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Forecasting hurricane Sandy

8 April 2013

Advance weather warnings made all the difference in reducing the impacts of hurricane Sandy that raged across the Atlantic late last year. Here, James Franklin, Chief of the Hurricane Specialist Unit at the National Hurricane Center (NHC), part of the US National Weather Service, recounts the challenges of forecasting for such a large, complex weather event — and how with the help of the Met Office and other agencies they were able to effectively track it.

James Franklin Hurricane Sandy will be remembered as one of the largest and most destructive storms in history.

Between 22 and 29 October the world watched on as it swept through Jamaica, Cuba, Haiti, the Bahamas and all the way up the Eastern US and even into Canada.

Dramatic scenes of flooded New York subways, destroyed homes and enormous queues of people waiting to stock up on petrol filled TV screens across the globe.

Destruction caused by hurricane Sandy There were an estimated 147 direct fatalities due to Sandy, and property damage in the United States alone was estimated at $50 billion.

But without the effective forecasting efforts of the NHC it could have been a lot worse. As James explains:

"This storm had huge media coverage yet there were still people who didn't heed the warnings and evacuate. But you can only imagine how much worse it would have been if the warnings were not in place."

"This storm had huge media coverage yet there were still people who didn't heed the warnings and evacuate. But you can only imagine how much worse it would have been if the warnings were not in place."

Alongside media briefings, NHC liaises directly with decision makers like President Obama, the Governor of New Jersey, and senior executives at the Federal Emergency Management Agency to help them understand the potential threat and how it's developing. "After all, they're the ones making the decisions to evacuate places, not us", says James. But forecasting Sandy, and everything it would bring with it, wasn't as straightforward as with most hurricanes.

"Sandy was very large, with a broad spectrum of weather hazards, from storm surge, to rainfall, to blizzard conditions. In fact, it's the first time ever that we had a snowfall hazard statement in a National Hurricane Center advisory."

This complexity was produced by the fact that the storm changed over time. As it moved from the Caribbean towards the United States, it made the transition from a hurricane to an extra-tropical cyclone.

"Sandy was very large, with a broad spectrum of weather hazards, from storm surge, to rainfall, to blizzard conditions. In fact, it's the first time ever that we had a snowfall hazard statement in a National Hurricane Center advisory."

"The practical importance of this," says James, "is to expand the scope of the impacts. Hurricanes generally are smaller, with the strongest winds and the worst impacts relatively close to the centre. Extratropical cyclones, on the other hand, tend to be very large with, as we've seen, a wide array of hazards."

To forecast the storm's path and strength the NHC used weather models from the Met Office alongside those of other domestic and international agencies. Combining different models in this way is known as 'ensemble forecasting' and taking an average forecast from several reliable models usually produces more accurate readings than using a single model alone.

Track of hurricane Sandy Improvements in these models over the past decade enabled James and his teams to give emergency managers and the general public a couple of days extra lead-time that they wouldn't otherwise have had - which, given the destructive force of Sandy, made a world of difference. It allowed the NHC to brief President Obama, the emergency services and other decision makers on the regions to be affected and the magnitude of the threat - giving people on the ground the best possible chance to prepare and get out of harm's way.

But for James and his teams, the storm's transformation from a hurricane to an extra-tropical cyclone also presented one of the greatest real-time challenges. In addition to the National Hurricane Center, the US has agencies that specialise in forecasting non-tropical weather events. When the storm changed and became an extra-tropical cyclone, issuing warnings came under another centre's jurisdiction. As James explains:

"Every storm is a learning experience for us and I think we did very well given the constraints. Sandy's transition from a hurricane to an extra-tropical cyclone was a wrinkle that we don't often come across. But we've already come up with plans to improve our services as a result."

Met Office and Sandy

An enormous weather event like hurricane Sandy always has widespread impacts and is of great scientific importance - which is why Met Office's forecasters closely tracked its progress. The data we collected was passed on to various bodies that used it to manage their responses and study the storm and its wider significance.

Met Office forecasters provided regular updates to the Foreign Office's Crisis Centre, which helps consular offices around the world deal with large-scale events that affect British nationals abroad.

After Sandy, the Met Office prepared reports to help the insurance industry deal with the numerous claims it was receiving. With the storm being technically reclassified from a hurricane to an extra-tropical cyclone just before it made landfall in New Jersey, forecaster's reports were essential to help insurers understand how claims met policy rules. But despite this transition, the combination of high tide, unprecedented amounts of rainfall, high winds and the sheer size of the Sandy meant it still wreaked havoc across the US.

Finally, the Met Office worked with the UK's Department of Energy and Climate Change (DECC) during and after hurricane Sandy, providing official guidance on hurricanes and studying the significance of Sandy in the context of climate change.

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