How we are able to receive weather forecasts, even offshore.

Before Sophie and I learned how to sail, I was trained as a pilot. Part of my training involved 5 years at university studying weather forecasts, their underlining models and their limitations for the aviation industry. So in this post, I thought I'd share with you what I've learned and how.

One of the questions I get asked the most is “what do you use to get weather when you plan a passage and when you are underway?”

Weather is extremely important for any sailor, as it will affect every aspect of our life on board: when we leave port, where we choose to anchor, but also sometimes how we eat, sleep, move and enjoy life :)

How is a weather forecast made?

Modern-day forecasting is very good, but it can get quite complicated.  With millions of data points like temperature, humidity, winds, ocean temperatures, satellite data from all over the world are getting processed by giant computers in order to produce a forecast that can provide some sense of reality in the future. 

For example, the MET office (the national weather service provider in the UK) takes in over 215 billion weather observations from all over the world, every. single. day.

Time is not on our side however, these data points can take anywhere from a few minutes to half a day to be collected and sent to a forecasting center with the capabilities of doing something meaningful with it.  In practical terms, this means some of the information could be half a day old before it is even processed. 

Once it does get to a center the models can take anywhere between 6 to 12 hours to run, even with the most advanced supercomputers in the world it can take half a day to process all this information. 

A few years ago the MET office in the UK announced they would spend 100 million pounds on a new supercomputer capable of processing 14,000 TRILLION algorithmic operations a second!  This, on top of the multi-million-pound computer they already have plus the hundreds of scientists and meteorologists they have on staff: a person can now begin to understand the complexities involved in global weather modeling.

MET office Super Computer

With this in mind we must understand that if a forecast model is within 10%, 20% or even 30% of the actual conditions, forecasters will call this a success (and a sailor may if they are pushing their personal limits, call it a very bad day)

It's important to remember this when looking at a forecast.  These forecasts are generally good out to about three days, from there the quality and accuracy start to degrade rapidly. 

Remember this when planning your passage or assessing your personal minimums!


Who produces the weather forecast?

Most industrialized nations have their own metrological organizations to provide their citizens forecast within their national boundaries (the UK has the Met office, France has Meteo France, Sweden has SMHI, etc...

That said, very few nations have the financial resources to produce models, deploy worldwide sensors or support global forecast models.  In the aviation industry only two nations, the United States Noaa and the MET office in the UK, have the mandate to provide global aviation weather.  

Almost no private companies have the resources to produce global meteorological modeling, though a few are trying.  IBM (who owns ‘The Weather Company’, weather.com and wunderground.com) has been applying significant resources into this space since 2016.  Panasonic also tried going down this path, though with less success than IBM.

The reality is that almost all online weather apps or programs get their weather information from the same forecast models.  Primarily these models are:

  • the GFS (American)
  • the ECMWF (European)
  • the WRF (American)

The GFS and ECMWF are considered the best two models out there. 

There are also regional models that regional maritime use. The US Navy offers a model and the French "Meteo France" office offers a very good model around Europe (though I can't recall what they are called)


How is the weather forecast delivered?

One term you might hear a lot is GRIB file or GRIB forecast, and that is just the format in which weather forecasts can be delivered. GRIB is to weather forecast what ‘.doc’ is to a word document. 

Most meteorological organizations will produce their weather forecast in these formats in order to keep transfer times to a minimum.  That is because a lot of weather data user need it in remote places, where the only way to get it is via satellite link or SSB (wink wink!).

It is amazing how we can take all that data and make it usable and transferable over a basic SSB connection or Satellite link.  GRIB and GRIB 2 are the standards that are currently out there.

Basic view of a GRIB file - SailDocs

PredictWind: how we get weather forecasts onboard

Most weather applications such as Predict Wind, Windy or Weather 4D are just fancy GRIB viewers. 

They provide an easy to use tool for the user to visualize and interpolate the weather forecast but the majority of these applications or programs are pulling their data from the same sources listed above. 

Some of these programs will provide added features like route/departure planning, astronomical information tides, and streams.  A few even provide their own forecast models (such as Predict Wind, who provides the PWG and the PWE), though I’m not exactly sure how these are being processed, so I take them with a grain of salt.

While there are a lot of good apps out there onboard Polar Seal I prefer to use Predict Wind. 

Their app provides the user with four forecast options: The GFS, the EMCWF, the PWG and the PWE (their own models, as mentioned above)

Predict Wind Interface

Predict wind also has a few different visualization options like tables or visual maps which I find useful when digging deep in to forecast. 

They provide information such as real-time observations (good when trying to asses if a forecast is accurate or not), wave/swell forecast and air/Sea temperature. 

Predict wind also provides tools such as departure planning, weather route planning and GPS tracking (in connection with Iridium Go, more about that later).

The Departure planning and weather routing are two tools that have me addicted to Predict wind.  They help to make decisions as to when and where to go and I use them absolutely every day.

Predict Wind offers a free version that we've been using ever since we got the boat and until we started going seriously offshore. The free version of Predict Wind features everything a day sailor or short coastal cruiser would need:

Iridium Go via PredictWind: how we get weather forecasts offshore!

The other reason I tend to lean with Predict Wind is they are a dealer/supplier of Iridium Go!  If you are interested in how the Iridium Go via PredictWind works, I have written an in-depth article of how it works , as well as the kit and plans available here.

Before we departed on our first long offshore passage between Malta and Spain, Sophie and I purchased a Go! Through Predict Wind in order to communicate and receive weather information while our of 4G range.

While you don’t need to use PredictWind weather application if you do purchase the Go! through them, it is nice to use a service that can seamlessly provide a user with communication equipment and a weather tool/tracking tool… it keeps life easy for us!

Iridium Go! Mounted inside Polar Seal

So that is how we are able to receive our weather forecasts onboard Polar Seal, even while sailing offshore! What are you using yourself? Let us know in the comment section below!

If you are interested in acquiring the same solution we have, check out this other article that I wrote, about how Iridium works, what is available through Predict Wind and what to think about before you purchase your kit.

And if you have purchased your unit and are looking at how to set it up, check out my article about how to set up the Iridium Go via PredictWind on your own boat!

If you have a comment or question about this post or you think I missed something or got something wrong please leave it in the comment section below.

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How to set up the Iridium Go! with PredictWind