The mariners 1-2-3 rule for tropical cyclones

In the last newsletter, I promised to go through a method to analyze the threat posed by tropical cyclones (tropical depressions, tropical storms, and hurricanes) by hand plotting information that is readily available in public advisories. This method is called the 1-2-3 rule.

First, it is important to understand that the most critical piece of information for a mariner to focus on with any tropical cyclone is NOT the position (and forecast positions) of the center of the system, and also is NOT the strength of the system (strongest winds, or Saffir-Simpson category). Of course, these parameters do give some indication of the severity of the system and the area that it will affect, and are the most often quoted measures of the storm in the popular media. What is most important to the mariner, though, is the size of the wind field associated with the tropical system. 

In particular, it is best for most vessels to focus on how far the sustained tropical storm force winds extend (and are forecast to extend) from the center of the system. This is called the 34-knot wind radius, and is provided in all tropical cyclone advisories for the current time, as well as for future forecast positions of the storm. The reason for focusing on the area of 34-knot sustained winds is that for most moderate and larger vessels, it is this wind speed, and the accompanying sea state, that begins to limit the navigation options for the vessel. In other words, with the conditions typically generated by sustained 34-knot winds, depending on the characteristics of the vessel, proceeding directly into the wind may not be possible. It just may be that proceeding in the difficult direction is what is needed to move away from the storm, thus staying away from even more perilous conditions. Therefore, by staying out of the area of sustained 34-knot winds, most vessels will be able to avoid damaging and perhaps catastrophic situations.

For every system, the 34-knot wind radius is different, and it is not necessarily related to the strength of the system. For example, it is possible that two systems with nearly identical central pressures and maximum sustained winds could have very different 34-knot wind radii. It is also possible that a weaker system (i.e., lower maximum sustained winds) could have a larger 34-knot wind radius than a stronger system. It should also be noted that the 34-knot wind radius is often not symmetrical. That is, the 34-knot wind radius can extend farther from the center of the system in one direction than in the other. This is accounted for in the advisories by providing the 34-knot wind radii in four quadrants around the system. 

Forecasting tropical cyclones is not an exact science, although significant improvements have been achieved in recent years. Therefore the 1-2-3 rule, in addition to taking account of the 34-knot wind radius, also factors in an average forecast position error to construct a danger area to avoid. When the rule was devised in the 1990s, it was reasonable to use as error estimates 100 nautical miles for the 24-hour forecast, 200 nautical miles for the 48-hour forecast, and 300 nautical miles for the 72-hour forecast. In fact, these error estimates (100, 200, and 300 miles) are what gives the 1-2-3 rule its name. 

To construct the danger area to avoid, on a plotting chart simply plot the current position of the tropical cyclone along with the 24, 48, and 72-hour forecast positions. For each position, sketch in the 34-knot wind radius as given in the advisory. Keep in mind that the area may not be a perfect circle if the radii are different in the different quadrants. Now, for each forecast position, expand the radius by the appropriate error estimate (100 miles at 24 hours, etc.). Usually the areas will overlap, especially with the larger error estimates for the later positions. Then simply outline the entire region defined by the separate areas, and this will be the danger area to avoid through the next 72 hours. 

There is an article available at the National Hurricane Center (NHC) Web site that has a nice diagram showing the 1-2-3 rule graphically. Here is the link: &mdash go to page 50. The entire article is worth reading, too. 

A couple of final comments: 

First, the 1-2-3 rule is available on the NHC Web site for each advisory, but, as indicated in the previous newsletter, there is great value in taking the time to plot it by hand.

Second, the forecasting accuracy has improved in recent years, and it is actually now more appropriate to use as error estimates 1, 2, and 3 degrees of latitude (60, 120, 180 miles) instead of the 100, 200, 300 miles used previously. For smaller vessels, though, as an extra measure of safety, it might be better to stick with the older error estimates. 

So get yourself a hurricane plotting chart, and sharpen your pencil to help stay out of the way of tropical storms and hurricanes this summer and fall.

About the Author 

Ken McKinley earned a bachelor’s degree in atmospheric science from Cornell University in 1980, and attended graduate school in meteorology at the Massachusetts Institute of Technology. After working as a meteorologist for nearly 10 years he founded his own meteorological consulting firm, Locus Weather, in Camden, Maine in 1991. 

A large portion of his business at Locus Weather involves providing custom weather forecast services for oceangoing yachts, both racers and cruisers. Ken serves as an instructor for the Ocean Navigator School of Seamanship, and also as an adjunct instructor at the STAR Centers for Professional Maritime Officers in Dania, Fla., and Toledo, Ohio, and for MITAGS in Baltimore, Md. He has also taught meteorology at Maine Maritime Academy. He resides in Rockport, Maine with his wife and two sons. Ken’s Web site is:

By Ocean Navigator