratropical over the southeast U.S., an extension of its associated frontal boundary dipped into the southern Gulf of Mexico. On September 6, the interaction of this front with a trough of low pressure caused a low pressure system to form in the Bay of Campeche. Over the next day, the low hardly moved, and organized rapidly. On September 7, the low-level circulation of the low became more well-defined and the system was upgraded into Tropical Storm Nate.

Nate formed with a very slow motion to the southeast, and did not move significantly overnight. Despite a low shear environment, there was one inhibiting factor to strengthening: a large area of dry air to the system's northwest, occupying the entire northern Gulf. During the day of September 8, the center drifted further southeast, becoming closer to the convection, which had been displaced to the southwest of the center by moderate wind shear. Nate was sheltered somewhat, and strengthening commenced. By the evening of that day, Nate was completely stationary, and at its peak intensity of 75 mph and a pressure of 994 mb*.

Dry air entered the system during the early morning of September 9, however, and weakening took place. Later that day, Nate began to move slowly northwest, away from the Yucatan Peninsula, as further weakening occurred. By the morning of September 10, rainbands had recovered somewhat on the periphery of the cyclone, but the center remained devoid of convection, giving Nate a hollow appearance. The tropical storm finally adopted a definite motion later that day, moving due west. As it did so, conditions improved slightly and re-strengthening occurred afternoon. Nate reached its secondary peak intensity of 65 mph winds before convection decreased once again early on September 11.

The position of the center became very uncertain as Nate approached the Mexican coast, and the definition of the circulation decreased, lowering Nate's intensity to only 45 mph as it made landfall in Veracruz. The cyclone quickly weakened to a remnant low that night. Not much convection was associated with Nate over its lifetime, but its slow movement still allowed prolonged periods of gusty winds and rain along many parts of the coast of the Bay of Campeche. 7 fatalities were the result of Nate.

*Nate was upgraded to a hurricane during the 2011 postseason analysis, its maximum winds having previously been recorded as only 70 mph.

Tropical Storm Nate at peak intensity on September 8. At this time, Nate was nearly stationary.

Track of Nate.

Wednesday, September 7, 2011

Hurricane Maria (2011)

Storm Active: September 6-16

On September 4, a tropical wave off of the African coast began to generate shower activity south of the Cape Verde Islands. This activity became more widespread and more organized with the westward moving wave on September 5, as surface pressures began to drop over the region. A low pressure center developed in association with the wave that day, and the circulation became closed on September 6, announcing the formation of Tropical Depression Fourteen. The depression slowly organized over the next day, as it moved quickly west-northwest. A rapid intensification occurred during the morning of September 7, as an area of deep convection appeared northeast of the center, and the cyclone was upgraded to Tropical Storm Maria, with 50 mph winds.

High wind shear affected the system from its formation, however, and a powerful ridge to Maria's north imparted it with an extremely rapid westward motion of 23 mph during the night and into the morning of September 8. The storm struggled to maintain a circulation during the afternoon, and began to weaken as a result. All but minimal central convection disappeared during the early evening, and Maria became a minimal tropical storm. Overnight, however, a huge area of convection appeared. Despite this new thunderstorm activity, Maria did not intensify significantly on September 9, as the low level center was ill-defined and difficult to locate on satellite imagery.

On September 10, all convection was displaced to the northeast of the center by wind shear. So although being in close proximity to the northeast Caribbean Islands, the storm caused nearly no rain there as Maria moved northwest. The cyclone began to recover organization once again during the morning of September 11, and became a strong tropical storm as it moved west-northwest past the U.S. Virgin Islands that afternoon. A powerful trough coming off of the United States east coast started to affect Maria that evening, and it took a turn to the north on September 12, though slowing to nearly stationary. On September 13, the system began to accelerate, and finally moved away from the upper-level low that had been affecting it for so long with wind shear.

The center of Maria reformed within an area of deep convection late that night, and banding features increased in organization. During the morning of September 14, intensification began, and shear decreased. The decrease was gradual, however, and no further change occurred during the afternoon. By this time, Maria had turned to the north-northeast and already was accelerating extremely rapidly. Conditions in Bermuda deteriorated late that night and into the morning of September 15, as Maria passed to its west at near hurricane strength. Deep convection appeared near the eye that afternoon, and an eye structure began to form. Maria was therefore upgraded to hurricane status.

That night, Maria reached its peak intensity of 80 mph winds and a pressure of 979 mb. During the morning of September 16, the cyclone's forward speed exceeded 50 mph. Its closed circulation vanishing, Maria made landfall in Newfoundland as a minimal hurricane on the cusp of extratropical transition that afternoon, followed by absorbtion by a front a few hours later.

Hurricane Maria moving rapidly northward. The cyclone is already exhibiting some extratropical traits.

Track of Maria.

Friday, September 2, 2011

Tropical Storm Lee (2011)

Storm Active: September 1-4

During the day of August 30, shower activity increased with a tropical wave moving west-northwestward through the western Caribbean Sea. Meanwhile, a stationary low pressure trough was situated over the Gulf of Mexico. During the next few days, the tropical moisture of these two systems combined, forming a very large area of strong thunderstorms over the eastern Gulf, stretching from the Yucatan Peninsula to Florida. Shear from the west affected the system, but began to slowly abate during the day of September 1. During that afternoon a circulation appeared on the western edge of the clouds, becoming closed by the evening. Tropical Depression Thirteen had formed.

During the night, the depression's center reformed farther south and lost nearly all of its initial motion to the northwest, Due to weak steering currents, the system remained nearly stationary through the morning of September 2. Also, an area of deep convection developed near the center, despite the center itself remaining rather elongated. Due to the system's large size, rain and gusty winds already were moving into southeastern Louisiana. The circulation of the system became more well-defined that afternoon, and Thirteen became Tropical Storm Lee.

Lee had an unusually large windfield even at the time of its formation, and it continued to expand later that day. The cyclone itself also slowly strengthened, moving erratically, but generally northward. Lee still exhibited some characteristics of a non-tropical cyclone even into September 3, with the southwest quadrant still devoid of convection. During that morning, heavy rain continued across Louisiana and Mississippi, and tornadoes were reported within the heaviest bands, located in southern Louisiana.

Lee strengthened further as it slowly moved toward the Gulf coast, reaching its peak winds of 60 mph during the day. However, a second low-level center developed within the system early that afternoon, the original over land, and the second still off the coast. The new formed center took over the circulation while the other dissipated, and Lee therefore stalled off of the coast of Louisiana, with tropical storm conditions still extending far inland all across the central Gulf states. During the night, convection decreased, with the only rain band near the center extending to the southwest. Gradual weakening occurred, but Lee still did not assume any definite motion, and was still hovering over the coast, continuing the already severe flooding of the surrounding areas. Despite the winds having fallen, Lee's central pressure decreased to 987 mb early on September 4. The system finally made landfall in Louisiana later that morning.

The circulation actually became better defined for a brief period over land that afternoon, but the cyclone quickly degenerated, weakening further that evening, and began extratropical transition late that night. The system was fully extratropical by the next day, but the rain was by no means over. During the day of September 5, the remnants of Lee combined with a powerful cold front. With the addition of tropical moisture, a region of heavy rain formed from the tail end of Lee, in Louisiana, through the end of the front, in Canada! Lee's remnants lost their well-defined center on September 8, but rainfall continued for two more days, finally ending on September 10. The several days that Lee spent moving up the coast saw unprecedented flooding in the the mid-Atlantic states, New England, and even in some areas as far north as Canada. Rainfall totals from the combined storm system exceeding 6 inches in widespread areas, with local amounts significantly greater. 21 fatalities resulted from Lee, along with over $250 million in damages.

Lee strengthening over the northern Gulf of Mexico.

Track of Lee.

Note: In post-season analysis, Lee was confirmed to have been subtropical from September 3 up to the points of extratropical transition. This means that the cyclone transitioned from tropical to subtropical, a very rare event.