I have recently discussed the errors associated with track forecasts and how to use the Track Forecast Cone. As mentioned on the last blog, the cone graphic is useful for determining the most likely track area of the center, but it says nothing about the Tropical Cyclone (TC) intensity or size.
To deal with this problem, the National Hurricane Center (NHC) now issues Wind Speed Probability Graphics. These graphics account for the uncertainty in the track, the uncertainty in the intensity, and the variability in the size of the TC. The NHC website states that these graphics show probabilities of sustained (1-minute average) surface wind speeds equal to or exceeding 39 mph (tropical storm force), 58 mph (strong tropical storm force), and 74 mph (hurricane force). The graphics are based on the official NHC track, intensity, and wind radii forecasts, and on the NHC forecast error statistics for those forecast variables during recent years. Each graphic provides cumulative probabilities that wind speeds of at least those thresholds will occur during cumulative time periods at each specific point on the map. The cumulative periods begin at the start of the forecast period and extend through the entire 5day forecast period at 12-hour intervals. An individual graphic is produced for each cumulative interval, and the capability to zoon and animate through the periods is provided on the NHC website. To assess the overall risk of experiencing winds reaching one of the given thresholds at any location, the 120-hour graphic is recommended. For example, the cumulative 120-hour Tropical Storm Force Wind Speed Probability Graphic on Tropical Storm Fay from the 11:00 pm EDT August 16, 2008 forecast is seen below.
A companion product, the Wind Speed Probability Text Product is also issued along the each TC forecast. That product is recommended to more easily assess when winds of each threshold are most likely to start at any specific location, helping answer the question, “How long do I have to prepare?”