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Coconut Milk Run weather

Apr 13, 2012

Resources for collecting weather data when crossing the Pacific

A voyaging boat gets into a South Pacific lagoon and anchors ahead of bad weather.

A voyaging boat gets into a South Pacific lagoon and anchors ahead of bad weather.

Jeff Williams

Crossing the Pacific Ocean even on the straightest path is a long voyage. You can visit a dozen countries and travel more than 5,000 miles. Getting weather information is challenging in part because of the great diversity of communities that you pass through on your trip.

The so-called Pacific Coconut Milk Run generally starts at the Marquesas in eastern French Polynesia. The majority of voyagers arrive either from the U.S. West Coast directly or from Panama/Galápagos after transiting the Panama Canal. This 3,000-mile leg may be the longest in a west-about circumnavigation and will generally take around three weeks to complete.

Following an 800-mile run from the Marquesas, the Milk Run continues through French Polynesia touching at some of the Tuamotu Islands and the dramatically high and rugged Society Islands. The Tuamotus are only a day sail from each other, and most of the Societies are the same. The run from the Tuamotus to the Societies is only about 200 miles.

Leaving French Polynesia, the Cook Islands are next — perhaps the most widely spread and sparsely extant country in the world. Voyagers visiting the Northern Cooks often then call at Samoa before continuing to Tonga; those touching at the Southern Cooks may stop at Beveridge Reef and the unique island nation of Niue instead. A typical leg in the Cooks is 800 to 1,000 miles; from Samoa to Tonga, or Niue to Tonga, is little more than 200 miles.

Tonga, stretching more than 300 miles north to south, offers two excellent cruising grounds: Vava’u in the north and Ha’apai in central Tonga. About 250 miles west from Tonga are the myriad islands of Fiji with incredible cruising clustered into socially distinct groups.

Like any ocean area, the Pacific has its share of unsettled weather. Getting good weather information can make a Pacific crossing easier.

Jeff Williams

Five hundred miles beyond Fiji is the fascinating country of Vanuatu with its unique heritage of combined French and English colonial rule. Here you walk from village to village, alternating French and English as you go.

The Milk Run concludes 250 miles southwest of Vanuatu at the French territory of New Caledonia. Alternatively, the lightly-cruised Solomon Islands and the islands of Papua New Guinea are a slight diversion to the north.

Truthfully, it shouldn’t all be done in one season. People veer out of the cyclone belt each Austral summer, heading south to New Zealand to refit, refuel, and relax for a couple months before returning to the tropics to continue for another season. Many cruisers make New Zealand their home-away-from-home, basing here each summer and cruising each winter.

There are many resources available for route planning. My favorite two have been Jimmy Cornell’s World Cruising Routes and the RCC Pilotage Foundation’s Pacific Crossing Guide. Both offer insights into the weather and sea conditions prevalent in the various parts of the South Pacific, information essential to planning the overall strategy for each cruising season.

Milk Run weather patterns
The bulk of the Milk Run is under the influence of the southeast trade winds. While the wind near the Galápagos Islands can be light and variable, once you reach about two degrees south latitude, the trade winds begin to be felt. You can choose the strength of the wind you want by picking the latitude that suits you best. This past trip, we did most of our westing in about 4° south enjoying 15- to 20-knot winds and about one knot current boost for almost two weeks.

Once in the Marquesas, the trade winds are much steadier. They are occasionally perturbed by cold fronts passing south of the region, but they can generally be depended upon throughout French Polynesia. Cold fronts are significant features that are well forecast; you can choose safe anchorages if you see a wind shift coming in the next day or two. Strong cold fronts can bring westerly winds, so be cautious since most popular anchorages are exposed to the west.

Even absent a cold front, there are often showers around this area. During a passage, as an averse passes you, expect the wind to pipe up in front of the rain shower and then die away once it’s gone. We frequently had showers bringing short periods of 30 to 32 knots with higher gusts.

Farther west, the trades continue but the South Pacific Convergence Zone (SPCZ) becomes a major player in the daily weather as well. The SPCZ can extend from the Solomon Islands all the way to the Marquesas at times, though it is generally most active from the Cooks and west. When the SPCZ is active, it brings very unsettled, squally weather with heavy rains and sometimes violent lightning cells. Although several weather products identify the current location of the SPCZ, none give a day-to-day forecast of its movements.

When the SPCZ is active over your area, it’s a good opportunity to fill the water tanks and read a good book. If you encounter it during a passage, expect shifty winds and the potential for lightning.

To understand the general weather patterns of the South Pacific, I strongly recommend Mariners Met Pack: South West Pacific compiled by Bob McDavitt and published by MetService New Zealand. This book is invaluable for understanding the unique weather features of the South Pacific.

Big picture weather
There are several excellent resources for big picture views of the South Pacific. The most unique is McDavitt’s weekly Weathergram issued by MetService New Zealand. Sources of more conventional daily analyses and forecasts are the National Oceanic and Atmospheric Administration (NOAA), MetService New Zealand, Fiji Meteorological Service, and GRIB files.

Many weather products are available by e-mail. Users of Winlink, SailMail, and MailASail services can avail themselves of the relevant catalogs of products either through AirMail or directly by e-mail to the service provider. Be careful, however, when using catalogs — during my research I found dysfunctional entries in all three systems. The worst cases are those that return a seemingly good weather product that just happens to be out of date by a few years.
 

This weather chart produced by the Fiji Meteorological  Service provides a localized view of the waters around Fiji, including a closer look at Tropical Storm Vania passing over New Caledonia. Mariners Met Pack: South West Pacific by Bob McDavitt is a good book for learning South Pacific weather.

The closer you are to the source of a weather product, the less likely errors are to creep into the delivery. The National Weather Service FTPmail facility provides access to all relevant NOAA products, but it is more complicated to use. Saildocs can directly access the needed NOAA websites. Queries to the NOAA website are case sensitive, e.g. “TIF” not “tif.”

1) Bob McDavitt’s weekly Weathergram: McDavitt is the South Pacific weather guru from MetService New Zealand in Auckland. Each Sunday evening (Saturday evening east of the dateline), he sends out his weekly Weathergram — a summary of his thoughts on the development of weather conditions for the South Pacific for the coming week. He discusses tropical features — including the SPCZ location and activity — and the movement of the controlling highs and cold fronts south of the region. He includes commentary on good times for passagemaking for the longer legs (Galápagos-Marquesas or tropics-New Zealand, for example) and discusses New Zealand regional marine weather as well.

Weathergrams are distributed free of charge and are best received by e-mail subscription from the server at Pangolin (www.pangolin.co.nz). This service sends the Weathergram to subscribers as soon as each new issue is available. McDavitt also has weather routing services available. E-mail bob.mcdavitt@metservice.com.

2) NOAA: NOAA divides its South Pacific forecast products into two areas: the eastern region from South America to 120° W and the western region from 120° W to 160° E. Basically, you transition from the eastern region to the western region as you make landfall in the Marquesas.

The High Seas Forecasts are text products that describe current conditions and provide 24- and 48-hour forecasts. Since these products are aimed at users on the open ocean, only areas of wind greater than 20 knots or seas greater than eight feet merit notice. The position and size of highs, lows, and fronts are described. Any areas of strong convection are also noted.

High Seas Forecasts for the Pacific including FZPN03 and FZPS40 are broadcast by U.S. Coast Guard station NMO in Honolulu at 0600 UTC and 1200 UTC on 6501 kHz and 8764 kHz and at 0005 UTC and 1800 UTC on 8764 kHz and 13089 kHz.

Weatherfax charts covering the same regions are also produced by NOAA. The suite of charts includes a 72-hour forecast, one of the few available in the region. I prefer charts because of the wealth of information available there: subtle bends in isobars flag areas of stronger winds or the development of new features before they’re ever mentioned in a text forecast, for example. All NOAA charts are available for email download from the NOAA website. Unless you have access to cheap bandwidth, radio weatherfax is the best way to get these products.

Suites of eastern South Pacific charts are broadcast by U.S. Coast Guard station NMC (Pt. Reyes, Calif.) starting at 0215 Z, 0945 Z, 1435 Z, and 2150 Z on 8682 kHz, 12786 kHz, and 17151.2 kHz. NMC can be picked up from near the Galápagos and through eastern French Polynesia.

A thorough suite of charts is broadcast by Department of Defense station KVM70 (Honolulu, Hawaii) starting at 0701 Z on 9982.5 kHz and 11090 kHz and at 1901 Z on 11090 kHz and 16135 kHz. The Hawaiian broadcasts can be reliably received from French Polynesia to Fiji.

3) Fiji Meteorological Service: The Fiji Meteorological Service produces daily weather bulletins for the islands area from French Polynesia to New Caledonia. These bulletins provide current warnings and the position and intensity of all fronts, convergence zones, and troughs with more detail than the NOAA products. The bulletins do not provide forecast information; they are a statement of current conditions and need to be monitored daily.

Fiji Met Service also produces a daily surface analysis, distributing it in two formats: a weather chart and Fleet Code. A text product, Fleet Code contains a long sequence of numbers that encode the weather data. A viewer program, such as Pangolin’s PhysPlot is needed to interpret the message and render a viewable weather chart. PhysPlot is available free from Pangolin.

4) MetService New Zealand: The MetService New Zealand produces a number of text and graphical forecast products that are useful in the tropics and for transiting between the tropics and New Zealand.

Forecast area ‘subtropic’ covers the ocean area between the tropics and New Zealand’s North Island. Taupo Maritime Radio (ZLM) broadcasts the warnings, synopsis, and forecast for area Subtropic daily at 0803 UTC and 2003 UTC on 6224 kHz and 12356 kHz and at 0833 UTC and 2033 UTC on 8297 kHz and 16531 kHz.

MetService New Zealand also issues graphic products covering the southwest Pacific and broadcasts them as weatherfaxes on ZKLF. These include a sea level analysis and a suite of three forecasts: H+30, H+48, and H+72 (another source for a three-day prognosis!). These weatherfaxes are excellent information sources since they cover the southern ocean areas and show the various weather features that are driving the changes in the tropics. You can reliably receive these faxes from Niue to New Caledonia.

5) GRIB files: GRIB files are available to anyone with e-mail or Internet access. Users need to be aware of limitations in the use of GRIB files, especially in the tropics. First, GRIB files contain the output data from computer model runs; there’s no human interpretation in the content and no collation of data from different sources. Second, the GFS model which is the source of GRIB data does not do a good job of predicting wind speeds in areas of low pressure gradient, which are often found in the tropics; areas showing five to seven knots are often filled with pockets of weather packing squalls and gale-force winds. Third, the GFS model does not do a faithful job of representing small tropical systems since the coarse resolution of the model misses small features.

You often hear people lamenting GRIB forecasts. It’s not the model that’s usually the problem, it’s the application by the user.

GRIB files can be accessed by radio e-mail systems using a map-driven request from AirMail or through the MailASail request server.

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Jeff and Raine Williams circumnavigated on their J/40, Gryphon, from 1998 to 2004. They’ve once again sailed off and are presently in New Zealand.

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