Sunday, August 11, 2013

Comments on the Mariners Danger Rule for Hurricane Avoidance

 The original 1-2-3 and 34 kt Rule at the Mariner's Weather Log is here (pdf file).

The Mariners Danger  Rule is actully two rules - The 1-2-3 Rule and the 34 kts Rule. The 34 kt Rule is the recommendation that deep-sea ocean going ships stay outside the 34 knot wind field.  The 1-2-3  Rule is a method for plotting a danger area using a margin for forecast errors. The Mariners Dangers Rule  allows mariners to plot the danger area based upon forecasts.

Bill Bishop at The Marine Installers Rant put up an interesting post this past June - 123 TD Andrea about the Mariners 1-2-3 with links to the National Hurricane Center's National Hurricane Center Forecast Verification  with some charts showing the errors in hurricane forecasts.

Bill makes some good points about average errors  compared to the errors in the forecasts of individual hurricanes. 

  The 24 hour position forecast is pretty much on the money, the 48 hour position isn't quite as good, and at 72 hours you can see the outliers really start to appear, and by 120 hours the data permutations are biting hard. It's pretty much hit or miss, with a lot of misses. The intensity forecasts that impact the size of 34 kt wind fields are even more difficult to get right.

Bill makes another point about being inside the  35 knot field as too much. If 35 kts is too much just adjust accordingly. However besides the issue of the accuracy and reliability of the track forecast there is the issue of the forecast wind intensity and variations in wind field size. . For me  if I'm in 35 kts it is not usually an issue but if it was I'd add some distance from the system.

. From the NWS:
The 1-2-3 rule establishes a minimum recommended distance to maintain from a hurricane in the Atlantic. Larger buffer zones should be established in situations with higher forecast uncertainty, limited crew experience, decreased vessel handling, or other factors set by the vessel master. The rule does not account for sudden & rapid intensification of hurricanes that could result in an outward expansion of the 34 KT wind field. Also, the rule does not account for the typical expansion of the wind field as a system transitions from hurricane to extratropical gale/storm.

There is also a post at the Art of Dredging about the  1-2-3 rule which makes remarks about the dangers of using average tracks for hurricane avoidance.  That's a good point, using average tracks is a very bad idea,  but I don't agree with this:

The 1-2-3 system assumes that the cyclone will follow the textbooks.
Here is something new: tropical cyclones do not read textbooks.
And the chances you take (by assuming that cyclones stick to the rules) are huge.
Guessing that cyclones will follow the rules is close to a game of Russian roulette.
That's incorrect, no assumptions about cyclones following textbooks are made, the 1-2-3 Rule uses the forecast track with a margin of error. There is an important difference between the average track and the forecast track.

 In most cases the forecast is going to be the best information you have. Staying out of the area where the hurricane is forecast to go seems smart to me. The post mentions "modern techniques" but doesn't say what they are.

  This is what the original  article says in the case your vessel is in the danger area:

when a hurricane’s track is plotted, a 100-mile error for each 24-hour period must be applied and a vessel within this adjusted area must take action as if a hurricane were bearing directly toward them, which may become the case  

Forecasts are issued every six hours and the danger area can be replotted each time a forecast is received. The idea that the 1-2-3 rule uses textbook assumptions is nonsense.

All rules, including the Mariners Danger Rule are simplifications of more complex realities - knowing the rule alone may not be sufficient in all cases. When  there is any elevated risk or a complex situation  I get advice  from professional weather routers. Information about model agreement and confidence in the predicted track could be critical.  It's all about using more expertise if required to stay safe.

My post: Guidelines for Avoiding Hurricanes at Sea is here and How Accurate are Weather Forecasts? is here 

The NWS National Hurricane Center Marine Safety  is here. 

Mariner's Guide For Hurricane Awareness
in the North Atlantic Basin



Reid Sprague said...

Excellent explication, KC!

I review this every year even though I no longer go to sea. Maybe one year I'll take that dreamed-of trip south and need to use the Rule. Thanks!


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