Saturday, May 15, 2010

Which Way is the Wind Going to Blow?

Or... we finally meet "The Beast"

Our Weather Right Now

Next time all you East Coasters are sitting waiting for a cold front to pass, thank your lucky stars for the millions of dollars that our government puts into the NOAA weather service. Between GRIB files, scatterometers, and high resolution IR satellite photos, all available at your fingertips, and NOAA Weather Radio, you have everything you need to figure out what the weather is doing..

Out here in the paradise of French Polynesia, we are struggling to get believable weather forecasts. The French forecast only gives 'today and tomorrow', in French, and it is similar to the Bahamian weather forecasts 'Wind SE to NE at 10-20 knots with higher gusts in squalls'. And 'tomorrow' is really 'this afternoon' by the time we get the forecast in the morning.

There are some products from the Hawaii NOAA Office and the Fiji Met Office. There are almost no satellite photos available, even if I had the internet to get them. And our O-Holy-GRIB files, which were uncannily accurate when we were down near Easter and Pitcairn, and have generally been pretty good, are just flat wrong right now.

The problem is, there is a very complex weather picture, and the GFS computer program that generates the GRIB files doesn't deal with it well. What's buggering up our weather here is called the South Pacific Convergence Zone, sometimes called "The Beast" by knowledgeable cruisers. A convergence zone is a quasi-stationary area of very unstable warm moist air. What happens in this area is that cold fronts from New Zealand, moving east and bringing cold unstable air, stall out on top of a Convergence Zone, and then you mix in 2 strong high pressure areas side-by-side down south, with a trough in between, and the weather picture is VERY unstable and very unclear.

We are sitting in an atoll that is about 10 miles across and about 20 miles long. It is rimmed by coral and sand motus, but seems to be very deep in the middle. As far as we know, there is no protection in the interior of the atoll, but reasonable protection along the coral rim. There is no place at this atoll (Tahanea) to get 360 degree protection, and the next atoll that *might* have better protection is over 50 miles away over open ocean.

As this weather approached us, the GRIB files were forecasting almost a 180 degree wind switch, but not very strong, with a period of light and variable weather in between. We were watching it approach pretty carefully, and thought we were on top of the situation.

Well, 2 nights ago, the forecast was for the wind to clock from ENE-N-NNW through the night. So we picked a spot accordingly. As it approached and the wind started to go NE, we snuggled up to the coral rim in a place giving us good protection from ESE - N - NNW. Well, the wind went S-SE instead, and we were completely exposed across miles of 'fetch' on a lee shore, with our backs to the coral reef.

To complicate matters, there is no good sand anchorage here. Anchorages either have sand and lots of coral heads or sand and not so many coral heads. So it totally complicates the situation when a wind switch is expected. There is just no way to keep your chain from wrapping in the heads if you swing much, eliminating your 'scope', and making re-anchoring in the middle of a blow almost impossible. This adds a whole 'nother dimension of 'fun' to the bad weather equation. (See our next blog post 'Anchoring in Coral').

Anyway, the wind switched, putting us on a lee shore, with 15 miles of 'fetch' in 20-30 kts winds. We eventually had breaking waves over the bow due to the wind-driven waves. Even though our anchor was well set, we still broke loose in the wild conditions and dragged about 100' toward shore. Fortunately we finally stopped and held for the rest of the night.

Yesterday, we re-positioned to a better spot close by, and the wind went back to a more easterly direction, in which we had better protection. We and the 2 other cruising boats here, and 3 more cruising boats scattered among atolls within a few hundred miles of us, have been anxiously gathering weather, trying to figure out where the next bad blow will come from. The GRIBS are still not showing the weather we are seeing, so we do not trust their prediction.

Because we have explored the atoll some in good weather, and laid down GPS 'tracks' we can follow back to other anchorages, we do feel we can move around if need be. We just can't figure out which way to go. The GRIBS are showing that our wind now should have been clocking around through the north, when we actually experienced very strong southerly winds. 36 hours later, they are still predicting moderate northerly winds and we've mostly had strong SE winds. Do we head for the southern rim of the atoll to protect us from SE-SW winds, or the northern rim to protect us from NE-NW?? We are still pondering that question.

We did have a few days of fun here in Tahanea, in gorgeous weather, and I will blog about them later... but for now, off to go collect some more weather (and share weather data via radio with other boats).
At 5/14/2010 7:44 PM (utc) our position was 16°51.06'S 144°41.55'W

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