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Hurricanes

Predicting the Unpredictable

How exactly are hurricanes forecasted? Come inside the National Hurricane Center to find out.

By Sar Perlman — August 2003

 

 More of this Feature
• Hurricane Prediction
• Crystal Ball, Tuning In
• Nature’s Fury
• On The Web
• Tropical Cyclone Facts

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• Feature Index

Hold onto your skipper’s hat. Hurricane season is here, and to help us prepare for it, the National Hurricane Center (NHC) has unveiled new four- and five-day hurricane forecasts, extending the traditional three-day forecasts it had issued since 1964. In doing so, the NHC went through a rigorous set of experiments during the 2001 and 2002 hurricane seasons to test its capability, and according to NHC director Max Mayfield, results indicate that the five-day track forecast will be as accurate as the three-day forecast.

“These experiments were successful largely because of improved modeling techniques developed jointly by NOAA’s Geophysical Fluid Dynamics Laboratory, Environmental Modeling Center, and other researchers,” Mayfield says. Such modeling was made possible by vast improvements in the computational speed and memory capacity of supercomputers.

As late as the 1990’s, computer models took so long to develop that forecasts were being produced hours or even days after they were needed; the storms simply moved faster than the computers could calculate their likely paths. In order to speed up the process, scientists were often forced to eliminate many important equations, which resulted in significant errors and severely degraded model forecasts.

“Today’s supercomputers allow modelers to produce higher resolution grids with more forecast points and use all of the physical equations they need,” says Mayfield. “And as a result, we have seen more than a 20-percent decrease in 72-hour errors during the past decade alone.”

Here’s how the NHC keeps boaters and other concerned citizens apprised of an impending storm.

The Nature of the Beast
First, a bit of background on the kind of severe weather that NHC forecasts. Hurricanes, tropical storms, and tropical depressions all fall under the heading of a tropical cyclone and are essentially categories of the cyclone’s intensity as measured by the one-minute mean-surface wind speed. A tropical cyclone is defined by NHC as a wide, warm-core, nonfrontal low-pressure system that develops over tropical or subtropical waters and has an organized surface counterclockwise circulation.

Depressions contain winds of up to 34 knots and typically develop from an easterly tropical trough (a tropical low) that gathers enough convection to exceed the threshold wind speed. Once winds reach 35 knots, the depression is upgraded to a tropical storm and given a name to simplify communication to the public.

A tropical storm becomes a hurricane when winds exceed 63 knots (72.4 mph). Hurricanes are further categorized according to wind speeds using the Saffir-Simpson Hurricane Scale, which ranks them according to strength from one to five. The wind speeds of Category One storms typically range from 64 to 82 knots (74 to 95 mph), while Category Five hurricanes pack winds of 135 knots (156 mph) and higher. Hurricanes vary widely in size, and they can easily span 300 NM in diameter.

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This article originally appeared in the July 2003 issue of Power & Motoryacht magazine.

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