1 – 2 – 3  RULE OF TROPICAL CYCLONE AVOIDANCE

(Content and graph from NOAA, provided here by WeatherCarib.com)

 

For vessels at sea, avoiding the 34 KT wind field of a tropical cyclone is paramount.  Any ship in the vicinity of a tropical cyclone should make every effort to remain clear of the maximum radius of analyzed or forecast 34 KT winds associated with the tropical cyclone.  Knowing that the area of 34 KT around tropical cyclones is rarely symmetric but instead varies within semi-circles or quadrants is important.  Understanding that each tropical storm or hurricane has it own unique 34 KT wind field are necessary factors to account for when attempting to remain clear of this dangerous area around a tropical cyclone.  NHC forecasts attempt to define the structure of this wind field and use of the latest TCM in determining the maximum radius of 34 KT winds is necessary when trying to avoid this dangerous threshold.

 

 

1.   Plot the current and forecast tropical cyclone positions taken from the latest TCM.

2.   Find the maximum radius of 34 KT winds at the current and each forecast time period of the TCM out to 72 hours.

 

For example, the radii of 34 KT winds given for the 24 hour forecast position

in the latest TCM are:

 

34 KT...175NE 150SE 150SW 150NW

 

Therefore, the maximum radius of 34 KT winds associated with the tropical

cyclone at its 24-hour forecast position is 175 NM.

 

3.   Next apply the 1-2-3 rule to each of the radii at the 24, 48, and 72 hour forecast positions.

 

At the 24-hour forecast position (1 day): add 100 NM to the maximum radius

of 34 KT winds found in the 24 hours forecast of step two.

 

>>> 175 NM (Forecast radius of 34 KT) + 100 NM = 275 NM

 

At the 48-hour forecast position (2 days): add 200 NM to the maximum radius

of 34 KT winds found in the 48 hour forecast of step two.

 

At the 72-hour forecast position (3 days): add 300 NM to the maximum radius

of 34 KT winds found in the 72 hour forecast of step two.

 

4.   Now draw a circle around the 24, 48, and 72 hour forecast positions of the tropical cyclone using the radii found in step 3.

 

5.   Connect a line tangent to each circle constructed in step 4.  The area enclosed by these tangent lines is known as the danger area of the tropical cyclone and must be avoided as a vessel attempts to navigate in the vicinity of the tropical cyclone.

 

 

Note of caution. This rule establishes a minimum recommended distance to maintain from a tropical cyclone in the Atlantic Basin.  Larger buffer zones can and should be established in situations of tropical cyclones with large forecast uncertainty, limited crew experience, decreased vessel handling, or other factors as determined by the vessel master.  The 1-2-3 rule does not account for sudden & rapid intensification of tropical cyclones that could result in a rapid outward expansion of the 34 KT wind field.  Also, the 1-2-3 rule does not account for the typical outward expansion of the wind field as a system transitions from tropical cyclone to extratropical gale or storm in the North Atlantic.  Finally, mariners should not equate the radius of 34 KT winds with the area of 12-foot seas in the vicinity of a tropical cyclone.  The 1-2-3 rule relies solely on avoiding the radius of 34 KT winds in a tropical cyclone and does not take sea heights into consideration. Vessels with lower sea keeping limits should also make adjustments to the 1-2-3 rule in order to minimize exposure to seas that will dangerously hamper ship stability and maneuverability.  The radius of current 12-foot seas is issued in the TCM and can serve as a gauge for vessels with lower sea keeping limits in order to remain clear of potentially damaging higher seas.  Further guidance on forecasted seas in excess of 12 feet in the vicinity of any active tropical cyclone is available in the Atlantic High Seas Forecasts issued by TPC and MPC.