Interpreting NHC Graphics of Probabilistic Forecasts for Wind Speeds (Prior to 2006)

In order for you to gain an understanding of the graphical product NHC once used for predicting the intensity of tropical storms and hurricanes (prior to the start of the 2006 hurricane season), let me pick up with my story about Hurricane Isabel. Before making landfall as a Category 2 hurricane (and causing extensive damage (high-resolution image) along the Outer Banks), Hurricane Isabel reached Category 5 strength (satellite; advisory). While forecasters expected the storm to eventually weaken (the northernmost location on the U.S. mainland ever to be stuck by a Category 5 hurricane is Pass Christian, Mississippi, on the Gulf Coast -- check it out), they didn't know exactly how much weakening would occur. To convey the uncertainty of their predicted intensity, NHC forecasters issue wind charts, which are essentially cones of uncertainty for the maximum wind speed of a hurricane or tropical storm. More succinctly, the intensity cone indicates the probabilities of possible maximum wind speeds (in miles an hour) as a function of forecast time (up to 72 hours). Speaking in practical terms, these images helped to better visualize the uncertainties of forecasting the intensity of a hurricane or tropical storm. In the example below, the thick blue line represents the official NHC forecast for the maximum sustained winds of Hurricane Isabel (NHC issured the forecast at 5 A.M. on September 15, 2003). The thinner lines, labeled 10% and 20%, indicate the probability that the maximum wind speed will be stronger or weaker that the NHC forecast.

This wind chart, issued by NHC at 5 A.M. on September 15, 2003, shows the official forecast for the maximum sustained winds of Hurricane Isabel (thick blue line). The thinner lines, labeled 10% and 20%, indicate the probability that the maximum wind speed will be stronger or weaker that the NHC forecast.

Let's interpret this wind chart (remember, you'll probably run across this product if you investigate storms prior to 2006). Clearly, forecasters expected Isabel to weaken over the next 72 hours (Isabel's maximum sustained winds were forecast to weaken from 150 miles an hour to 135 miles an hour over the 72-hour period according to the official forecast). But, there was a 20% chance that maximum sustained winds could decrease to about 97 miles an hour, and a 10% chance that they could decrease to about 87 miles an hour. These probabilities were based on verifications of NHC intensity forecasts between 1988 and 1997. In the interest of full disclosure, Isabel's maximum sustained winds dropped to 105 mph by 72 hours from the time NHC issued this forecast. So the official forecast was somewhat of a bust, lending credence to the difficulty of forecasting hurricane intensity.

Clearly, the general public would have some difficulty interpreting this product. There had to be a easier way, and NHC found it!