Changes to NHC / TAFB's Tropical Cyclone Danger Graphic

Hugh Cobb
Tropical Analysis and Forecast Branch
National Hurricane Center, Miami, FL
NOAA/National Centers for Environmental Prediction

The Tropical Analysis and Forecast Branch (TAFB) of the National Hurricane Center (NHC) has been providing the Tropical Cyclone Danger Graphic for both the Atlantic and East Pacific basins since the 2003 hurricane season. The graphic depicts the danger area associated with tropical cyclones from the equator to 60°N between 0° and 100°W, including the Pacific east of 100°W, and from the equator to 40°N between 80°W and 175°W, including the Gulf of Mexico and Western Caribbean. The tropical cyclone danger graphic depicts the forecast track and corresponding area of avoidance for all active tropical cyclones through 72 hours, and areas for which tropical cyclone formation is possible within the next 48 hours over the Atlantic and East Pacific waters between May 15 and November 30. Traditionally, the three-day forecast track of each active tropical cyclone is depicted along with a shaded "danger" region, or area of avoidance. The danger area is determined by adding 100, 200, and 300 nautical miles (nmi) to the tropical storm force wind radii (34 knots) at the 24-, 48-, and 72- hour forecast positions, respectively (hence the "1-2-3" nomenclature or the "mariners 1-2-3 rule"). Figure 1 illustrates the "mariners 1-2-3 rule".

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Figure 1. Illustration of the Mariners 1-2-3 Rule.


The Tropical Cyclone Danger graphic in Figure 2 valid 0300 UTC 25 August 2011 depicts the area of avoidance associated with Hurricane Irene advancing along the United States east coast and an area of possible tropical cyclogenesis within 48 hours over the southwest of the Cape Verde Islands.

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Figure 2. Tropical Cyclone Danger Graphic valid 0300 UTC 25 August 2011.


There have been significant improvements in hurricane track forecasting over the past five decades. Figure 3 shows the improvement in average forecast track errors from the 1970s through the current decade. The 100-nmi, 200-nmi and 300 nmi errors comprising the "mariners 1-2-3" rule reflect forecast track errors observed in the 1980s. In the current decade, average forecast track errors have been reduced by over 60% from what they were in the 1980s. Because of these improvements in tropical cyclone track forecasting, the "mariners 1-2-3 rule" methodology depicts excessively large potential tropical cyclone danger areas and leads to "over-warning" of tropical cyclone avoidance areas. Wind speed probabilities offer a way to convey uncertainty in experiencing specific wind speed thresholds in a quantitative sense. The advantage of this approach is that it allows the depiction of any particular desired level of risk. In addition, wind speed probability calculations consider the spread of the track model guidance and therefore has some situational variability. It also considers uncertainty in the forecasts of tropical cyclone size and intensity as well as the track of the cyclone. Thus each set of wind speed probabilities and their conveyance of risk are unique to each advisory issuance. This is an additional advantage the wind speed probabilities have over the empirically based Mariners 1-2-3 rule. In 2012, the National Hurricane Center developed an alternative experimental version of the Tropical Cyclone Danger graphic based on the wind speed probability calculations discussed above. Two avoidance thresholds were developed for the experimental wind-speed based product. The avoidance area encompassed by the 5% 34-kt wind speed probability swath conveys a low risk of experiencing tropical storm force winds within the area through 72 hours and is denoted with a dashed line and hatched. The avoidance area encompassed by the 50% 34-kt wind speed probability swath conveys a high risk of experiencing tropical storm force winds through 72 hours. This area is denoted within a solid line with solid cross-hatching. Figure 4 is a wind speed probability- based Tropical Cyclone Danger graphic valid 2100 UTC 1 October 2015 depicting the avoidance areas associated with Hurricane Joaquin over the Bahamas and off the southeastern United States. TAFB is replacing the empirical Mariner's 1-2-3 rule with the wind speed probabilities in the Tropical Cyclone Danger Graphic on or around 15 July 2016. This product will be available on the National Hurricane Center's website at http://www.nhc.noaa.gov/marin e/ and on the New Orleans, LA, Point Reyes, CA and Honolulu, HI marine radio fax.

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Figure 3. NHC Official Average Track Errors by decade 1970s-2010s.


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Figure 4. Wind Speed Probability-Based Tropical Cyclone Danger Graphic valid 2100 UTC 1 October 2015.


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