Understanding hurricane forecasting

Hurricane season officially starts June 1, but the season doesn't heat up in South Carolina until August. That means now is the time when S.C. residents start paying attention to weather patterns in the tropics.

This year, scientists at the National Hurricane Center in Miami stretched their official forecasts of a storm's track from three days to five days. They are convinced technology now makes those forecasts accurate enough to be used in planning evacuations.

Here is a primer on how meteorologists come up with hurricane forecasts:

What are hurricanes?

Hurricanes are severe storms that form in the southern Atlantic Ocean, Caribbean Sea, Gulf of Mexico and eastern Pacific Ocean. Low atmospheric pressure creates winds that rotate counterclockwise around a center called an "eye." When the winds reach at least 74 mph, the storms are considered hurricanes. Hurricanes gather heat and energy through contact with warm ocean waters.

What determines the movement of hurricanes?

They are steered by surrounding weather, from the surface up to eight miles high. Essentially, hurricanes float like a leaf in a stream. In tropical regions, the dominant air current is to the west and slightly to the north. When a hurricane runs into an obstacle -- another weather system, an abrupt change in water temperature on the ocean surface or a land mass -- it slows down, speeds up or turns.

How are the strengths and paths of hurricanes forecast?

Detailed information about water and air temperature in and around a hurricane, barometric pressure and the storm's wind speed and direction at the surface and in the upper atmosphere are entered into computer models. Those models are based on various ways to interpret what current weather information means for the future of the storm.

There are two main types of computer models: statistical and dynamic.

In the statistical computer model, the hurricane track is compared to similar movements by past hurricanes and tropical storms as far back as 1871. These forecasts bet a current hurricane will react like a hurricane in a similar position in the past.

In dynamic models, atmospheric conditions surrounding the hurricane are more important. These forecasts bet the hurricane will react to a high-pressure system to the north or south of it, the water temperature in front of it and the direction of winds in the upper atmosphere.

A third type of model, called a combination model, combines the strengths of statistical and dynamic models.

How do forecasters gather their information?

Many details come from weather satellites. But to get the most accurate readings in and around a hurricane, aircraft fly into the heart of a storm. The specially equipped planes measure wind, pressure, temperature and dew point. They also drop devices called sondes into the storms to collect more data. Two different organizations fly hurricane hunter planes: the U.S. Air Force Reserve and the National Oceanic and Atmospheric Administration. Air Force planes check out developing low-pressure waves over the oceans, checking for signs of development. The NOAA planes, which have more technical weather equipment, fly into developed storms as they threaten landfall.

Why did the National Hurricane Center go from a three-day forecast to a five-day forecast this year?

Forecasters finally felt comfortable that the five-day forecasts were accurate. Computers first were used to project hurricane behavior in the 1960s. As more powerful computers have been built, more statistics have been included in the predictions, and they've become more accurate. But they still are far from perfect. The average storm tracking error in a five-day forecast, for instance, is about 370 miles. That means a storm projected to hit Savannah in five days could hit anywhere from Jacksonville, Fla., to Cape Hatteras, N.C. The three-day forecasts remain much more accurate. For instance, the location of Hurricane Lili's landfall in Louisiana last year was forecast accurately nearly three days in advance, but intensity forecasts did not capture Lili's rapid weakening from a Category 4 to a Category 1-2 storm in the 12 hours before landfall.

To find out more

http://www.nhc.noaa.gov/

twister.sbs.ohio-state.edu/tropical.html

cimss.ssec.wisc.edu/tropic/tropic.html

www.state.sc.us/emd/

hurricane.accuweather.com

www.weather.com/newscenter/tropical/