Using PredictWind

We have had a standard subscription with PredictWind since we started our long-distance sailing. You need such a subscription to be able to download GRIB files via IridiumGo. You also get a tracking page and are allowed to download route suggestions. In this article, I write about how we used PredictWind and what we learned from it.

Using an iPad and IridiumGo, we downloaded GRIB files from PredictWind two times every day. I think it took an unnecessarily long time since the download stopped after 20-40 kB and then it took about 5 minutes before it started again. This happened even though IridiumGo reported full signal strength all the time.

Every morning I downloaded a route suggestion as well and a more detailed forecast for a smaller area around where we would be the next 3-4 days. In the evening, I fetched a new detailed forecast for the local area and a less detailed forecast for a larger area for the coming 7 days. It would have been good to be able to show the detailed forecast even after I had downloaded the overview forecast.

GRIB-files PredictWind

PredictWind delivers the weather forecast for GRIB offshore in several different GRIB files. There is one set of files with a 5 km detail level and another for 10 km. There is also an even better resolution of GRIB files in some areas, but it is not practically possible to download these with IridiumGo. Within the 5- and 10 km groups are the following files. The logical file name also contains information about the resolution.

  • GFS Global .. km
  • ECMWF Global .. km
  • PWG Global .. km
  • PWE Global .. km
  • ECMWF Global Wave .. km
  • PWG Global Wave .. km
  • PWE Global Wave .. km

Depending on the weather model or models and weather information you choose to download, you will download files according to the table below. You can see that the GFS model has the same information about waves as the PWG model, while the PWE model has its own GRIB file for waves.

WindGFS Global ECMWF Global PWG Global PWE Global
RainGFS Global ECMWF Global PWG Global PWE Global
CloudGFS Global ECMWF Global PWG Global PWE Global
TempGFS Global ECMWF Global PWG Global PWE Global
PressGFS Global ECMWF Global PWG Global PWE Global
CAPEGFS Global ECMWF Global GFS Global ECMWF Global
GUSTGFS Global ECMWF Global GFS Global ECMWF Global
WavePWG global Wave ECMWF Global Wave PWG global Wave PWE global Wave
GRIB files from PredictWind

Actual file names on a PC

If using the PredictWind Offshore app on a PC, you can choose where the downloaded files are stored. In that way, you can use another GRIB-viewer with a better presentation of weather data. If you open that folder, you will see all GRIB-files that have been downloaded. When using an IPad, this setting is not available.

Changing download location

The filename contains information about the weather model, area, data, and so on. Below is 2 examples of such file names.

  • Model: GFS
  • Data: Wind, Rain, CAPE, Temperature, Pressure, Cloud, Gust
  • Resolution: 100 km
  • Length: 10 days
  • Interval: 12 hours
  • Area: 61N to 6S, 16E to 84W
  • Date when the data was calculated: 2020-05-29
  • Time when the data was calculated: 01:15 UTC
  • Model: PWG
  • Data: Wave
  • Resolution: 100 km
  • Length: 10 days
  • Interval: 12 hours
  • Area: 61N to 6S, 16E to 84W
  • Date when the data was calculated: 2020-05-29
  • Time when the data was calculated: 00:55 UTC

GRIB-File sizes

How much information will there be from the various choices you make before downloading GRIB files with PredictWind?

How much data there will be in the files, depends on how large the area is, what the resolution is, how often, how long and what different weather data you choose. The file size also depends on how much the weather information varies from one forecast point to another. This means that if no precipitation is expected within the selected area and the selected time, then the amount of rain data will be almost zero.

In the tables below you can see approximately how much data there will be for the different choices that can be made if you fetch an area that is 5 x 5 degrees large. If you increase the area to 10 x 10 degrees, there will be about 4 times as much data.

Data/Grid, Time5 km, 24 h5 km, 12 h5 km, 6 h5 km, 3 h
Wind0.85 1.9 4.28.7
Rain0.450.9 2.04.2
Temp0.5 1.0 2.24.6
Press0.8 1.6 3.57.4
CAPE0.8 1.6 3.57.4
Gust0.8 1.6 3.57.4
Wave1.4 2.3 5.110.6
Size of 5 km GRIB-data in kB for 1 day by different intervals
Data/Grid, Time10 km, 24 h10 km, 12 h10 km, 6 h10 km, 3 h
Size of 10 km GRIB-data in kB for 1 day by different intervals

Weather routing

PredictWind Standard offers the service to get a suggested route with regard to wind and waves.

Before you can fetch route suggestions, you need to enter the boat’s polar diagram. It is a diagram that shows the boat’s maximum speed in different wind strengths on all bows with different sails. This is theoretically calculated data and PredictWind has diagrams for a lot of boat models. If your boat model is missing, you can upload your own polar diagram.

During the crossing, I fetched a new route proposal starting in the current position and to a position approximately 450 M away, in the direction of the final destination. Since you do not do this in a navigation program, some manual work is required to retrieve coordinates for the endpoint, and once you have received a suggested route, put it in the chart plotter as a route to sail after.

Before downloading a route proposal, you must decide how fast the boat is sailing in relation to the polar diagram. I used 80% of the polar chart’s performance and that was true for about as long as we sailed in a little harder wind. At lighter winds, we sailed 65% of maximum performance because we did not use any sails for light wind. To have any benefit from the route proposal, you need to learn how your boat sails in relation to the polar diagram under different conditions.

You must also state how big waves you want to sail in and how strong the wind can be. Our maximum limit is 25 knots of wind and 3 m wave height. 25 knots average wind often means that it blows closer to 30 knots most of the time and 3 m wave height means that some waves can be around 6 m.

This resulted in a route proposal for the next 3 days (we sail about 150 M on average per day in the trade wind) with a file size of 6 kB.

At the downwind sailing during the crossing, we always got a route proposal that tell us to gybe several times each day. We tried to follow the proposal a few times, but with three crew members will be too much work to gybe as often as the route proposal said. Changing the side of the whisker pole takes time and we needed to be at least 2 to do it, and each gybe is a risk. As we sailed downwind in big waves, we had preventer set on the mainsail and the genoa was sheeted hard against the whisker pole. Without this arrangement, the sails slam hard when the boat rolls from side to side in the waves. For our part, it was better to sail longer on each bow than what the route proposal said and in some cases, it was better to sail with the sails wing on wing straight towards the goal.

Weather overview each morning

I have not understood in what way PredictWind’s forecast data based on GFS and ECMWF would be better so therefore I have chosen to only use GFS and ECMWF when I retrieved weather forecasts. It is possible to retrieve the forecast for 14 days ahead, however, the uncertainty is very big. If you need such a long forecast, it may be better to have contact with a meteorologist who can help find a good weather window and send updates for a longer crossing. Personally, I would like such a contact if the crossing takes place in a low-pressure area and it is estimated to take more than a week.

We used this forecast to decide if and how much we would deviate from the straight path to the goal. By comparing two forecast models, I was able to get an idea of the uncertainty in the weather development in the slightly longer term. Slight difference in the forecasts, then I think the chance is lesser that the weather develops in this way. Big difference, then I have less trust in the prognosis.

  • Weather models: GFS och ECMWF
  • Area: Aprox 30 degrees north-south and 50 degrees east-west
  • Matrix: 10 km
  • Forecast length: 7 days
  • Interval: 24 hours
  • Weather data: Wind, Rain, Pressure

This resulted in the download of 2 files, GFS Global 10 km, and ECMWF Global 10 km, about 150 kB each.

Detailed forecast morning and evening

I used this weather forecast to decide on the route and sailing for the next 24 hours. In principle, it decided whether the whisker pole should be placed on the starboard or port side or not at all.

  • Weather models: GFS och ECMWF
  • Area: Aprox 7 degrees north-south and 10 degrees east-west
  • Matrix: 5 km
  • Forecast length: 3 days
  • Interval: 3 hours
  • Weather data: Wind, Gust, Rain, Pressure and Waves

This resulted in the download of 4 files.

  • GFS Global 5 km, ECMWF Global 5 km, approx 60 kB each.
  • PWG global Wave 5 km och ECMWF Global wave 5 km, approx 25 kB each.

Lessons learned

I do not need a routing function because it becomes too detailed and did not suit us on this type of sailing. For longer crossings, I will take the help of a meteorologist instead and get a more strategic route planning that allows you to position yourself correctly in relation to major weather changes.

When we continue sailing after the hurricane season, my goal is to

  • Reduce the size of GRIB files by reducing the area in both the overview and the detailed forecast.
  • Find a GRIB viewer that can
    • switch between different forecasts
    • show isobars for air pressure no matter how you choose to show wind
    • show the center for low pressure and high pressure respectively with information on air pressure
    • display differnt kind of weather data at the same time, eg
      • wind and rain to more easily identify fronts
      • waves and current to see where they ”collide”
  • Check if there is any service other than PredictWind that can deliver GRIB files based on the ECMWF model
  • Find out if there is a cheaper service than PredictWind that can deliver GRIB files with offshore currents. This is extremely important information at sea, as wind against current can give very steep and dangerous waves, or simply to avoid sailing against the current if possible.
  • Check if there is another tracking service that is not due to having a standard subscription with PredictWind running all the time. PredictWind also deletes tracking data that is older than 12 months and we would like to keep it as long as we sail long distances.

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