Tracking Hurricanes - An Inside Look
UPDATED June 27, 2010
By WeatherBug Meteorologists
Meteorologists at the National Hurricane Center (NHC) need to look well beyond the United States to track tropical systems that may threaten the country.
Every year, forecasters track hundreds of tropical disturbances (clusters of clouds and thunderstorms) in the Atlantic, Caribbean and Gulf of Mexico. Most form near the west coast of Africa. Only about ten develop into tropical storms each year, with winds of more than 39 mph and a closed circulation.
Satellite analysts at the NHC can objectively estimate the intensity of these disturbances by closely examining the organization in their cloud patterns. This is known as the Dvorak Technique.
Sometimes disturbances are upgraded to tropical depression or storm status purely based on satellite observations. However, when reports from ships and buoys over the ocean are available, these can help to confirm a storm`s intensity.
Once a disturbance develops into a tropical depression or storm, forecasters run specialized computer forecast models that predict the storm`s path and intensity up to five days in advance.
A few forecast models (called "statistical models") use the climatology of past storms in a similar time and place. The vast majority of the models (called "dynamical models") incorporate input from the global forecast models that are used for everyday weather forecasts.
These global forecast models take into account current temperature, moisture, pressure and wind observations and use the known laws of physics, chemistry and thermodynamics to produce a forecast. They tend to steer the tropical cyclone in the direction that the overall winds are blowing. Weaker storms are steered more by winds near the ground, while stronger storms are steered more by upper level winds.
Forecast models sometimes have difficulties simulating the structure of a hurricane in the upper levels of the atmosphere. This is in large part due to the lack of data over the oceans. These simulation difficulties can lead to significant model errors.
As tropical systems continue to develop and strengthen, the NHC usually sends its reconnaissance planes to gather detailed data on upper level winds and temperatures. These data have been shown to improve the performance of forecast models.
Forecasting intensity is more difficult than predicting the track of a hurricane. Additional models are used for this task. They take into account information on sea surface temperatures and upper level winds. The warmer the waters are and the smaller the wind shear (the difference in wind speeds and directions with height) surrounding the tropical cyclone, the more likely it is to intensify.
These models sometimes disagree considerably, and forecasters must critically examine their past performance and decide whether they match up with the expected weather pattern around the hurricane. The average track error for a three-day forecast is about 230 miles, with an average intensity error of about 25 mph.
Forecasters at the NHC issue four Tropical Weather Outlooks per day from June 1st to November 30th. Each outlook is intended for the general public and contains information on areas of disturbance and potential development in the next 48-hour period.
When a tropical cyclone has been established, there are also a variety of other products that the NHC will issue. These include Public Advisories, Forecast/Advisories, Wind Speed Probabilities and Discussions. These are issued every 6 hours (or as needed) and are intended for the general public as well as other forecasters. More information on the storm and expected hazards as well as anticipated wind speeds and landfall sites are included in these reports.
This system works so well due to the coordination between the NHC and other agencies and emergency personnel, as well as with other countries to gather information and send hurricane advisories out.
Story image is courtesy of NOAA.
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