Pilot Charts, GRIB Files, and Wind Patterns: Understanding weather reports, patterns, and forecasts makes cruising safer and happier.
Studying the weather forecast is such a part of the morning ritual on our boat that we automatically associate the smell of coffee with weather reports and surface analysis charts. No matter where we are – near a city with Wi-Fi and fast downloads, anchored in a remote corner of the globe with text bulletins, or on the way with the Pactor modem screaming and rumbling for half a time to get a GRIB file — time and morning coffee go hand in hand.
As sailors, we live much closer to the elements than most people. Keeping an eye on the forecast is imperative for the safety of the boat and the well-being of the crew. It is also a fun and interesting hobby.
Take your cues. When you start exploring a sailing area, the local weather can seem indecipherable. Get a good introduction to the prevailing systems by reading a few regional weather articles. What is the rainy season? Is there a hurricane season?
The guides provide a useful, albeit simplified, overview. For example, guides for French Polynesia, where we sailed on our 1988 Sparkman & Stephens 41, Pitoufa, over the past eight seasons, describe the region as having two distinct seasons: a dry and windy season between May and October, and a hurricane season from November to April with generally rainy and warm weather. This is true for Tahiti and the Society Islands, but the Marquesas are generally dry with two short rainy seasons in spring and autumn, while the Gambier and the Austral Islands have suitable seasons with hot summers and cool winters. If you do not study this kind of information beforehand, you will have some surprises.
Driver boards. Also take a look at the driver boards. What is the strength and direction of the prevailing winds? How often do passing low-pressure systems interrupt them? Get an overview, then choose a few reliable weather sources from the jumble of information available. Check them daily.
Look at the big picture. When we have a good internet connection, we download wind forecasts for a large area to observe distant systems behind local winds. This technique makes it possible, for example, to monitor the passage of troughs that interrupt the trade winds, or to locate an acceleration zone at the top of an anticyclone.
We like to compare the US model of the Global Forecast System with that of the European Center for Medium-Range Weather Forecasts. This way we can see how they differ. We also like to check the models after the fact and keep track of which one was the most accurate.
Surface analysis maps allow us to visually interpret isobars, and they transform features such as fronts, troughs, and depressions into understandable images.
Some cruisers crop too small a frame when forecasting and upload data to an immediate area without seeing the big picture. They are often flabbergasted (and damn the stupid forecast!) if the wind is blowing the opposite direction. On the other hand, if you download a larger GRIB file, you might see a convergence zone with northwesterly winds on one side and southeasterly winds on the other. It’s important to understand the big picture, as minor inaccuracies in the forecast can lead to a sticky situation that leaves you pitching and rolling on a downwind bank.
Looking at the big picture also means you can prepare for multiple scenarios and have alternative anchors in mind. We like to explore sailing areas thoroughly and have our own GPS tracks to follow to a safe anchorage should the wind change – a particularly useful tool if a move needs to be made during a gust or in the middle of the night .
Get an idea of the weather conditions. During a prolonged stay in a protected anchorage, it is interesting to watch the forecast every day. What is typical of the season and the region? Do forecasts tend to overestimate or underestimate meteorological characteristics? How do systems evolve? What can you expect from wind shifts in certain directions, sunny skies or squalls? If you know your weather, you can take advantage of fair weather sailing downwind or use the wind shifts generated by passing troughs to make miles against the prevailing trades.
Wait for the windows to be appropriate. Losing patience and navigating a sub-optimal weather window is tempting, but it often leads to frustration. Whipping sails in fickle winds, or too much wind in the wrong direction, is worse than waiting.
Once you’ve found your window, make sure you have a reliable source for current forecasts. Whether you’re using an SSB or an Iridium, forecast monitoring means you can change course if necessary or head to another destination.
Every year we sail east from Tahiti to the Tuamotus in September or October, and every year neighbors ask us which island we are going to. The answer is always the same: “Where the wind will allow us to go. Starting with winds from the north quadrant, we go for kilometers towards the east and try to go as far as possible. If the wind returns to the east sooner than expected, or has less of a northerly component than expected, we have half a dozen atolls to choose from.
Keep it up. Keep watching the forecast even at anchor and be ready to move. What looks like a benign calm in the anchorage could, in fact, be an approaching squall-laden system. Being prepared with information is better than trying to navigate unpleasant conditions on the wrong side of an atoll. The fetch can rise quickly and the waves can reach surprising heights. We were recently at anchor on the east side of Maupihaa in the Society Islands when we noticed that the GRIB files showed a significant wind shift in the following two days. These changes are often accompanied by squall fronts, so we warned our neighbors of the anchorage and set out to explore other places on the atoll that might be safer in the winds. When the front arrived with 30 knots from the northwest, we were comfortably anchored behind a beautiful motu on the northwest side. Sadly, the boats that decided to exit the team on the east side had ‘the worst night of their cruising life’, according to the skipper of a 50ft catamaran, even though they were anchored just 2 miles away from us on the other side of the small lagoon.
Birgit Hackl and her companion, Christian Feldbauer, have been criss-crossing French Polynesia for eight seasons. For more information, visit their blog.
Check charts for patterns
Cross pilot charts, local guides and anchor guides can offer a good overview and overview of an area’s weather conditions.
View our interactive global wind atlas based on satellite data at pitufa.at/oceanwinds.
The climate in the South Pacific is dominated by two major highs – the Kermadec High and the Easter Island High – with the South Pacific Convergence Zone in between. The trade winds here are not as stable as the Atlantic and Caribbean trade winds, and can be frequently interrupted by troughs that move along the convergence zone. Every two weeks a passing trough lets the winds turn, which is annoying when passing west but handy when hopping east along the islands.
We use weather reports and surface analysis from Météo France, the surface analysis and cyclone warning site from NOAA, and Bob McDavitt’s weekly summaries from MetBob. All these sites are integrated here: pitufa.at/weather-fp.
For Fiji, check the weather forecast on the official government website (met.gov.fj), which includes marine forecasts.
These sources provide a general overview:
Pacific Crossing Guide (Adlard Coles Nautical); South Pacific anchorages (Imray); cruising world, July 2019, “Pacific Weather Routing”; and cruising worldJuly 2019, “Planning for the Pacific Passage.”
We use an SSB radio in combination with a Pactor modem to download current or remote forecasts. SSB propagation is limited to certain times of the day, so some cruisers prefer a satellite phone. Whatever medium you use, saildocs.com provides an excellent free service for downloading the forecasts, using small files to record the data. For French Polynesia, visit pitufa.at/weather-fp.