Cone of Uncertainty

BATON ROUGE, LA (WAFB) - Drones are one of the newest tools being used in hurricane research.

NASA's Global Hawk drone flew over the eye of Hurricane Edouard at an altitude of more than 60,000 feet. The drones gather key weather data from the core of the storm, providing forecasters with a more detailed look at its intensity and the factors influencing its strength and motion.

Over the years, you may have become quite familiar with the various forecast maps. Three maps representing the National Hurricane Center's forecast projections for Katrina, Rita and Gustav two to three days before landfall show the well-known forecast cone of uncertainty.

Each cone is based on the National Hurricane Center's average forecast error over the previous five years for specific forecast times out for five days. It may sound complicated at first, but it's not.

For the official three-day forecast for Hurricane Katrina, which showed the average error from 2000 to 2004, the average error was more than 200 miles at three days. The envelope of the average error at three days stretched from near Lake Charles almost to Apalachicola, Florida. The average error also extends north to south. In this case, the error could range from still-not-onshore to inland as far as north central Mississippi.

Fortunately, there have been some marked improvements in forecasting over the past 10 years. If this year's average error was applied to Katrina, there would be a substantial reduction, almost a 50 percent reduction, in the size of the cone at three days.

However, the average error at three days is still rather large, at roughly 120 miles. Also, remember that on average, roughly one-in-three storms ends up outside of the forecast cone. The forecast based on the cone of uncertainty is far from a sure thing.

This might have you asking: if the forecast error for every storm in a season is exactly the same for every forecast time, why do the cones often appear so very different?

Obviously, the shape of the cone is dependent on any turns the system may make. But, even storms that are expected to follow along a straight forecast path can end up with five-day cones of different shapes.

Why? Because of the storm's forward speed.

For example, a storm with an expected forward speed of 16 mph, using the average error over the course of five days, produces an elongated appearance to the cone, making the cone look rather thin. The cone is constructed by connecting the outer edges of each forecast circle.

Compare the storm to a storm with a forward speed of just 10 mph. The forecast circles are exactly the same size for each time interval. Yet, the final cone has a 'fatter' appearance, a result of the slower forward speed.

While the science behind forecasting storms is important, having your family ready for a hurricane is the most important part of storm preparedness. You may want to add to that hurricane survival pack.

Copyright 2015 WAFB. All rights reserved.