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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

September 28, 29°53.8’S, 121°22.8’W

>> Synopsis: As the high-pressure system (center pressure 1029 millibars) moves farther north and east, the northerly winds will drop to 10 knots in the next 24 hours. (See 09/30 0600 (24 hours).) A low-pressure system (center pressure 1015 millibars) will approach from the west. this will bring a nice band of moderate northwest winds along 30 degrees S, with gale-force winds between 30 degrees and 35 degrees S over the next 36 hours. (See 10/01 0600 (48 hours).) After that, a high-pressure system (center pressure 1023 millibars) is forecast to bring light and variable winds to the entire area between 30 degrees and 40 degrees S, followed by a weak low-pressure system (center pressure 1014) that continues the westerly flow around 30 degrees S. (See 10/02 1800 (84 hours).) The forecast period ends with a confusion of weak systems and the possibility of winds from just about any direction. (See 10/04 0600 (120 hours).)

>> Analysis: While the GRIBs have been consistent in their forecasts for September 28 through October 3, with two relatively strong and well-defined systems bringing a good band of westerly wind to the area centered around 30 degrees S, the remainder of the forecast has been changing daily for the past five days. The current version shows a large area of light and variable winds starting late on October 1, but most of the GRIBs over the past few days have shown a narrow band of westerly winds of 10 to 15 knots continuing along 30 degrees S. this forecast ends with what we call a “mushy” period, with many small, weak systems, any one of which could strengthen, displacing the rest. In this situation, we only really trust the first 48 hours or so of the forecast. Continuing southeast, along our current course, will put us right in the center of the next high-pressure system by October 2.

>> Strategy: We decide to turn due east along latitude 30 degrees S when the winds shift to the west, at least for the next 48 hours. We hope to stay in the band of 10- to 25-knot westerly winds along the top edge of the approaching low-pressure system. This should keep us out of the gale-force winds near the center of the low and north of the light winds at the center of the high pressure behind the low. Given the mushiness of the forecast at the end of the period, we’ll re-evaluate this strategy with each new forecast.

>> Actual: Over the course of the next six days, we stay within 30 miles of either side of 30 degrees S and manage to hold on to the westerly wind. We sail dead downwind in winds ranging from 8 to 25 knots true and motor during two periods for a total of about six hours when the wind falls below 5 knots true.

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