[16] 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.

[8][12] 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.

[39] 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.

[53] Much of the East Coast of the United States, in Mid-Atlantic and New England regions, had a good chance of receiving gale-force winds, flooding, heavy rain and possibly snow early in the week of October 28 from an unusual hybrid of Hurricane Sandy and a winter storm producing a Fujiwhara effect.

Power companies from the Southeast to New England alerted independent contractors to be ready to help repair storm damaged equipment quickly and asked employees to cancel vacations and work longer hours.

[61] President Obama signed emergency declarations on October 28 for several states expected to be impacted by Sandy, allowing them to request federal aid and make additional preparations in advance of the storm.

[70] By October 29, the state of emergency was extended to 24 counties in western North Carolina, with up to 12 in (30 cm) of snow attributed to Sandy anticipated in higher elevations.

[78] In addition, Washington D.C. Metro service, both rail and bus, was canceled on October 29 due to expected high winds, the likelihood of widespread power outages, and the closing of the federal government.

[72] By the following day, Smith Island residents were evacuated with the assistance of the Maryland Natural Resources Police, Dorchester County opened two shelters for those in flood prone areas, and Ocean City initiated Phase I of their Emergency Operations Plan.

[86][87] Officials warned that more than a hundred million tons of dirty sediment mixed with tree limbs and debris floating behind Conowingo Dam could eventually pour into the Chesapeake Bay, posing a potential environmental threat.

[97][98][99] Governor of New Jersey Chris Christie ordered all residents of barrier islands from Sandy Hook to Cape May to evacuate and closed Atlantic City casinos.

[72] Mayor of Philadelphia Michael Nutter asked residents in low-lying areas and neighborhoods prone to flooding to leave their homes by 1800 UTC October 28 and move to safer ground.

[110] By October 27, major carriers canceled all flights into and out of JFK, LaGuardia, and Newark-Liberty airports, and Metro North and the Long Island Rail Road suspended service.

[109] On October 28, Mayor Michael Bloomberg ordered that all public schools be closed on Monday[120] and announced a mandatory evacuation of Zone A, which comprised areas near coastlines or waterways.

[121] Additionally, one of NYU Langone Medical Center's backup generators failed on October 29, prompting the evacuation of hundreds of patients, including those from the hospital's various intensive care units.

[149] On October 29, Environment Canada issued severe wind warnings for the Great Lakes and St. Lawrence Valley corridor, from Southwestern Ontario as far as Quebec City.

[8] In Haiti, which was still recovering from both the 2010 earthquake and the 2010s cholera outbreak, a total of 75 people died,[175] and approximately 200,000 were left homeless as a result of four days of ongoing rain from Hurricane Sandy.

[178] Most of the tents and buildings in the city's sprawling refugee camps and the Cité Soleil neighborhood were flooded or leaking, a repeat of what happened earlier in the year during the passage of Hurricane Isaac.

[220] Two panther kittens escaped from the White Oak Conservation Center in Nassau County after the hurricane swept a tree into the fence of their enclosure; they were missing for 24 hours before being found in good health.

Bob McDonnell announced on October 30 that Virginia had been "spared a significant event", but cited concerns about rivers cresting and consequent flooding of major arteries.

[239] Interstate 68 in far western Maryland and northern West Virginia closed due to heavy snow, stranding multiple vehicles and requiring assistance from the National Guard.

[264] The Atlantic Ocean storm surge also caused considerable flood damage to homes, buildings, roadways, boardwalks and mass transit facilities in low-lying coastal areas of the outer boroughs of Queens, Brooklyn and Staten Island.

[277] The contamination caused by this incident resulted in the EPA issuing a warning that all individuals should avoid coming into contact with the water in Newark Bay and New York Harbor, due to the increased presence of fecal coliform, a bacteria that is associated with human waste.

[278] Sandy's rain became snow in the Appalachian Mountains, leading to blizzard conditions in some areas, especially West Virginia,[8] when a tongue of dense and heavy Arctic air pushed south through the region.

Various television channels in the United States and internationally aired the four-hour concert which was expected to reach over 1 billion people worldwide, featuring many famous performers including Bon Jovi, Eric Clapton, Dave Grohl, Billy Joel, and Alicia Keys.

[304] Gathering humanitarian supplies and disaster recovery equipment, the DOD prepared to carry out DSCA (Defense Support of Civil Authorities) operations across the eastern seaboard.

[313] On December 6, 2013, an analysis of Federal Emergency Management Agency data showed that fewer than half of those affected who requested disaster recovery assistance had received any, and a total of 30,000 residents of New York and New Jersey remained displaced.

November 2012 Representative Henry Waxman of California, the top Democrat of the House Energy and Commerce Committee, requested a hearing in the lame duck session on links between climate change and Hurricane Sandy.

"[330] On January 30, 2015, days after the U.S. Army Corps of Engineers released a post-Sandy report examining flood risks for 31,200 miles (50,200 km) of the North Atlantic coast, President Obama issued an executive order directing federal agencies, state and local governments drawing federal funds to adopt stricter building and siting standards to reflect scientific projections that future flooding will be more frequent and intense due to climate change.

[356][357][358] As Hurricane Sandy approached the United States, forecasters and journalists gave it several different unofficial names, at first related to its projected snow content, then to its proximity to Halloween, and eventually to the overall size of the storm.