WASHINGTON — It was the moment a run-of-the-mill hurricane mutated into a monster named Sandy.
Paradoxically, it was the same time Sandy lost much of its wind power, dropping from a hurricane to a tropical storm. It was a Friday night and Sandy had just passed the Bahamas and was being enveloped by an ordinary cold front coming off the Southeast. It was changing how it got its power, where its highest winds were and even what it looked like.
But mostly it was getting bigger. Dangerously large. And then it merged with a second storm, turned record huge and pivoted toward the nation’s largest city.
It was that enormousness that set off alarms in the people who knew weather, especially those living in the New York area. For a week forecasts placed Sandy on its path toward New York and it was it sticking to it.
Months earlier, Princeton University professor Michael Oppenheimer had written a scientific study about the dangers of storms hitting the nation’s largest city, and now he was watching one develop. He was enthralled but fearful, hoping that the forecasts would change. ‘‘It was just this monster coming at us,’’ he said.
In the year since Sandy blew through the East Coast, meteorologists have pored over forecasts, satellite photos, computer models, and even the physical damage to try to get a sense of what made Sandy the demon it was.
Put simply, what made the storm dangerous and freaky in more than a dozen different ways was a meteorological trade in: The hurricane lost some oomph in winds in return for enormous size.
And just like Katrina seven years earlier, Sandy caused so much havoc because of its record girth, National Hurricane Center director Rick Knabb said, adding: ‘‘Smaller versions of those same storms would not have had the same scope of disasters.’’
Sandy’s breadth pushed much more water into New Jersey and New York, dropped 3 feet of snow in West Virginia, caused 20-foot waves on the distant Great Lakes and registered other records reflecting whopping energy. It meant at least 182 deaths and $65 billion in damage in the United States, the second-costliest weather disaster in American history behind only Katrina, according to the National Oceanic and Atmospheric Administration.
‘‘I don’t know if we’ll ever see another storm like this,’’ said James Franklin, hurricane center chief forecaster. ‘‘The atmosphere can do a lot of weird stuff. I don’t want more like this.’’
The question is, How likely is it that there will be more? Researchers continue to study whether Sandy was a forerunner of similar storms.
‘‘One of the major lessons scientifically is that there could be these configurations of meteorological events that can combine to be very damaging in ways that are surprising,’’ said NASA and Columbia University climatologist Cynthia Rosenzweig, who warned in a 2001 report of the kind of flooding that New York endured from Sandy.
She said other cities around the world have learned from New York’s Sandy experience and are better preparing for flooding worsened by climate change and rising sea levels.
A September study by NOAA found that sea level rise triggered by global warming is making Sandy-type flooding more likely.
For example, the flooding that swamped Sandy Hook, N.J., last year would have been considered a once-in-435-year event in 1950, but it is now a once-in-295-year event. By 2100, it could become a once-in-20-year event, the study said.
In the past century, sea level in the New York region has risen about a foot, two-thirds of it caused by man-made climate change, said Princeton’s Oppenheimer.