What will this tropical weather do?

What will this tropical weather do?
Thread Fin Butterfly Fish (Chaetodon Auriga)

23:09.61S 135:02.32W

I mentioned before the fact that we had been struggling to get our heads around tropical weather and its forecasting. Last weekend had not just us but every boat in the area studying the weather situation very closely. A tropical depression was coming right over us (TD18F was its label) and as it was our first experience of one we were not sure what to expect.

The VHF radio was playing a constant stream on conversations between other boats in various languages; English, French, Norwegian, German all discussing whether to leave to another anchorage or stay put, how bad it would be etc..

We were in Rikitea so had rudimentary internet access via WiFi and used that to pull in weather data from all the sources we could think of. At sea we mainly use GRIB data which is the output of various computer models. We can pull in data from four different models. Normally we use just the GFS model via Saildocs and if we want a second opinion get the European model via MaxSea Timezero which provides free access to this otherwise paid service.

Most of the weather models are seeded with real world observations at 00:00 and 12:00 UTC each day and are available for download about five hours later. Some models such as GFS also run at 06:00 and 18:00 but my understanding is that those runs are not seeded with real world data but with the results of the previous runs and therefore can be less accurate.

All of that is rather academic as with the depression approaching none of the models agreed on what we could expect, or when, if we looked at the forecast more than about 12 hours hence. When we doubt the validity of the model forecasts we can we also try to take forecasts prepared by human hand. This normally means looking at the weather charts disseminated by various meteorological offices covering the region that we are in. In this area we can get weather charts via HF fax from Australia, New Zealand, Hawaii, and of course the French Polynesian meteorological service (www.meteo.pf).

Weather chart from Meteo Polynesia

Of course the French weather service provides most of its output in French so we have had to learn basic weather terminology in French and can now follow the forecasts on the radio and read the text forecasts online. The graphics on the French weather charts were also confusing to begin with all their variations on the fronts such as a “Pseudo-quasi-stationnaire” front.

Some of the weather models have certain characteristics; for example the GFS model is known to be fairly pessimistic with its longer range forecasting of winds around depressions. In this case GFS was forecasting greater than forty knots and the European model forecasting 30knots. The French meteo service was showing a peak of 50kmh (about 27knots) with gusts of 80kmh (about 43knots). In the higher latitudes of South America, the weather that we are used to, a GRIB forecast of 40knots would be accompanied by warnings of gusts in the 70knot range.

WindHighColour2 by .
One of the outputs from the GFS model

All in all it didn’t look too bad to us and as we were tucked in nicely between the town of Rikitea and some coral reefs we had no danger of suffering large waves no matter from where the wind blew. The downside was that if we did drag anchor we only had about 40m of room before we were on the beach or a reef. We decided to stay put and as a precaution, and something of a drill, decided to put out a second anchor to the NE where we expected the strongest winds to come from.

We prepared our kedge anchor, a 20kg French FOG model, on 15m of 10mm chain (which also weighs about 20kg), with 100m of 22mm nylon rope. The whole lot was lowered into the tender and we rowed out against the chop paying out the rode behind the dinghy. Once we got to the spot where we wanted to drop the anchor we payed the chain out over the stern of the dinghy and then launched the anchor. Well that was the plan, we got into position a couple of times before we managed to get everything to pay out smoothly and drifted back downwind while we sorted out the snags but eventually we got everything set. The only casualty was Paula’s favourite T-shirt that suffered a fatal rip.

As it was the French meteo service were spot on with their forecast as the highest steady wind that we experienced as around 27 knots with the highest gust that we recorded at just 32.8knots. Although out of the shelter of Mangareva we could see lots of white water. The drill was worthwhile as we now have a much better idea of the setup for setting the kedge with the dinghy, especially for Paula as it was her first experience of such an operation. Having the two anchors well set allowed us to sleep easily as the gusts swirled around the bay. When we recovered the second anchor we had to unwrap four turns of rope from the main chain so those swirling gust really did swirl us around.

Thread Fin Butterfly Fish
Thread Fin Butterfly Fish (Chaetodon Auriga)

Anyway all of that was last weekend. We are now off exploring the islands again and enjoying some snorkeling

2 Comments on “What will this tropical weather do?

  1. I am really happy to see that you have some good stuff in french on your website Chris !
    Thanks !!!
    Yoann .

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