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The dark science of weather forecasts

When you live on a boat it’s very important for your safety to constantly monitor the weather.  For this reason, the first task of the day for us is checking the weather forecasts.

On Oroboro we developed a routine that so far it’s working very well.

Synoptic charts

If we have cellular coverage, we download the synoptic chart from the Brazilian Navy:

Synoptic chart from the Brazilian Navy

GRIB files

After checking the synoptic chart, we check the weather models GFS and ECMWF by downloading GRIB files.  These files are special binary format files and are highly compressed, so they are ideal for downloading.  To download them when we don’t have cellular coverage, we use my satellite IridiumGo! device, and to view them we use a software called PredictWind.

At this particular latitude and in this particular time of the year (winter in Brazil), you have regular cold fronts coming up from the south that you need to be aware of.  Sometimes you can also have tropical depression.  Below you can see GFS and ECMWF models side by side.

PredictWind screen

GFS (Global Forecast System) is the American model.  WCMWF (European Center for Medium-Range Weather Forecasts) is the European model.

Which one of these models is more reliable is debatable.  Americans swear by their model, so do Europeans.

Since I am both European and American, I tend to use the best of the two worlds:  So we compare them side by side to see the difference.

GMDSS bulletin

Once that is done, we check the GMDSS bulletin (Global Maritime Distress and Safety System), that we download together with the GRIB files using the satellite connection:

Global Maritime Distress and Safety System

This bulletin contains important warnings and it’s easier to read after you check the GRIB files.


Another important thing to be aware of – especially if you are planning a landfall – are tides.  For example, because most of the anchorages in this part of the world are behind a coral reef or in the mouth of a river, it’s important to time your arrival with a rising tide.  You don’t want to run aground!

But also when you are safely at anchor, knowing the tides it’s important.  For example, you may go on shore at high tide and leave your dinghy on the river’s bank.  Then when you come back to the shore it’s low tide and now you need to push your dinghy a long way to the water, often through sticky mud…


Currents can be very strong.  If you’ve ever sailed in the San Francisco Bay under the Golden Gate bridge, I’m sure you are aware of it.  Here in Brazil if you’re planning to navigate up or down a river, you may have to fight against 3 or 4 knots of current during flooding or ebbing.  But also when planning a passage like the one that we’ll need to do at some point in the future to reach the Caribbean, knowing the currents is paramount.  For example,  when we’ll start sailing west from the easternmost tip of Brazil, we’re going to get up to 75 nautical miles a day for free, thanks to the strong equatorial currents that can reach up to 3 knots.  This is a lot!

Equatorial currents


One of the easiest ways to predict weather is to look directly at the sky. There are many different types of clouds and each different kind of cloud means different types of weather:

Type of clouds


Out of necessity, during the course of the centuries sailors came up with sayings or proverbs such as, “Distant shores loom up nearer before rain because of thinning of the air”.   Many of these sayings have proven mostly untrue over the years, but there is one that often proves to be accurate: “Red sky at night, sailor’s delight. Red sky in the morning, sailor’s warning”.

Accuracy of weather forecasts

Weather models are what they are: just models.  Only you can know for sure what’s the weather like in the exact location where you are.

Especially when navigating close to the coast, there are so many different factors that can affect weather.  For example, winds and currents near capes can often be stronger.  Narrow passages between two islands can create a venturi effect with the wind accelerating dramatically. Wind against the current can create big waves. And so on…  So don’t forget to keep your eyes open and take weather forecast with a pinch of salt!

If you think I forgot something, or if you have suggestions, please leave a comment below!

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