Friday, October 26, 2012

Tropical Storm Tony (2012)

Storm Active: October 22-25

A tropical wave formed in association with an area of disturbed weather accompanying an upper-level low about midway between the western coast of Africa and the Lesser Antilles on October 18. This interaction sporadically produced concentrated thunderstorm activity as it moved to the northwest over the next several days.

Atmospheric conditions improved markedly on October 21, and allowed the system to organize rapidly into Tropical Depression Nineteen on October 22. The cyclone began to curve towards the northeast later that day, as a front approached from the west. By late on October 23, convection had developed close enough to the center of circulation that the cyclone was upgraded to Tropical Storm Tony.

Despite shear from the front to its west, Tony experienced modest strengthening as it accelerated to the east-northeast, and the storm reached its peak intensity of 50 mph winds and a pressure of 1000 mb on October 24. Meanwhile, Tony was beginning to exhibit nontropical characteristics; the banding features became more linear, and the circulation elongated. The transition was very gradual, however, and the system remained a tropical storm through October 25, at which time it lost any remaining tropical characteristics and was downgraded to a remnant low.

Tony as a moderate tropical storm moving rapidly to the northeast over the Central Atlantic.

Track of Tony.

Wednesday, October 24, 2012

Hurricane Sandy (2012)

Storm Active: October 22-29

A low pressure trough embedded in the Intertropical Convergence Zone moved into the Caribbean sea on October 18, and began to increase in shower activity the next day. On October 20, as the area of disturbed weather moved west, the pressures in the area dropped precipitously, and the circulation became much better organized.

Deep convection did not consistently accompany the system on October 21, but conditions continued to be favorable as the disturbance moved southwest, bringing some showers to Jamaica and neighboring areas. By October 22, a swirl was evident amid the clouds, and the low was classified as Tropical Depression Eighteen. The system drifted southward and organized further later that day, and strengthened into Tropical Storm Sandy.

Sandy adopted a slow but accelerating northward motion early on October 23, as a front lifted out of the northwestern Caribbean. Meanwhile, convection steadily increased with the system, and became closer to the center by later that day, causing steady strengthening. In the evening, the cloud tops of Sandy's central dense overcast cooled considerably, and the first hints of an eye feature appeared, indicating that the cyclone was undergoing rapid strengthening. Meanwhile, the outflow had improved, with heavy rain bands sweeping across Jamaica, Hispaniola, and eastern Cuba as Sandy approached. These factors caused the cyclone to be upgraded to a hurricane later that morning.

During the afternoon, the center of Sandy passed directly over eastern Jamaica, but the land interaction did almost nothing to disrupt the circulation and the system continued strengthening, as an eye appeared on infrared as well as visible satellite imagery. Over the next twelve hours, Sandy put on a burst of extremely rapid strengthening, bringing its pressure down to a value of 954 mb. Very early on October 25, the cyclone made landfall in eastern Cuba with its peak winds of 110 mph!

Sandy weakened slightly as it moved over Cuba, but emerged over water still maintaining Category 2 intensity. The cyclone slowed down considerably and turned to the north-northwest that night as it interacted with an upper-level low. Higher shear weakened the system as it lashed the Bahamas, but the structure of the storm also underwent a transformation. Convection became displaced from the center in all but the northwestern quadrant, the windfield broadened, and the outflow became more extratropical in appearance on October 26.

However, shear declined somewhat, and thunderstorm activity more completely covered the center by early on October 27. By this time, Sandy had begun to moved towards the north-northeast, fluctuating in intensity but remaining near minimal hurricane strength.

By later that day, rain bands associated with the combination of a front stalling near the U.S. east coast and the circulation of Sandy swept across numerous states, causing tropical storm force winds in the North Carolina and heavy rain in localized areas up through Virginia. Dry air also invaded the circulation of Sandy, creating a narrow ring devoid of thunderstorm activity between the central convection and outer bands. However, this did not weaken Sandy, as the system was exhibiting some subtropical behavior.

Early on October 28, the central pressure of Sandy dropped again as the cyclone deepened further, plunging to a new low of 951 mb. Meanwhile, the cyclone accelerated to the northeast, and gale force winds expanded even further, stretching from North Carolina all the way to Bermuda, and rain bands moved further up the coast, sweeping across Pennsylvania and New Jersey.

During the night, Sandy began a highly unusual turn towards the northwest, under the influence of an exceptionally strong high pressure ridge over northeastern Canada. This ridge caused an inversion in the normal path of the jet stream, diverting it so that it doubled back on itself. The cyclone began to be drawn in by this feature, and so curved in the opposite direction that tropical cyclones typically turn.

Meanwhile, as Sandy traversed the warm waters of the Gulf Stream, it actually intensified somewhat, despite being at a fairly high latitude. In addition, the pressure continued to drop. Conditions deteriorated rapidly along the Delaware and New Jersey coastlines that afternoon as the central bands of the cyclone came onshore. Hurricane force wind gusts and storm surges in excess of 5 feet were recorded up and down the coast. Sandy accelerated rapidly that afternoon, and was losing tropical characteristics as its central band became frontal in nature. Early that evening, the system recorded its minimum pressure of 940 mb, and winds of 90 mph.

Shortly afterward, around 7:00 pm EDT, Sandy was recognized as an extratropical cyclone, and the remnants of Sandy made landfall in southern New Jersey an hour later. High wind and occasional heavy rain continued as the low crossed into Pennsylvania late that night and weakened to the equivalent of a tropical storm early on October 30. The low continued westward and weakened, still causing rain and snow in the Appalachian areas until it dissipated on October 31. The remnants still caused shower activity for another few days as they moved northeast away from the United States.

Hurricane Sandy set a new record for the largest Atlantic hurricane, with a gale diameter of 945 miles a few hours before landfall in New Jersey, and was one of the costliest in U.S. history. Sandy caused widespread damage in a large swath extending from Jamaica, through Cuba and the Bahamas, and up the east coast from North Carolina to New England.

Sandy near peak intensity near landfall in eastern Cuba.

Track of Sandy.

Saturday, October 13, 2012

Hurricane Rafael (2012)

Storm Active: October 12-17

On October 5, a tropical wave emerged off of Africa, but remained weak for the next several days as it traversed the eastern Atlantic. The system gradually moved over warmer waters and increased in shower activity. By October 9, the system had a well-defined low pressure center associated with it. Shear out of the west still impacted the low, but atmospheric conditions improved over the following few days.

Rain and windy conditions began to affect the Lesser Antilles and surrounding areas during the day of October 12. At this time, the convection was becoming concentrated at the center of circulation, but was still somewhat displaced to the east. Later that day, aircraft indicated that the cyclone had developed a closed center, and was thus classified Tropical Storm Rafael. At the time of its formation, the convection was still distributed linearly along the former trough boundary, and the circulation remained slightly elongated. However, the thunderstorm activity was very vigorous; there were widespread areas of heavy squalls and tropical storm force wind gusts.

Rafael moved generally to the north-northwest over the following day, and very slowly organized, with a more defined region of cold cloud tops appearing near the center during the afternoon of October 13. This initiated a period of strengthening as the center moved closer to the Virgin Islands. The system passed close to the northeastern Caribbean islands late that night with maximum winds of 50 mph as the cyclone continued its trek north.

On October 14, Rafael took a slight turn towards the northwest as the ridge over the north-central Atlantic strengthened, slowing as it did so. By this time, the cyclone had assumed a more symmetric appearance, and on October 15, the circulation finally achieved gale force winds on all sides of the center, and was near hurricane intensity. Later that night, a flare up of very strong convection appeared at the center, prompting the upgrade of Rafael to a Category 1 hurricane.

Early on October 16, the system began to accelerate northward in the flow of a trough emerging off of the U.S. coast, and vertical shear increased. Despite the intense shear, however, outflow remained remarkably healthy in all quadrants, and Rafael strengthened further, reaching its peak intensity of 90 mph winds and a pressure of 969 mb that morning. By the evening, the cyclone was approaching Bermuda as an impressively large cyclone.

The system made its closest approach to the island that night, and continued to accelerate to the north-northeast, moving away from the island at over 25 mph. Early on October 17, Rafael's circulation assumed an extratropical appearance, with a very large area of gale force winds and bands extending many hundreds of miles from the center. However, the hurricane maintained a small amount of deep convection near the center until that afternoon, at which time is was pronounced extratropical, still producing hurricane-force winds as an extratropical low. The low continued northeast before combining with another powerful system over the north Atlantic the next day.

Rafael as a Category 1 hurricane moving north into open waters.

Track of Rafael.

Friday, October 12, 2012

Tropical Storm Patty (2012)

Storm Active: October 11-13

On October 11, a low pressure center formed at the tail end of a frontal boundary extending from the northeast Atlantic down to near Hispaniola. Disturbed weather increased near the low during the following day, as the low became disassociated with the trough to its northeast.

Over the next several days, another frontal boundary began to approach the low, causing a sharp increase in wind shear. However, the low did not get caught in the flow ahead of the front, but instead remained nearly stationary just to the northeast of the Bahamas through October 10. Despite being expected to merge with the front, the system maintained its identity, and in fact became more organized, as thunderstorm activity concentrated near the center.

By the afternoon of October 11, the low had achieved enough deep convection to be considered a tropical cyclone and so was classified Tropical Depression Sixteen. That evening, the convection increased and covered the exposed circulation, and the cyclone was therefore updated to Tropical Storm Patty. Late that night, Patty unexpectedly strengthened further, and reached its peak intensity of 45 mph winds and a pressure of 1005 mb.

On October 12, a combination of strong southwesterly upper-level winds and a northeasterly low-level flow started to pull the circulation apart. Patty weakened to a tropical depression that evening as the center once again became completely exposed. By the morning of October 13, the circulation was no longer closed, and Patty was declared a remnant low. The remnants combined with a trough of the U.S. east coast shortly afterward.

Patty as a disorganized tropical storm struggling to survive just north of the Bahamas.

Track of the short-lived Patty. Most of the positions indicate occurred when the cyclone was non-tropical (triangles), with only the tiny clump of circles accounting for Patty's time as a tropical cyclone, in which it moved little.

Saturday, October 6, 2012

Tropical Storm Oscar (2012)

Storm Active: October 3-5

On September 30, a tropical wave off of the coast of Africa began to show signs of an organized circulation, though convection remained limited. The disturbance moved generally west-northwest over the following two days, and increased markedly in thunderstorm activity. The circulation remained slightly elongated into October 3, but the system was sufficiently organized to be designated Tropical Depression Fifteen.

The cyclone had internal structure issues ab initio; at its formation, convection was displaced to the east and south of the circulation, and the center featured multiple vortices that only gradually consolidated. A trough descending into the central Atlantic had displaced the Bermuda high by this time, and Fifteen began to turn northward.

Overnight, winds increased slightly, and the system was upgraded to Tropical Storm Oscar. Strong upper-level winds kept the center of the cyclone exposed all through October 4, but deep convection moved a little closer to the center, and the winds within the area of shower activity increased. This brought Oscar to its peak intensity of 50 mph winds and a pressure of 997 mb.

By October 5, the trough was encroaching on the circulation of Oscar, and the rapidly deteriorating cyclone was accelerating to the northeast. By late that morning, the cyclone's elongated circulation combined with the trough of low pressure, and the system was announced dissipated.

Oscar as a strongly sheared cyclone over the far east Atlantic.

Track of Oscar.