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The power and reach of Hurricane Sandy came as no surprise to government forecasters. They told people where the storm was headed days ahead of time. As NPR's Jon Hamilton reports, recent improvements in hurricane forecasting helped the government get it right.

JON HAMILTON, BYLINE: For several days before Sandy arrived, the National Hurricane Center was broadcasting a consistent message to the public. This is forecaster James Franklin.

JAMES FRANKLIN: We're going to have a very, very large area that's affected by strong winds, storm surge, heavy rainfall and inland flooding, and, in fact, even snowfall.

HAMILTON: The Hurricane Center also told New York residents to expect an extreme storm surge. Those forecasts were accurate even though Sandy was a very odd storm. It turned left when most hurricanes turn right. It maintained its strength even as it struck land. And it joined forces with a winter storm.

SHARAN MAJUMDAR: I think National Hurricane Center did a fantastic job with its forecasts.

HAMILTON: Sharan Majumdar at the University of Miami says one reason for the success is that the models used to predict weather are more accurate now and run on much faster computers than they used to.

MAJUMDAR: What that means is that we can run the global models at a much higher resolution. So in the same way that our TV sets are going more high def, so computer models of the globe are also becoming more high def.

HAMILTON: And higher definition leads to higher accuracy. A week before Sandy arrived, one of the major computer models was already showing that Sandy would make a sharp left turn toward New Jersey. Majumdar says computer models also get more and better data than they used to. He says one source of this data is dropsondes, capsules dropped from an airplane into a hurricane. As they fall, they transmit a constant stream of weather data.

MAJUMDAR: The dropsonde data tend to improve hurricane forecasts by up to 30 percent.

HAMILTON: Government scientists put a lot of extra dropsondes into Sandy to help resolve early differences in their forecast models. Tom Renkevens from the National Oceanic and Atmospheric Administration says satellites have also played a big role in improving forecasts. Renkeven says during Sandy, satellites that sit over the equator were churning out high-quality pictures.

TOM RENKEVENS: We were taking images every 7 1/2 minutes continuously through the storm. So that's something we cannot do a long time ago.

HAMILTON: A long time ago being the 1990s. Renkevens says special satellites that orbit the Earth's poles are also much better than their predecessors. He says these satellites report things like sea surface temperatures and humidity in the atmosphere at different altitudes.

RENKEVENS: So while you don't see the imagery typically on TV from the polar orbiting satellites, it's the information from these satellites that really is what drives a lot of the forecast models and the hurricane models.

HAMILTON: All these technological improvements allowed the Hurricane Center to predict an extraordinary event in Sandy's life. Adam Sobel of Columbia University says as the hurricane's rotating winds moved north, they began to approach the rotating winds of a cold winter storm.

ADAM SOBEL: And as the two got close to each other, they both got caught up in each other's circulation.

HAMILTON: Sobel says this interaction helped steer Sandy sharply to the left and transform it into a monstrous winter storm. Sobel says it's remarkable that the Hurricane Center was able to see this coming several days before it happened. And he doubts it would have been possible just a couple of decades ago.

SOBEL: The models could have simulated it in some sense. But we wouldn't have seen a forecast being accurate that far ahead of time, no.

HAMILTON: Jon Hamilton, NPR News.

SIEGEL: Many people are mourning the loss of people killed during the storm. And throughout our program, we are remembering some of them.

AUDIE CORNISH, HOST:

William Sword Jr. died Monday in Princeton, New Jersey. He was 61 years old, a lifelong Princeton resident, a graduate of the university there and managing director of a Princeton-based investment baking firm. Better known as Bill, he was an avid golfer, a volunteer for Habitat for Humanity and a Little League coach.

SIEGEL: On Monday, he was hit by a falling tree outside his home. William Sword Jr. is survived by his wife and three children.

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