• Sandy got its energy from multiple sources throughout its life cycle.
  • Hurricane Sandy transitioned to a superstorm as it gained energy from multiple troughs.
  • Each transition caused the storm to grow in size.

Sandy’s beginnings were fairly typical of a tropical system in October. A late-season tropical wave in the Caribbean developed into a Category 3 hurricane near Cuba. Nothing too unheard of there – 18 Category 3 or stronger hurricanes have done this in October or November.

But as Sandy tried to escape northeastward, like most systems do during the course of a hurricane season, a nasty game of meteorological pinball between a blocking ridge of high pressure over the North Atlantic bumped Sandy to the northwest and toward the U.S. East Coast, instead of northeastward and out to sea. A deep droop in the jet stream over the Southeast and southern Appalachians also helped pull Sandy westward and into cooler air.

Sandy was a superstorm or “frankenstorm” in two ways: its size and its structure.

Sandy’s wind field doubled in size in the 48 hours leading up to its approach to the coast of New Jersey. At maximum size, Sandy’s tropical-storm-force winds were 870 miles across – roughly the distance between New York City and St. Louis. This size remains a record for a tropical cyclone dating back to at least 1988, and likely beyond.

You can see this growth in the size of the wind field below. The orange colors are the extent of the tropical-storm-force wind field. It went from a more typical size near Cuba to a giant before landfall.

Sandy’s wind field over its lifecycle.

(National Hurricane Center)

T​he large storm size of Sandy produced an enormous storm surge in the Northeast and violent waves in the Great Lakes. B​ut Sandy was going to have to change in order to produce snow.

Structurally, Sandy underwent two distinct changes as it moved from Cuba to New Jersey that helped lead to its superstorm title.

First, as Sandy moved out of the Bahamas, it began to bump into a drier continental airmass. Along the boundaries between Sandy’s moist, tropical airmass and this new continental airmass, noticeable fronts developed and winds accelerated around those fronts. These fronts were reinforced by the warm Gulf Stream current that runs northward off the East Coast, or, in this case, under Sandy. These developments caused the first growth in Sandy’s wind field.

At this point and closer to landfall, Sandy took energy from these fronts, the warm Gulf Stream and the gradient between the warm Gulf Stream and the cooler waters off the mid-Atlantic coast with the balance teetering toward more winterlike sources rather than tropical air and water.

Sandy later found a second, larger change in airmass associated with a deeper trough of low pressure in the southeastern U.S. and underwent a second structural change. This time, frontal boundaries strengthened and reached the center of Sandy.

Sandy remained warm-core like a hurricane, but cooler air surrounded the storm and eventually overwhelmed its warm-core processes. Energy from the jet stream aloft took over. At this point, Sandy became a post-tropical storm just off the New Jersey coast on Oct. 29, 2012.

Surface map of Sandy on October 29, 2012, near the time of transition to post-tropical.

(National Hurricane Center)

In essence, Sandy’s core cooled, and its energy was being drawn more from aloft than from the ocean. Impacts along the coast and points inland remained unchanged, and Sandy began to weaken after landfall. Sandy crossed southern and western Pennsylvania and the circulation became ill-defined over northeastern Ohio.

The cooler air on the storm’s western side caused snowy conditions over the central Appalachians. Snow in itself is rare in hurricanes, but this superstorm produced multiple feet of snow from western North Carolina to West Virginia.

Sandy Rewrites NOAA’s Hurricane Playbook

Following Sandy’s untimely transition from a hurricane to post-tropical just prior to landfall, lessons learned from confusion in the media and general public led the National Hurricane Center (NHC) to change some of their practices. This important new precedent based on a meteorological transition became known as the “Sandy Rule.”

The Sandy Rule allows the NHC to retain control of tropical watches and warnings and allows the NHC to continue to forecast the future track and intensity of the post-tropical storm or hurricane.

Hurricanes Hermine (2016) and Fiona (2022) are two of several systems that have since used this precedent since it was brought into practice in 2013.

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