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Posted at 01:22 PM ET, 11/01/2012

The National Hurricane Center’s strikingly accurate forecast for Sandy


Average 72 hour forecast track errors for hurricane in 1970 were 518 miles and 345 miles in 1990, but just 71 miles for Sandy. Some forecasts even beyond 72 hours were that accurate (or even a bit more). (National Weather Service)
Almost five days prior to landfall, the National Hurricane Center pegged the prediction for Hurricane Sandy, correctly placing southern New Jersey near the center of its track forecast. This long lead time was critical for preparation efforts from the Mid-Atlantic to the Northeast and no doubt saved lives.

Hurricane forecasting has come a long way in the last few decades. In 1970, the average error in track forecasts 3 days into the future was 518 miles. That error shrunk to 345 miles in 1990. From 2007-2011, it dropped to 138 miles. Yet for Sandy, it was a remarkably low 71 miles according to preliminary numbers from the National Hurricane Center (NHC).

Within 48 hours, the forecast came into even sharper focus, with a forecast error of just 48 miles, compared to an average error of 96 miles over the last 5 years.

Track forecast for Hurricane Sandy, from its birth as a tropical cyclone through landfall in Atlantic City. Southern New Jersey was positioned in the center of track guidance by Thursday at 5 a.m., roughly 108 hours prior to landfall...although there were some minor fluctuations in the forecast .

Yet not all storms are equally predictable. The error for the five day forecast Hurricane Isaac, which struck southeast Louisiana in late August, was 253 miles, slightly above the modern day average error (of 245 miles). It was not until within 72 hours (watch animation) that the NHC correctly placed southeast Louisiana squarely inside its track projection. By 48 hours - though - the NHC had Isaac just about right - its landfall forecast just 70 miles off.


Track and intensity errors for Hurricane Sandy compared to the 2007-2011 average. (National Hurricane Center)

While considerable strides have been made in track forecasts, intensity forecasts have not progressed in recent years.

“According to NOAA, during the past 15 years, hurricane track forecasts have improved by 50 percent, while the accuracy of intensity forecasts have not budged,” Climate Central’s Andrew Freedman reported.

Why haven’t intensity forecasts kept pace? Bringing small scale storm details into prediction systems has proven elusive. Freedman wrote the following:

[NHC hurricane specialist James] Franklin said ... predicting storm intensity requires knowing lots of small-scale details that computer models have trouble capturing, from the dynamics of a storm’s structure to the characteristics of air masses being pulled into a storm’s circulation.

. . .

“We now have to telegraph in to much, much smaller scales in the atmosphere” where predictability is less, Franklin said, “Where we’re working blind, essentially.”

But a piece in IEEE Spectrum says considerable progress is being made in this area as of this year due to improvements in the resolution of regional forecast models. The article, which featured remarks from Robert Gall, technical development director for NOAA’s Hurricane Forecast Improvement Project, reveals gains in intensity forecasts are already being realized :

Gall explained that the resolution, that is, the spacing of the points on the map to which the weather data is attached, was increased to 3 km from the previous 9 km. Researchers had amassed evidence that such a change in resolution could improve forecasts in storm intensity, something that hadn’t improved in two decades. Gall credits the team at the National Center for Environmental Prediction for pushing hard to make the calculations of the regional model more efficient in order to allow the increase in resolution without significantly extending the computer time needed to run the calculations. This change, Gall says, is causing a 10 to 20 percent improvement in the accuracy of hurricane intensity forecasts. And experiments underway now to incorporate data gathered by flights into hurricanes into the intensity forecasts should bring another 10 percent improvement, he says.

The intensity forecast for Sandy proved to be quite accurate with average errors below recent averages by as much as 33 percent.

A new NASA initiative to send unmanned Global Hawk drones into the environment around hurricanes may allow for some additional improvements in hurricane track and intensity forecasts. These aircraft will collect high resolution data that will enable researchers to better understand hurricane development.

By  |  01:22 PM ET, 11/01/2012

Categories:  Latest, Tropical Weather

 
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