Hurricane Sandy began as a low pressure system which developed sufficient organized convection to be classified as Tropical Depression Eighteen on October 22 south of Kingston, Jamaica. It moved slowly at first due to a ridge to the north. Low wind shear and warm waters allowed for strengthening, and the system was named Tropical Storm Sandy late on October 22. Early on October 24, an eye began developing, and it was moving steadily northward due to an approaching trough. Later that day, the National Hurricane Center (NHC) upgraded Sandy to hurricane status about 65 mi (105 km) south of Kingston, Jamaica. At about 1900 UTC that day, Sandy made landfall near Kingston with winds of about 85 mph (140 km/h). Just offshore Cuba, Sandy rapidly intensified to winds of 115 mph (185 km/h), and at that intensity it made landfall just west of Santiago de Cuba at 0525 UTC on October 25. Operationally, Sandy was assessed to have peaked as a high-end Category 2 hurricane with maximum sustained winds of 110 mph (175 km/h).
A time-lapse video made from NASA satellite images of Hurricane Sandy
After Sandy exited Cuba, the structure became disorganized, and it turned to the north-northwest over the Bahamas. By October 27, Sandy was no longer fully tropical, as evidenced by the development of frontal structures in the outer circulation. Despite strong shear, Sandy maintained convection due to influence from an approaching trough; the same that turned the hurricane to the northeast. After briefly weakening to a tropical storm, Sandy re-intensified into a hurricane, and on October 28 an eye began redeveloping. The storm moved around an upper-level low over the eastern United States and also to the southwest of a ridge over Atlantic Canada, turning it to the northwest. Sandy briefly re-intensified to Category 2 intensity on the morning of October 29, around which time it had become an extremely large hurricane with a wind diameter of over 1,000 miles (1,609 km), and an unusually low central barometric pressure of 940 mbar, possibly due to the very large size of the system. This pressure set records for many cities across the Northeastern United States for the lowest pressures ever observed. The convection diminished while the hurricane accelerated toward the New Jersey coast, and the cyclone was no longer tropical by 2100 UTC on October 29. About 2½ hours later, Sandy made landfall near Brigantine, New Jersey, with winds of 80 mph (130 km/h). During the next four days, Sandy's remnants drifted northward and then northeastward over Ontario, before merging with another low pressure area over Eastern Canada on November 2.
On October 23, 2012, the path of Hurricane Sandy was correctly predicted by the European Centre for Medium-Range Weather Forecasts (ECMWF) headquartered in Reading, England nearly eight days in advance of its striking the American East Coast. The computer model noted that the storm would turn west towards land and strike the New York/New Jersey region on October 29, rather than turn east and head out to the open Atlantic as most hurricanes in this position do. By October 27, four days after the ECMWF made its prediction, the National Weather Service and National Hurricane Center confirmed the path of the hurricane predicted by the European model. The National Weather Service was criticized for not employing its higher-resolution forecast models the way that its European counterpart did. A hardware and software upgrade completed at the end of 2013 enabled the weather service to make more accurate predictions, and do so far more in advance than the technology in 2012 had allowed.
Relation to global warming
According to NCAR senior climatologist Kevin E. Trenberth, "The answer to the oft-asked question of whether an event is caused by climate change is that it is the wrong question. All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be." Although NOAA meteorologist Martin Hoerling attributes Sandy to "little more than the coincidental alignment of a tropical storm with an extratropical storm", Trenberth does agree that the storm was caused by "natural variability" but adds that it was "enhanced by global warming". One factor contributing to the storm's strength was abnormally warm sea surface temperatures offshore the East Coast of the United States—more than 3 °C (5 °F) above normal, to which global warming had contributed 0.6 °C (1 °F). As the temperature of the atmosphere increases, the capacity to hold water increases, leading to stronger storms and higher rainfall amounts.
As they move north, Atlantic hurricanes typically are forced east and out to sea by the Prevailing Westerlies. In Sandy's case, this typical pattern was blocked by a ridge of high pressure over Greenland resulting in a negative North Atlantic Oscillation, forming a kink in the jet stream, causing it to double back on itself off the East Coast. Sandy was caught up in this northwesterly flow. The blocking pattern over Greenland also stalled an Arctic front which combined with the cyclone. Mark Fischetti of Scientific American said that the jet stream's unusual shape was caused by the melting of Arctic ice. Trenberth said that while a negative North Atlantic Oscillation and a blocking anticyclone were in place, the null hypothesis remained that this was just the natural variability of weather. Sea level at New York and along the New Jersey coast has increased by nearly a foot (300 mm) over the last hundred years, which contributed to the storm surge. Harvard geologist Daniel P. Schrag calls Hurricane Sandy's 13-foot (4 m) storm surge an example of what will, by mid-century, be the "new norm on the Eastern seaboard".