By rights, Hurricane Sandy ought never to have had a chance to form. Back in May, just before the 2012 hurricane season began, the National Oceanographic and Atmospheric Administration said this was likely to be an average year, with perhaps nine to 15 named storms.
Instead, there have been 19 - 10 of them hurricanes - and it is not over yet. In terms of numbers of storms, this will go down as the third busiest Atlantic hurricane season since record-keeping began in 1851, exceeded only by 2005 (the off-the-charts year of Katrina) and 1933.
So what happened? The original outlook by NOAA's Climate Prediction Center was about as accurate as it could have been at the time, all the way back last spring. But the oceans and the atmosphere make for a complex system, and the Northeast is suffering now because of patterns that formed as far away is Indonesia.
In the air over southeastern Asia is a phenomenon called the Madden-Julian Oscillation, or MJO - air currents that can shift back and forth in 30 cycles of 30 to 60 days. It usually means very little to Americans, as it mostly affects Asian monsoons. But late this summer it shifted to feed increased amounts of warm, moist air to the east, toward the Pacific, and ultimately toward the United States.
"We had a busy start to the hurricane season - 13 named storms by the end of September, though only Isaac did anything to the U.S.," said Paul Knight, a meteorologist at Penn State University. "After that there was a lull. Then the MJO began to act up."
On top of it, there was El Nino, the giant strip of warm water in the Pacific that periodically forms along the equator, rearranging jet streams that flow over it. An El Nino began to weaken just in the past few months - bad news for people along the Atlantic 10,000 miles away. For lack of a strong El Nino, there were not the usual high-level air currents that would help break hurricanes apart as they tried to move toward the U.S. coastline.
Those and other factors helped worsen an already busy year. Sandy formed in the Caribbean, a breeding area for late-season storms, and when it began to move north, there was little to stop it.
Ironically, Hurricane Sandy comes a little more than a year after Irene - physically smaller but still destructive - which also hit the heavily populated northeastern U.S.
"I think it's pretty much chance," said Hugh Willoughby, a veteran hurricane forecaster now at Florida International University. "There seems to be a tendency for the crosshairs to shift around, Gulf Coast for a few years, then peninsular Florida, then the Mid-Atlantic and New England. I'm not sure if this pattern is real or just the human tendency to see patterns in randomness."
Willoughby quoted an old colleague, Jim Lushine, who said, "Hurricanes are like bananas; they come in bunches."
In other words, fearsome as Sandy may be, forecasters said it would not be terribly unusual if not for the way it coincided with an Arctic cold front, coming across the U.S. from the west at the same time. The front, said Knight, is what pulled the hurricane in toward shore. The two together have made for a large, dangerous, cold storm.
"Had they not merged," said Knight, "this would just have been a cold period." Instead, "the hurricane didn't have a chance to run out of steam."
Sandy's path is bringing it onshore on the same day as a full moon, but forecasters said its tides only add incrementally to the storm's effect. Eqecat, a catastrophe risk modeling firm, said the storm would affect 20 percent of the U.S. population, with economic damages likely to be $10 billion to $20 billion.