Sandy crashed ashore near Atlantic City, New Jersey, on Oct. 29, 2012, packing maximum sustained winds of 80 mph – equivalent to a Category 1 hurricane on the Saffir-Simpson Hurricane Wind Scale, though it wasn’t meteorologically considered a hurricane when it made landfall.
This so-called superstorm had a unique life cycle that was both climatologically normal and abnormal.
Sandy originated as a tropical wave that organized into a tropical depression on Oct. 22, 2012, over the southwestern Caribbean Sea, where late-season tropical cyclones tend to develop. It moved northward and intensified into a Category 1 hurricane before striking Jamaica, and then strengthened further into a Category 3 hurricane by the time it made its next landfall in eastern Cuba.
By October, the midlatitude westerlies – prevailing winds that blow from west to east between 30 and 60 degrees latitude – tend to increase as the atmosphere begins its transition from summer to winter. These stronger winds typically steer late-season tropical cyclones out into the open waters of the Atlantic Ocean by the time they reach the latitude of about Cape Hatteras, North Carolina, nudging the storms away from the mid-Atlantic and Northeast coasts of the United States.
The difference with Sandy was that the weather pattern over the northeastern U.S. was not typical of late October.
A southward dip in the jet stream, or a trough, was plunging into continental North America. This brought unseasonably cold air and snow cover all the way down to the Canada-U.S. border – in late October. To the east of this trough, a northward bulge in the jet stream, or a ridge, was parked over the North Atlantic Ocean.
"That combination of the ridge in the North Atlantic, with its clockwise motion around it, and the trough coming in, which brings kind of counterclockwise winds with it, was essentially serving as both a push and a pull for that storm as it came up the coast," said Dr. David Robinson, New Jersey State Climatologist. "So the ridge over the North Atlantic wouldn’t allow that storm to head out to sea and actually helped to keep it hugging the coast, and that trough off to the west was helping to actually pull the storm up the coast, and ultimately, the two combined to bring it inland perpendicular to the East Coast – and from east to west, when our weather is generally west to east."
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There are many tropical cyclones in recent history that have moved from south to north up the East Coast before eventually curving out to sea. Hurricane Irene in 2011 and Hurricane Floyd in 1999 are just two notable examples.
"But to see one come up the coast and then hang a left and head into the mainland is climatologically quite unusual," Robinson said.
Many forecasters believed the combination of the trough over North America and the time of the year (late October) would cause Sandy to lose its tropical characteristics and transition into a system more typical of a nor’easter.
The National Hurricane Center was only issuing forecasts for this storm to about as far north as Hatteras, North Carolina, with the individual National Weather Service offices then responsible for the forecasts in their areas of responsibility. Without an official NHC forecast, no hurricane watches or warnings were able to be issued along the mid-Atlantic coast.
Sandy did begin to transition more toward a nor’easter-like system once it interacted with the trough. This caused it to expand in size, but it still retained its warm tropical core until within hours of making landfall in New Jersey.
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"Ultimately, it was only about two to three hours before it made landfall in New Jersey that it was declassified as a hurricane," Robinson said. "It stayed a tropical system – a hurricane – farther north than originally anticipated."
Meanwhile, wrapping around it was this developing nor’easter, so it was a true hybrid storm.
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"Colloquially, it became known as a superstorm, which is not a formal definition – but truly, it was a superstorm because, by the time it got north of Hatteras, it was maintaining its winds and its tropical core," Robinson said. "It was expanding in size, so you had gale-force winds extending farther from that storm than any storm on record."
Clouds from Sandy extended from southern Greenland down into Georgia, from the mid-Atlantic coast over toward Chicago. Very cold air was wrapping around the storm all the way from the Arctic, resulting in snow in the mountains of West Virginia. Meanwhile, there were severe thunderstorm warnings from Massachusetts to Montreal, Canada.
The New York City metropolitan area and the Jersey Shore were in the core of Sandy, but the superstorm’s effects extended thousands of miles away from its center.
"The winds were blowing over such a huge expanse of the western Atlantic that, as a result, it was pushing water up against the Jersey coast, New York, into southern Connecticut and Long Island Sound, equivalent of what would be a Category 2, maybe even a (Category) 3 hurricane," Robinson said. "So your winds didn’t say hurricane, but your storm surge said borderline major hurricane, and that’s all because it became such an enormous storm."
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When adjusted for inflation, the superstorm was responsible for $75.4 billion in damage, according to NOAA’s National Centers for Environmental Information. The National Hurricane Center said in its final report that Sandy, at the time, was the second-costliest tropical cyclone to hit the U.S. since 1900, behind only Hurricane Katrina in 2005. However, it now ranks as the fourth-costliest tropical cyclone after being topped by hurricanes Harvey and Maria in 2017.
Sandy killed 159 people (72 directly, 87 indirectly) during its journey from the Caribbean to the United States, according to NCEI. As of 2021, that ranked as the 12th-deadliest billion-dollar weather disaster to hit the U.S. in records dating to 1980.