Now that the tropics have become active, I would like to say a word about tropical cyclone forecasting. Although it would be nice if we could always have a perfect forecast, that is not going to happen. The atmosphere is unbelievably complex and we will always have limitations with observations and computer models.
Uncertainty is a fundamental characteristic of weather prediction, and no forecast is complete without a description of its uncertainty. That is the bottom line of a National Research Council report published a few years ago.
One way the National Hurricane Center (NHC) attempts to deal with uncertainty in track forecasting is by always producing a “cone of uncertainty” along with each five-day track forecast. This cone is designed such that the actual track of the tropical cyclone will stay within the cone approximately two thirds of the time.
The above graphic (courtesy of the National Hurricane Center web site) shows the current track forecast along with the track cone on Hurricane Bill. Again, the actual track of Bill is expected to stay within the cone two thirds of the time. I’ll talk more about how the cone is constructed in future blogs. It is important to understand that the cone is not an impact graphic. The impacts (strong winds, rain, tornadoes, waves, storm surge) can extend well beyond the cone. But the cone at least gives you an idea of the most likely track of the center of Bill.
One way the National Hurricane Center deals with uncertainty in intensity forecasting is by producing a “maximum wind speed probability table.” This table shows the probability that the maximum sustained wind speed of the tropical cyclone will be within various intensity ranges during the next five days.
The above graphic (courtesy of the National Hurricane Center web site) shows the current Maximum Wind Speed Probability Table on Hurricane Bill. Although the NHC gives specific values for the maximum wind speed, those should be thought of as the most likely scenario. The table helps give a feel for the uncertainty in the intensity. For example, the NHC is forecasting Bill to become a major hurricane with Category 3 intensity of 115 mph by 36 hours. But the table indicates the probabilities at that time period are 17% for a Category 1 and 32% to be a Category 2. The probability for being a major hurricane is 39% at that time.
Although Hurricane Bill is currently not forecast to have a direct impact on the United States, it helps to understand the uncertainty involved in the forecasts.