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.

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