The tropical wave started off Africa's west coast in early October. It took its time, perhaps two weeks, to make the 7,200-kilometre trek to the warm waters of the Caribbean. The turbulence it stirred didn't catch the attention of weather-watchers such as Jeff Masters until Oct. 15.
The National Hurricane Center in the U.S. soon designated it an Invest, a low-pressure system innocuous yet worthy of investigation. Even after meteorologists elevated it to TD 18, shorthand for a tropical depression, they assumed it would fizzle.
"We expected it to lose strength after Jamaica, and it didn't," said Masters, founder of Ann Arbor, Mich.-based Weather Underground, a commercial weather service. "We expected Cuba to tear it apart, and it didn't. As it headed north we expected it to decay some, as most storms do, but it didn't."
On Oct. 22, Sandy was officially born, troublesome beyond belief -- "a freak storm, unprecedented in so many respects," Masters said. The destroyer that came out of nowhere produced strong winds reaching farther than any known Atlantic hurricane. It took dozens of lives, cost billions of dollars, plunged millions into darkness and created nightmare scenarios that not even the disaster planners anticipated.
Whatever the final cost, it already has renewed debate over both the possible influence of climate change on the storm's movement and the preparedness of states and cities to handle increasingly violent weather events.
"We see 10 to 12 named tropical storms per season on average," said Annes Haseemkunju, an atmospheric scientist at Eqecat, a risk-management company in Oakland, Calif. "This season we already have 19 named storms, and there's one more month to go."
Greater frequency will force authorities to bolster the defences of their communities, New York Gov. Andrew Cuomo said at a news conference Tuesday in Manhattan, where the city's subway system is flooded and may take weeks to recover.
"We have a new reality when it comes to these weather patterns and we have an old infrastructure, and that is not a good combination," Cuomo said. "We have 100-year floods every two years now."
U.S. President Barack Obama noted the increasing frequency of such events in his remarks Tuesday at Red Cross headquarters in Washington.
"Sadly, we are getting more experience with these kinds of big-impact storms along the East Coast, and the preparation shows," Obama said. "Were it not for the outstanding work that they and their teams have already done and will continue to do in the affected regions, we could have seen more deaths and more property damage."
A 2008 study concluded Manhattan was vulnerable to storm-surge flooding from even a moderate nor'easter and recommended local authorities build protections such as Rhode Island's Fox Point Hurricane Barrier, which spans the Providence River.
"Both state and city authorities in NYC and coastal northern New Jersey should begin exploring the feasibility of constructing European-style storm-surge barriers across major connections of New York Harbor to the ocean as protection against serious storm surge flooding," according to a study in the June 2008 issue of the Bulletin of the American Meteorological Society.
Sandy's path, aided by the warm water and low air pressure, defied expectations. In doing so, it prompted public discussion about the potential causes, including climate change.
The percentage of Americans saying there is "solid evidence" of global warming has increased in the past six years, according to an Oct. 15 poll from the Pew Research Center for The People & The Press. Two-thirds agreed the earth's average temperature has been rising during the past few decades, up four percentage points since last year and 10 points since 2009.
Linking a single storm to climate is very difficult, though climate change itself is well understood on a global level. Earth's average temperature has warmed about 0.7 degrees Celsius since 1900, most of which is very likely due to human greenhouse gas emissions, according to the Intergovernmental Panel on Climate Change's 2007 report.
Identifying causes of a storm as complex as Sandy, and determining whether climate change was among them, is challenging, said Randall Dole, deputy director for research at a division of the National Oceanic and Atmospheric Administration. Scientists hunt for a climate-change fingerprint by experimenting with computerized models.
"You have to reconstruct a world that might have been," Dole said. "And to do that you need to estimate the human influence on sea-surface temperatures" and any other trends showing human influence.
"Where you might make a case is what the trend in the sea level rise has been," he said. "There's a pretty strong attribution that can be made, but the weather itself -- it's almost certainly natural."
The confluence of unusual events -- a string of violent storms in the past two years and the worst drought since 1956 -- convinced Masters the forces governing storms have been altered.
"Freak weather events happen, right, but in the last two years?" Masters said. "I think something's up. I think we've crossed over to a new climate state where the new normal is we're going to be getting these intense weather events that are very destructive to our economy and kill lots of people."
Others aren't prepared to draw that conclusion.
Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology in Cambridge, Mass., said Sandy was an unusual storm that will have to be studied thoroughly before a link can be established.
Until then, New York Mayor Michael Bloomberg said, the emphasis should be on improving preparedness.
"Is a storm like this that's so strong and unusual a global warming incident? What is clear is the climate is changing," he said in a news conference Tuesday. "I think each of these storms, we've got to learn to see if we can do some things better the next time."
-- Bloomberg News