Little Compass RoseCaribbean Compass   May 2018


More Thoughts on Hurricanes

by The Scottish Captain


Every year produces new data on hurricanes, and every year we should do our best to understand what that data is telling us about these storms - and what (if anything) we can do to prepare for them.

The old adage about hurricanes is that the only predictable thing about them is that they're unpredictable. It is also true that we will continue to encounter them in this region, so if you're not prepared for that possibility, you'd best not keep your boat "in the zone" over the summer. Last year also reminded us that there is a big difference between a Category 4 or 5 storm and a lesser one. Do not assume that if your favourite "hurricane hole" gave you protection from a Category 2 storm, it will do the same should one of the monster hurricanes decide to barrel through it in future.

Sometimes we are lulled into believing that the meteorologists have mastered the science of forecasting hurricane tracks. Certainly when storms form where expected and follow traditional routes (e.g. the Cape Verde storm), the models now tend to be very good. I sat on an island as a Category 5 storm headed straight towards us from the Atlantic for over 24 hours, but because the consensus of the computer models had it veering north before it reached us I was not panicking. I was concerned (because I'm a sceptic by nature), but was mostly concerned for the islands farther north - where it was predicted to hit. The problem is that as we see these models perform incredibly well sometimes (and Maria in 2017 did alter course as predicted, going on to devastate the island of Dominica, a mere 70 miles north of my location), we have a tendency to trust those forecast tracks more. It's a natural tendency, but one that we must consciously resist.

One storm that has been analysed extensively is Joaquin (2015). This was an unusual tropical cyclone because it formed in waters northeast of the Bahamas and was initially predicted to move toward the northwest (as would have been expected), but actually defied the models and moved toward the southwest, reaching as far as the Bahama islands. The reason that its origin, behaviour and evolution have been analysed so much is because it led to the sinking of the US container ship El Faro with the loss of 33 lives - the worst US maritime disaster in 30 years. Very early on in this tropical depression's development, no one had it heading south at all. The US weather reports were all discussing it in terms of how it would affect the Eastern Seaboard, and in particular the associated heavy rainfall to be expected.

Nowhere did it say to expect the hurricane to track southwest towards the Bahamas - so, expect the unexpected.
As the eventual hurricane actually created its own route - southwest - which made a mockery of those early predictions, the lesson must be reiterated: do not make assumptions based on the model track forecasts, particularly when a storm is in the early stages of development (its future life is even more unpredictable at this time), and also when such storms are developing outside of usual locations. Furthermore, treat any predicted change in movement with caution until such time that at least some actual change has been demonstrated. Always recognise the possibility that future movement could be based on an extrapolated course of the current movement.

Read up on Don Street's Ten-Degree Cone of Probability (see sidebar), and practise plotting storm tracks, so you can better assess where might be a safe location to hole up, and when early evasive action is required.
Familiarise yourself with "Bowditch", The American Practical Navigator (https:// goo.gl/tZG1wQ) and the intricacies of calculating your position relative to the centre of circulation using plots of wind direction, wind speed and pressure: "Avoiding Tropical Cyclones" (pages 513 to 520).

Download a copy of the NOAA Mariner's Guide (https://goo.gl/RWXc3L). In particular, study Chapter 4, "Guidance for Hurricane Evasion in the North Atlantic" (pages 41 to 57). Understand the 1-2-3-Rule. Learn how to find/escape the "Navigable" and "Dangerous" semi-circles.
Read as many accounts of storm encounters as you can (see footnote), and if you've experienced one yourself, write about it for others to gain the benefit of your wisdom.

Other anomalies (which should nevertheless be considered distinct possibilities): storms have been known to make right-angle turns in defiance of the forecast predictions, tracking into completely unprepared areas as a result (for example Hurricane Mitch in 1998, which sank the windjammer Fantome with all hands near Roatan). Some storms stall and intensify over one particular random location, exacerbating the effects of the storm (also see Mitch and Joaquin). Tropical cyclones can form quickly and seemingly without warning, given the right conditions (such as Tomas in 2010). "Wrong Way Lenny" (1999) and Klaus (1984) both caused havoc by appearing from the west and moving northeast into the Leewards. Devastating swells caught many by surprise in west coast anchorages throughout the Eastern Caribbean. Hurricane avoidance plans had to be re-orientated to allow for this uncharacteristic western approach. The west coasts of islands as far south as the Windwards suffered damage from freak westerly swells.

Sometimes it all comes down to luck. After Irma (2017) caused mayhem in Barbuda, St. Barts, Anguilla, St. Martin/St. Maarten and the Virgin Islands during September 5th and 6th, a catamaran from St. Lucia was one of many vessels that headed north with relief supplies. They off-loaded in the BVI just in time to receive the news that another storm (Maria) was on its way. With limited speed and no means to update weather while off shore, the decision was made to stay put in Tortola (it was not an easy decision to make, particularly as another, faster, cat headed south and would be tucked up safe in Grenada when Maria eventually came knocking). As it was, a slightly more southerly path close to St. Croix spared Tortola from a repeat performance, though the relief cat only just survived, tied up on one of the few remaining docks in Nanny Cay at the time, subjected to swells that had the mooring lines popping all night and into the next day. A slightly more northerly track and it could have been so different.

Local boats have strategies to deal with hurricanes which commonly involve tying up in the nearest "hurricane hole" of choice, which can work as long as the storm is a moderate one, or not a direct hit. The faster commercial vessels have a tendency to leave port in advance of a westward-moving storm that seems likely to pass through the Leewards. They run south and wait offshore in the lee of Martinique, monitoring the progress of the storm to decide if they have to keep heading farther south or not (you can see them on AIS).
Perhaps it goes with out saying that if you can stay out of the hurricane zone during the hurricane season from June 1st to November 30th, you might have a more relaxed summer. But then again, you will also miss the quieter anchorages and (generally) calmer seas. Another of life's wonderful dilemmas…

Selections from the Archives. For more thoughts on hurricane preparation, see the following articles in the Compass archives:
• ‘Hurricane Survival Strategy', by David H. Lyman (www.caribbeancompass.com/online/may13compass_online.pdf, page 22)
•  ‘Preparing for a Hurricane', by David H. Lyman (www.caribbeancompass.com/online/june13compass_online.pdf, page 27)
• ‘Windwards During Hurricane Season? Thoughts for Cruisers Planning to Remain', by The Scottish Captain (www.caribbeancompass.com/online/july15compass_online.pdf, page 39)
• ‘Managing Hurricane-Season Risk for Boats Stored in the Caribbean', by Don Street. (www.caribbeancompass.com/online/march18compass_online.pdf, page 24)

The Cone of Probability

by Don Street

Since hurricanes approaching the Caribbean virtually never alter course more than five degrees in 24 hours, I have formulated "Street's Law of Probability".

In the Dark Ages before the arrival of electronic navigation, making landfall in fog or in periods of poor visibility, with no sun sights available, was difficult. We homed in on, or tried to fix our position with, radio DF bearings. A bearing was taken, and the experienced navigator knew he did not have an exact bearing but rather a cone of probability. The size of the cone varied according to electrical reception, sea conditions, equipment available and the abilities of the DF operator. The cone could be as little as three to five degrees or as much as ten or more degrees. When you were farther out at sea the cone could be quite large, but as you approached the DF station the width of the cone became smaller and smaller, and you became more and more sure of your position.

Hurricanes can be plotted in similar fashion. With the predicted track or the direction of movement in its center, draw a 10-degree cone extending from the position NOAA weather gives for the center of the hurricane. When the hurricane is 600 miles away, the cone is 105 miles wide; when it is 300 miles away, the width of the cone will be 35 miles wide, etcetera. This will allow you to predict the probable strike area, and the danger zone.
 


 
 


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