Experimental Tropical Cyclone Surface Wind Speed Probabilities

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Contents

1. Overview
2. Description of the wind speed probabilities graphical products
3. Potential advantages as compared to existing products
4. Method for computing the Wind Speed Probabilities

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2. Description of the wind speed probabilities graphical products

Each wind speed probability graphic provides probabilities (in percent) that wind speeds of at least 34 kt (39 mph, tropical storm force), 50 kt (58 mph), or 64 kt (74 mph, hurricane force) will occur during cumulative time periods at each specific point on the map. The cumulative periods extend into the 5-day forecast period at 12-hour intervals (that is, 0-12 h, 0-24 h, 0-36 h, ... , 0-120 h). An individual graphic will be produced for each active tropical cyclone for each cumulative time period and for each wind speed threshold. The capability to animate through the periods will also be provided. These cumulative probabilities indicate the overall chances that the indicated wind speed will occur at any specific location on the map during the period between hour 0 and the forecast hour.

In other words, these cumulative probabilities tell decision-makers the chances that the event will happen at any point on the map within the time period stated on each graphic.

The example graphic below shows the hurricane force (64 kt, 74 mph) wind speed probabilities for Hurricane Charley (2004), based on advisory 14 issued at 5:00 pm Thursday August 12, the day before landfall in southwestern Florida. When this advisory was issued, the hurricane warning was extended northward along the west coast of Florida from Bonita Beach northward to Bayport (to include Fort Myers, Port Charlotte, Sarasota, and Tampa). The cumulative 0-120 hour values are shown here to capture the overall probability of the event (overall chance of getting hurricane force winds at any point on the map). While the exact official track forecast for this advisory goes over Tampa, it is clear from this graphic that the chances of experiencing hurricane force winds from this event are nearly the same (about 30%) over a large portion of the coastline, including the eventual landfall location at Port Charlotte, FL. This graphic is an excellent example of a situation in which the wind speed probabilities can help users to understand forecast uncertainties, such that they are not surprised by any relatively small changes in the track. This graphic shows why it is crucial to make proper preparations when a watch or warning is issued for your area.

Plot of probabilities (in percent) of experiencing wind speeds of at least 64 knots (74 mph, hurricane force) during in the 120 hours starting at 18Z (5:00PM EDT) on Thursday, August 12.

A complete example of a suite of wind speed probability graphics for an advisory package is provided here, with explanations provided below. Examples are provided for 34 kt (39 mph, tropical storm force) or greater, 50 kt (58 mph) or greater, and 64 kt (74 mph, hurricane force) or greater.

The example set of probability graphics above are based on Hurricane Ivan (2004) advisory number 47, at which time a hurricane watch was issued for the northern Gulf of Mexico coast, about two days prior to the eventual landfall of the center of the hurricane on the northern Gulf coast.

The probabilities show that the hurricane is not a point and that there is uncertainty in the forecast of track, intensity, and size. They also show that there is a chance at many locations that the hurricane's effects (for example, tropical storm or hurricane force winds) could be directly experienced. The Ivan example involves a large and intense hurricane. However, not all storms will produce probabilities this large over as wide an area.

The hurricane force (64 kt, 74 mph) probabilities are smaller than for tropical storm force (34 kt, 39 mph), primarily because hurricane force winds do not extend as far out from the center of the hurricane. Note in the Ivan example that the probabilities of hurricane force winds at 5 days (120 hours) are essentially the same along the entire western and panhandle coasts of Florida. Therefore, even though the exact track forecast may go over a particular location on the coastline, there are many other locations which have the same chance of experiencing hurricane conditions during this event. It is important to understand that the probabilities are for specific points on the map. The chances that hurricane force winds will occur somewhere along the Gulf coast in this Ivan case are much larger than the chances at any one point.

The hurricane force wind speed probabilities are still relatively small in magnitude along the Gulf of Mexico coastline by the end of the 5-day forecast period. It is important for users to realize that probabilities that may seem relatively small may still be quite significant. The probabilities may indicate there is a chance that a damaging or even an extreme event may occur at your location. As a storm gets closer to land, relatively small probabilities may warrant making preparations to protect lives and property. Users are urged to consider the potentially immense cost (in terms of lives, property, etc.) of not preparing for an extreme event, even if the chances at an individual point are only perhaps 1 in 20 (5%) or 1 in 10 (10%) that the event will occur.

While separate graphics are centered on each individual tropical cyclone, probabilities resulting from more than one active tropical cyclone may be seen on each graphic. Note in the Ivan example that probabilities resulting from Hurricane Javier (2004) in the eastern Pacific Ocean are also visible in the left (western) portions of the graphics.

Next: Potential advantages as compared to existing products