The six-day window I keep rehearsing: Tonga to Opua

The leg I've been studying this week is the one nobody on our route gets to skip: Tonga to New Zealand, roughly 1,050 nautical miles from Nuku'alofa to Opua, sailed sometime between late October and mid-November when the tropics stop being safe and the Tasman hasn't quite decided to be kind. It's the passage you plan around, not the one you plan.

The problem is arithmetic, and it's an unusual kind. On most ocean legs the passage is much longer than any forecast, so you accept that you'll get what you get after day four. Here the passage and the weather cycle are the same length. Fronts sweep across the north of New Zealand every five to seven days in spring. A 50-foot cat averaging 170 to 180 miles a day needs six days for the trip. So the entire game is leaving on the back of one front and getting in before the next one, and the forecast for the day you arrive is issued the morning you leave.

The strategy every source agrees on is to leave Tonga as a front clears and a high builds in behind it. The first two or three days are the easy part: southeast trades, then the wind lightening and backing as the high slides east. The question is what's arriving from the Tasman on day five. Ocean Posse's write-up mentions a spot cruisers call John's Corner, loosely 30°S 173°E, about 300 miles out from Opua, as the point where you look at the next front and decide whether to run straight for the Bay of Islands, slow down and let it pass north of you, or bear off toward Marsden Cove.

North Minerva Reef is the other lever. It's about 250 miles southwest of Tongatapu, a lagoon with no land in it, and stopping there splits the passage into a 250-mile hop and an 800-mile one. That 800-mile leg is what a cat can actually do inside a single window, five days at our speed, and you sit in the lagoon watching GRIBs until the pattern lines up. The tradeoff is that Minerva is still inside the box the cyclone season is about to close on, and you can lose a week waiting.

What I want the GRIB to show before we leave, whether from Tonga or Minerva, is three things. First, a front cleared the track within the previous 24 hours and the high behind it is centered at or above 1,020 mb and moving east at a normal pace, not stalled. Second, nothing below 1,000 mb forecast in the Tasman between 30°S and 40°S out to day six on both GFS and ECMWF, and the two models agree on the timing of the next front to within about a day. Third, a wind angle at John's Corner on the arrival day that's forward of the beam but not on the nose, because a northeast front arriving as we close the coast is the failure mode I've read about most. If the models disagree on the day-six front by more than 24 hours, we don't go. That's the rule I'll write down before we leave, because I know how I'll feel after ten days on a mooring in Tonga with the kids asking when we're leaving.

The number that surprised me in all this is 30°S. Most yacht insurers I've looked at want the boat south of that latitude, or out of the named-storm zone entirely, by a date around November 1. Tongatapu is 21°S. So the passage isn't just weather; it's a contract date with a ten-degree gap in it, and the window doesn't care about the date. The honest plan for a family boat is to be ready to leave Tonga October 20, with the understanding that the boat may sit in Minerva through the first week of November while the policy paperwork gets a phone call. That's a cost I hadn't put in the worksheet until this week: not a dollar line, but a week of schedule slack that, on this leg, isn't optional.