Hurricane Sandy was the eighth-costliest Atlantic hurricane on record. It lasted for over a week in late October-early November 2012. Classified as the eighteenth named storm, tenth hurricane, and second major hurricane of the annual hurricane season, Sandy originated from a tropical wave on October 22. Performing a small loop over the central Caribbean Sea, the system intensified into a tropical storm a day later and became the final hurricane of the season before briefly coming ashore the coast of Jamaica on October 24. After emerging between Jamaica and Cuba, Sandy began a period of rapid intensification into a Category 3 hurricane on the Saffir–Simpson hurricane wind scale, with maximum sustained winds of . It made landfall at this intensity near Santiago de Cuba on October 25.

An approaching trough over the central United States induced high wind shear over Sandy as it traversed the Bahamas, causing the hurricane to weaken to a tropical storm while turning more northeastward. The southern part of the trough detached, causing the shear to decrease late on October 28 and allowing Sandy to regain strength. It attained a secondary peak of Category 2 strength the following day, and later turned toward the west. During this change in direction, Sandy began to transition into an extratropical cyclone, a process it completed before making landfall near Brigantine, New Jersey, late on October 29. The extratropical remnants weakened gradually overland, and the center of circulation was declared indistinguishable over western Pennsylvania two days later. In addition to becoming the largest Atlantic hurricane, Sandy broke records for the lowest pressures ever observed in many cities across the Northeastern United States.

Origins

The origins of Hurricane Sandy trace back to a tropical wave that moved off the western coast of Africa and into the eastern Atlantic Ocean on October 11. As the wave tracked westward over subsequent days, it interacted with an upper-level trough over the Eastern Atlantic, resulting in the development of widespread shower and thunderstorm activity; however, strong wind shear prevented further development at the time. Increased convergence, likely as a result of Hurricane Rafael to the wave's west, hindered development as well. By October 18, numerous yet disorganized convective activity formed near the center of the disturbance despite moderate wind shear. Marked with an extended low pressure area, conditions were expected to gradually become more favorable for development. On October 20, following modest organization, the National Hurricane Center (NHC) assessed a high potential for it to become a tropical cyclone within 48 hours, tagging it "Invest 99L".

By the next day, the convection had decreased, although barometric pressures in the area remained low, a trademark of development. Convection gradually increased as the day went on, while the system slowed and became nearly stationary over the western Caribbean. By 1500 UTC on October 22, surface observations and satellite imagery indicated the system had developed enough organized convection to be classified as Tropical Depression Eighteen. At the time of the upgrade, the system was situated about south of Kingston, Jamaica The environment around the newly formed depression was characterized by an area of weak steering currents south of a ridge extending eastward from the Gulf of Mexico. Low wind shear and warm sea surface temperatures were conducive for strengthening, and perhaps rapid deepening. Late on October 22, a Hurricane Hunters flight observed winds of in a rainband well removed from the center of circulation, prompting the NHC to upgrade the depression to Tropical Storm Sandy. Outflow increased, while moist air helped the convection organize further. The NHC noted that "remaining nearly stationary over the warm waters of southwestern Caribbean Sea is never a good sign for this time of year." Still, the cloud pattern initially remained largely unchanged. Early on October 24, an eye began developing, as observed on microwave imagery, and Sandy was moving steadily northward, drawn by a trough approaching from the northwest. At 1500 UTC on October 24, the NHC upgraded Sandy to hurricane status after the Hurricane Hunters observed flight-level winds of . At the time, Sandy was located roughly south of Kingston, Jamaica.

Caribbean landfalls and The Bahamas

At approximately 1900 UTC on October 24, Sandy made landfall near Kingston, with winds near . After spending a short duration over the island, Sandy moved just offshore Cuba and began a period of rapid intensification, in which the cyclone strengthened into a Category 3 hurricane on the Saffir-Simpson hurricane wind scale, with 1-minute sustained winds at winds and a central pressure at ; operationally, Sandy was classified as a high-end Category 2 hurricane at landfall. Shortly thereafter, at 0525 UTC on October 25, the hurricane came ashore just west of Santiago de Cuba. At landfall, Sandy had a well-defined eye over in diameter, and flight-level winds reached . While over land, the structure deteriorated, and the eye was no longer visible. After exiting Cuba, a combination of dry air and increasing shear restricted the outflow of Sandy and caused the structure of the storm to become disorganized. A mid-level low over Florida and approaching trough turned the hurricane toward the north-northwest.

By early October 26, a majority of the convection in association with Sandy was located to the north of the center, primarily due to wind shear and dry air to the southwest of the hurricane. The size of the storm had increased greatly as well, with tropical storm-force winds extending out some from the center. As the day progressed, Sandy continued moving slowly to the north, and the strong wind shear caused the storm's intensity to decrease slightly. On October 27, the NHC remarked that Sandy was "showing characteristics of a hybrid cyclone...like a large occluded frontal low." However, the system maintained a warm thermal core, and despite strong 50 kt (60 mph) wind shear, continued to develop thunderstorms due to an abundance of divergence from a nearby trough; the same trough turned Sandy toward the northeast as the two began to phase and morph into what many called a "Superstorm". On October 27, Sandy briefly weakened to a tropical storm, after dry air became fully ingested into the mid- and upper-level circulations. Later that day, however, data received from the Hurricane Hunters indicated that Sandy had re-intensified into a minimal hurricane.

Post-tropical transition and final landfall

By late October 27, Sandy was moving steadily northeastward ahead of an approaching trough. Although it maintained winds of hurricane force, the entrainment of dry air and continued strong wind shear caused the inner area of convection to diminish. On October 28, however, thunderstorms increased over the center, and Sandy's upper-level circulation was better defined when opposed to 24 hours previous. As the day progressed, wind shear decreased, and a banded eye began redeveloping while the hurricane was still over the Gulf Stream. The convection organized further early on October 29. Around the same time, Sandy began transitioning into an extratropical storm after the western periphery of the circulation began interacting with a cold front. The storm revolved around an upper-level low over the eastern United States, and also to the southwest of a ridge over Atlantic Canada that the NHC described as "highly anomalous"; this caused Sandy to turn to the north and northwest. Maintaining an eye and deep convection, the hurricane intensified, reaching a secondary peak of and an unusually low central barometric pressure of , by 1200 UTC on October 29; at this time, the cyclone was moving over a small area of the Gulf Stream with waters in excess of . Around that time, Sandy had a gale-force wind field of over in diameter. Both a warm and cold front were located near the storm's center, and the storm was predicted to become extratropical before landfall.

The convection diminished while the hurricane accelerated toward the New Jersey coast, due to it becoming involved with the low to the west. The pressure continued to drop, which indicated the system was intensifying because of baroclinic instability. In an advisory issued by the NHC late on October 29, the NHC noted that, "all of these considerations lead us to conclude that the most appropriate classification at advisory time is extratropical." The agency declared Sandy a post-tropical cyclone at about 2100 UTC that afternoon, while located just offshore southern New Jersey. About 2½ hours later, the storm made landfall approximately northeast of Atlantic City near Brigantine. The intensity at landfall was estimated at , although the strongest winds were located offshore, east and southeast of the center.

Dissipation

After moving ashore, Sandy continued moving to the west, weakening below hurricane force by the time it reached Pennsylvania. Because the system was non-tropical, the Weather Prediction Center (WPC) – known at the time as the Hydrometeorogical Prediction Center (HPC) – took over the responsibility of issuing advisories on the low. The remnants of Sandy brought heavy snow and high winds to the central Appalachian Mountains, resulting in blizzard warnings being issued. The system continued to weaken as it moved across western Pennsylvania, and by 0300 UTC on October 31, the storm's movement had shifted to the northwest. Blizzard conditions continued in the Appalachians, bringing more snow to the region that had already seen high amounts the day before. By 0900 UTC on October 31, the circulation degenerated into a trough of low pressure, with no discernible center of low pressure. Later that day, the remnants of Sandy spread into the Great Lakes, and the WPC issued its last advisory. By the time the system had moved out of the region, nearly three feet of snow had fallen in some areas of West Virginia, Tennessee, and Maryland, with lesser amounts elsewhere in the region. During the next two days, Sandy's remnants drifted northward and then northeastward over Ontario, before merging with another low pressure area over Eastern Canada, on November 2.

Predictions As early as October 23, while Sandy was developing in the Caribbean, the European Centre for Medium-Range Weather Forecasts (ECMWF) predicted the storm would strike the East Coast of the United States, while most other tropical cyclone forecast models anticipated the storm would move out to sea. By the next day, various computer models agreed that Sandy would interact with a trough over the eastern United States and turn to the west. About five days before landfall, the ECMWF, Geophysical Fluid Dynamics Laboratory (GFDL), and Navy Operational Global Prediction System (NOGAPS) models predicted Sandy would strike the Delmarva Peninsula, while the American Global Forecast System (GFS) model anticipated the hurricane would move out to sea; the remaining models were between the two scenarios. By four days before landfall, the NHC was forecasting a landfall on New Jersey, as were most of the computer models. In general, the European computer models performed better than the United States ones, due to the European models' higher resolution. MIT professor Kerry Emanuel used the moment to call attention to the under-performance of US models, and to recommend a "dedicated effort" to reverse it.

Records