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Tropical Cyclone Wind Speed Probabilities Products


Updated 7 January 2009

Contents

  1. Overview
  2. Description of the tropical cyclone wind speed probabilities text product
  3. Description of the tropical cyclone wind speed probabilities graphical products
  4. Description of the tropical cyclone NDFD wind speed probabilities products
  5. Potential advantages as compared to previous products
  6. Method for computing the wind speed probabilities

5. Potential advantages as compared to previous products

The TPC/NHC has been issuing other products intended to convey the uncertainties in the track forecast. However, those products do not account for the uncertainties that also exist in the forecast of the cyclone's intensity and size.

The discontinued strike probabilities text product was a statement about the "close" approach of the center of a tropical cyclone. However, the wind speed probabilities products are about the weather. That is, the wind speed probabilities provide the chances of wind speeds equal to or exceeding familiar thresholds (for example, tropical storm force and hurricane force) at individual locations. Therefore, these probabilities likely have more direct meaning and impact to users. Also, while the previously available strike probabilities only provided forecast information out to three days for just the Atlantic basin, the wind speed probabilities provide information out to five days for both the Atlantic and Eastern Pacific basins.

TPC/NHC also continues to provide a watch/warning graphic that also displays the forecast track and a "cone of uncertainty". The cone depicts the long-term average error in the forecast of the track of the cyclone center. It is important to remember that the effects of a tropical cyclone can be experienced well away from the center of the cyclone and well outside of the cone of uncertainty, since the actual path of the center does not always stay within the cone, and since tropical cyclones vary in intensity and size. The wind speed probabilities provide more direct information about what wind conditions could be experienced at specific locations both inside and outside of the cone.

The NHC has issued the Maximum Wind Speed (Intensity) Table since 2003. Beginning in 2008, the methodology to produce these probabilities was changed to the same methodology that is used to create the location-specific probabilities. The new method uses errors from recent years in the official track and intensity forecasts issued by the TPC/NHC and accounts for track errors that can lead to errors in the intensity forecasts. For example, for a storm forecast to be very intense but just off the coast, the intensity probabilities would reflect the fact that the track forecast could be off a little and the storm could be inland instead of over water. This incorporation of track error and its impact on the intensity probabilities is the reason that using the intensity probabilities to infer the intensity of a cyclone at the time of landfall can be misleading (see section 3).

The Maximum Wind Speed (Intensity) Table provides users with the chances that a tropical cyclone.s maximum winds will fall within several categories. For example, a deterministic forecast of the maximum sustained winds of Tropical Storm Humberto for advisory number 2 (issued at 2100 UTC 12 September) was 55 kt at 12 h. The probabilities, however, shown in the new maximum wind speed table indicate that Humberto had a 7 % chance of becoming a hurricane at the 12 h forecast time.

The probabilities of the maximum wind speed along with the location-specific probabilities have the potential to provide users with information that enhances their ability to make preparedness decisions specific to their own situations.

Next:  Method for computing the wind speed probabilities



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Page last modified: Wednesday, 07-Jan-2009 16:48:04 UTC