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Boating Weather for the Long Haul

A cruising couple downloads and analyzes GRIB files to find the best route across the South Pacific.
Boating Safety

Anchored at Mangareva, Îles Gambier
September 21 23°07.9’S, 134°57.3’W

>> Synopsis: A large area of very light to no wind surrounds a weak high-pressure system (with a center pressure of 1017 millibars) over the Îles Gambier. (See 09/22 1800 (12 hours).) this persists until September 24 with the approach of a weak low-pressure system (center pressure 1017 millibars) moving southeast over the top of a strong high-pressure system (center pressure 1035 millibars) well to the south of the island. (See 09/24 1800 (60 hours).) These two systems form an extensive easterly flow as far north as 27 degrees S, less than 250 miles due south of the island. The high (center pressure 1037 millibars) continues to move east as the deepening low (center pressure 1007 millibars) pushes south, creating a squash zone between the two with easterly winds of at least 30 knots south of the island and a large band of moderate to strong north-to-northeast winds to the east of the island. (See 09/26 0600 (96 hours).) As the high (center pressure 1036 millibars) moves off to the east, the low (center pressure 1013) starts to fill, and another weak high (center pressure 1018 millibars) approaches the Gambiers. (See 09/27 0600 (120 hours).) the northeasterly flow persists to the northwest of the more southerly high-pressure system until the end of the forecast period, when the high is predicted to be centered around 46 degrees S, 103 degrees W.

>> Analysis: Except for strong southeast trades, this is the only other pattern we’ve been seeing for the past two weeks, and we can’t expect anything different to develop in the immediate future. Because the low passes to the west of the island moving from northwest to southeast and the strongest winds develop in the squash zone between the low and the high to the south of it, the system won’t bring us any westerly winds, and no favorable weather pattern develops for the entirety of the forecast period. Leaving the island in the light winds of the high and heading south would mean sailing directly into the strong easterly flow north of the high-pressure system in less than two days. If the low changes direction or strengthens even slightly, we could easily end up with gale-force east winds. While there are no more easterly winds below 25 degrees S in the forecast period, given the prevalence of easterly winds as far north as 30 degrees S over the past two weeks, we don’t want to run due south in the northerly flow trying to get to the westerlies on the back side of the high-pressure system and risk ending up in the path of another easterly gale.

>> Strategy: Rather than wait for the next weather pattern to develop in another 10 days, we decide to take advantage of the calm period to motor 200 miles due east of the island before the northerly winds from the approaching low fill in. If we can get east of 131 degrees W by the afternoon of September 24, we believe that we’ll be in a lighter, 15- to 20-knot band of northeast winds as the low moves from north to south. We could then turn southeast and sail on a close reach in the northerly flow from the high-pressure system through the end of the forecast period without going too far south until we see what winds the next set of systems are supposed to bring.

>> Actual: We’re able to sail for the first 12 hours, after which the wind drops to 2 to 3 knots. From there, we motor due east, reaching 23 degrees S, 131 degrees W on the afternoon of September 23. At that point, we turn off the engine, sheet in the main, lock the wheel, and wait for the northeast wind to fill in. By the early hours of September 24, we’re sailing close-hauled in light winds, and we spend much of the next four days sailing along at 40 to 50 degrees apparent, with 20 to 25 knots of apparent wind on a course of approximately 110 degrees magnetic, before the wind shifts to the north.

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