The Making Of A Superstorm

Published: January 2013

Let's be clear — when Sandy came ashore on October 29, it was not a hurricane. Technically, it was an extratropical cyclone. Factually, it was all but unique in the annals of weather history in this country. The meteorological ingredients that created Superstorm Sandy may never have come together in just this way before. So what went into making Hurricane Sandy into a "superstorm"?

To answer that, we need to start with some meteorological basics. Tropical cyclones, which include hurricanes, develop around a core of warm air and are not associated with frontal systems. They are fueled by oceanic heat and moisture and grow strongest when the surrounding air is uniformly warm and humid and upper-level winds are relatively weak. In these conditions, sustained wind speeds can reach 120 to 150 knots, but the storm center is usually very compact, often less than 100 miles in diameter. When separated from their warm-water energy source, whether by moving over land or over colder water, tropical cyclones quickly become disorganized and wind speeds drop to gale force. Damage at landfall is usually limited to the tight band around the storm center where the strongest winds and largest surges occur; once inland, most damage results from flooding as the tropical air cools and loses its moisture in the form of heavy rainfall.

Image of Superstorm Sandy from space Photo: NOAA

Superstorm Sandy on October 29, a few hours before landfall.

In contrast, extratropical cyclones, known as lows or nor'easters in the United States, develop around a core of cold air and are typically positioned at or near the intersection of a cold front and a warm front. They draw their energy from the temperature and pressure differentials across these front lines and can be strengthened by the strong winds of the polar jet stream. They average three to four times larger than tropical cyclones, but their wind speeds rarely reach half of the highest wind speeds of a fully developed hurricane. They can last for days and cover thousands of miles before the front lines dissipate and the low center "fills."

For Hurricane Sandy to become Superstorm Sandy, it had to undergo a process called extratropical transition — to switch from a warm-core, tropical cyclone fueled by the Gulf Stream to a cold-core, extratropical cyclone fueled by a complex frontal system and the jet stream. Meteorologists are aware of three ways this can occur, as described by Bob Henson in a blog post on the website for the National Center for Atmospheric Research:

  • As colder, drier air from the jet stream intrudes into the warm core, the storm typically loses symmetry and begins tilting toward the coldest upper-level air. In an average year, one or more hurricanes will evolve into extratropical storms this way as they move into the North Atlantic.
  • Once in a while, an extratropical cyclone will get a boost of energy by absorbing the remnants of a hurricane. In October 1991, well east of New England, the iconic "perfect storm," made famous by the Sebastian Junger book of the same name, was fueled by heat and moisture from the late Hurricane Grace. While it never came ashore, this powerful storm still pushed destructive surf into much of the US. East Coast, killed 13 people, and caused $200 million in property damage.
  • In the least understood case, a pocket of warm, moist air is drawn into the cold-core circulation, then pinched off through a complicated set of dynamics involving air pulled down from the stratosphere. The extratropical cyclone is said to have developed a "warm seclusion," and though not well understood, it is known that some of the Atlantic's most intense storms have emerged from this process.

Integrated Kinetic Energy and Intensity at Landfall Chart Source: Washington Post

The integrated kinetic-energy index, or IKE, (left axis and red bars) quantifies the overall power of a hurricane based on how far tropical storm-force winds extend from the center rather than the highest wind speed (right axis and purple diamonds)

Sandy's evolution appears to have involved elements of all three, meaning that meteorologists are still parsing the maps, trying to understand all of the dynamics at play. But to simplify it as much as possible, when Hurricane Sandy headed north after battering the Caribbean, a deep trough was moving across the northern United States, and the jet stream had dipped well south of its normal track. The cold air in the trough meshed with Sandy's circulation and pinwheeled around it, creating a nor'easter of unprecedented size and rarely seen power. The energy from the joining of the two systems fueled the hurricane at its heart: Even as the outer part of the storm increasingly resembled an extratropical cyclone, a thin eye-wall appeared at Sandy's center, a sign of hurricane intensification. In terms of its total energy, Superstorm Sandy was the second most powerful storm in history (see chart above).

None of this would have been of more than academic interest if Sandy had simply headed off into the Atlantic the way most hurricanes do. Instead of taking a right, though, the storm took a 90-degree turn to the left and made a beeline for the Jersey shore. That turn resulted from an additional meteorological twist — a strong high-pressure system blocking Sandy's path to the east. The rest of the story has been hashed over often enough: By the time the storm made landfall, it was more than 900 miles wide, and its coming coincided with a high lunar tide. The result was unprecedented storm surges and devastation along the coast from Cape May to the eastern end of Long Island and in parts of Connecticut.

Only one other storm in recorded history may have had a similar morphology — the Long Island Express hurricane of 1938. Incomplete weather records make it impossible to know for certain, but meteorologists have pointed out many similarities. Though forecasters were well aware of the storm's existence days before it made landfall, everyone expected it to curve off into the Atlantic. No one predicted it would cross Long Island to come ashore in Connecticut and Rhode Island as it did. Storm surges of 14 feet and more struck from Connecticut to Massachusetts. Estimates of total deaths range from 682 to 800.

Damage Estimates

Number of deaths: 106

Total economic losses: $60 billion plus

Total recreational boats damage or destroyed: 65,000

Value of recreational boats damage or destroyed: $675 million

Number of marinas seriously damaged or destroyed: 500 plus

Given all of this complexity, the accuracy of the forecasts for Sandy's track and potential impacts is astonishing. The European model was the first to predict this outcome, one week before the storm made landfall. The NOAA models took a few more days to come to the same conclusion, but did so in time to warn people along the coasts to evacuate. One cannot help but wonder what the death toll would have been if the forecasts had been less accurate. Superstorm Sandy was a meteorological anomaly. We can only hope we don't see its like again in our lifetimes.End of story marker



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