With two named tropical storms already on the books before the official start of the 2012 hurricane season I wanted to review what the Mariner’s 1-2-3 Rule is and if it should be revised. The most important ship routing tactic in dealing with tropical cyclones is avoiding them in the first place. The Mariner’s 1-2-3 Rule was developed as an aid for mariners to avoid tropical cyclones by accounting for forecast track errors and is a must for any mariner to know when navigating near a hurricane or tropical storm.
The Mariner’s 1-2-3 Rule originally was an adaptation made from a US Navy training film “A Time for All measures” in use during the early 1970â€²s. Since the 1990’s the Mariner’s 1-2-3 Rule has incorporated a “danger zone” by adding the 10-year average tropical cyclone forecast track errors which were at the time, approximately 100 nm for each 24 hour forecast period plus the radius of forecasted gale force (34 knot or higher) winds. The 34 knot or higher wind field was chosen as the critical wind speed because at this level or higher the wind and sea conditions significantly limit ship maneuverability. When ship maneuverability is limited, then course options are also significantly reduced.
There has been a significant improvement in cyclone track forecasting in recent years and we should now revise the Mariner’s 1-2-3 rule to better reflect this improvement. According to Lee Chesneau, owner of Chesneau Marine Weather and former Ocean Prediction Center meteorologist we should consider revising the rule as follows:
“The overall skills scores of the NHC for track location which the wind radii extends from, has vastly improved. The 1-2-3 Rule today should include the more accurate skill score data that adds up to about 1 degree of latitude (60nm) at 24 hours, 2 degrees (120nm) at 48 hours, and 3 degrees (180NM) at 72 hours.“
For example, at 24 hours (1 degree of latitude or 60 nm) would be added to the right and left of the track. At 48 hours the error is (2 x 1 degree or 120nm) and for 72 hours the error is (3 x 1 degree or 240nm). In addition to the allowance for track error, the forecasted radius of gale force (34 knot or higher) wind field is also added. Therefore, if the 24 hour forecast shows gale (34 knot or higher) winds will extend out to 120 nm, then the radius of danger would be 60nm (the 24 hour average track error) plus 120 nm (radius of forecasted gale or higher winds) or a total of 220 nm danger zone left and right of the forecasted track and position.
This then becomes the minimum distance to maintain from the hurricane center in 24 hours. Keep in mind that larger safety zones should be considered whenever there is a high degree of forecast uncertainty, limited crew experience, or limits on vessel handling. The rule also does not account for sudden & rapid intensification of hurricanes that could result in significant expansion of the 34 KT wind field or to the typical wind field expansion that occurs when a hurricane transitions to an extra-tropical storm.
The National Hurricane Center currently produces a “Tropical Cyclone Danger Graphic” depicting the “1-2-3 Rule” danger zone for tropical cyclones in both the North Atlantic and Eastern North Pacific oceans but this graphic still uses the older average track error of 100NM per 24 hours Perhaps it is time for NOAA to update this.
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