Grib Weather Plugin

OpenCPN has a grib weather file viewer plugin, aimed at being useful while under way.
The grib plugin is always distributed with OpenCPN, no separate download is required.


What is a grib file?

  • If you are not familiar with grib weather files, make sure you understand the basics, before you start to use them. It is essential to understand the limitations of weather forecasts that are distributed in the grib format. It is also worth pointing out that gribs are not reliable near tropical systems. Send a blank mail to Read the mail you get back carefully.
  • To get started with gribs and to find sources for downloads, check Franks-Weather.
  • A very good, up to date book, is "Modern Marine Weather, second edition" by David Burch, with a thorough treatment of the subject.
  • A few files including lectures on  tropical weather and gribs are available here.

Get started

  • To display grib files as an overlay on your normal charts you have to first activate the grib icon
  • Go to find the plugin tab. Click on the Grib plugin, then the "Enable" button. The "Preferences" Button brings up a small dialog. The settings in this dialog rarely needs changing after the initial setup.


Grib Preference

  • Use High Definition Graphics. The setting only concerns the barbed wind arrows.Tick the "Use High Definition Graphics" box, unless you are on old hardware. Not ticking the "High Definition Graphics" box can help to speed up the grib display, in such cases.
  • Use Gradual Colors. This setting applies to  all colored overlay maps. On many installations the difference in rendering is very small. The overlay maps built in color variation plays a part.
  • Copy First Cumulative Missing Record. This is about rainfall and cloud cover records .
As these records are "cumulative" , the first one does not exist in files from zyGrib and Saildocs. With this parameter unchecked, there will be neither rainfall and nor cloud cover displayed for the first date/time of the file. Ticking the box fixes the problem.
  • Copy Missing Wave Records. This only concerns wave records from zyGrib delivers files from two models ; FNMOC (WW3-...) for waves and GFS for all other parameters. These two forecast models does not have the same time spacing. For some dates/times, the wave data do not exist.This parameter allows this missing data to be reconstructed and avoiding the overlay (if set) to flicker.

Load File Options

  • Load the More Recent File in Directory. Loads the newest file in the grib directory on start.
  • Load the Last Open File. Loads the grib file used last time.

Start options

  • Start at the first forecast in GRIB file
  • Start at the nearest forecast to current time
  • Authorize interpolation to start at current time.

Time Options

  • Local Time
  • UTC


The Grib  Display Controls


The Standard Display Control

To start the Grib plugin press the button in the tool-bar. The "GRIB Display Control" then pops up.

This is a smart and compact dialog. For a quick start press the "Open File" button and navigate to your grib file directory and press the file to be displayed.
  • Previous/Next Go to the previous or the next record in the gribfile. This corresponds to the time "interval" chosen when the grib file was downloaded.
  • Data valid at The time of validity for all grib data on screen.
  • Open Grib file The name of the grib file in use.
  • Now Displays the record, nearest in time to "now" according to the Start option you have selected: ("Start at the nearest forecast to current time" or "Authorize interpolation to start at current time").
  • Zoom to Center This button will home in on the loaded area that the loaded grib file covers.
  • Play Goes through all records in turn, with the animation speed controlled in the "Grib Settings" dialog.
  • Progress Slider Just to the right of the "Play" button is a slider that is just another way of going through the grip records.
  • Open File Activates the operating systems standard file selector dialog to select a grib file. The plugin remembers the directory of this grib. This is used next time you start OpenCPN and activates the Grib plugin. See settings in the "Grib Preferences" dialog above.
  • Settings is where to control how the different grib data will be displayed. See more below.
  • Request Download grib files via email. More below.
  • GRIB Data at Cursor. This is where the "smart" part kicks in. Only entries for the data contained in the active grib file will be displayed. For example, if there is no wave data in the current grib file, there will be no signs of wave data at all. In the case above, where the the wave data is N/A, the grib file includes wave data, but not for the area where the cursor is. Compare with the picture below, where the grib only contains current data. The tick boxes next to each data entry, are used to activate the data-display on the screen. Exactly what will be shown is controlled in the "Grib Settings" dialog, which is reached by pressing the "Settings" button. Don't activate to many display features at the same time. The display quickly becomes very cluttered.

The "GRIB Display Control" loaded with a grib file only containing current data.
  • Advanced GRIB Data at Cursor. The display options for all the data shown, can be reached by right.clicking on the little squares where the "ticks" are shown. For the meaning of all these options, read on.

Right-clicking the "Air Temp" option square.

The Advanced Display Control

The plugin has an advanced mode were altitude data can be displayed. This mode requires some more knowledge than plain gribs. A good start is to read "Mariner’s Guide to the 500 – Millibar Chart".
If a grib file includes altitude data, the control will automatically include more options. See below.

If the dialog looks normal except for an extra selection box under "GRIB Data at Cursor", like this,

then the grib file contains altitude data. Selecting one of the altitudes, for example 500, changes the dialog to what you see above. The advanced data are displayed against a yellow background.
Wind. These are the wind data valid for the selected altitude level.
Grib Data at 500 hPa. The values at the selected level.

Weather Tables

Once a grib file is loaded in the current session, the "Weather table" appears in the right click menu.

Click the entry, and a Weather Table, valid at the cursor point, pops up. All available data in the current grib file, for the whole time span, are shown.



...controls how the grib files are displayed on the screen, and units used in the Grib Display control. Thing like Overlay colors, animation time interval and speed and much more.
The basic dialog looks like this.


The Loop Mode controls what happens when "Play" is pressed in the Grib Display Control.
  • Loop Mode. Sets the "Play" function into a loop mode. Otherwise, playback will stop when reaching the end of the file.
  • Loop Start. Were to start the loop. The option "Current time forecast" makes sure that only data now or in the future are shown
  • Updates per Second. Controls the speed of "play".
  • Loop Start. The loop can start either at the start of the grib file or from the current time forecast.
  • Interpolate between gribs. If you select this option, you will be able to choose your own time interval, but you have to consider that it can decrease data accuracy. To remind you, this info will pop up.
  • Time Interval. This is the time interval used for interpolation. The entry is only visible if "Interpolate between gribs" is ticked. This is connected tho the chosen time interval when requesting a grib file.


The lower part of the Grib Settings Dialog, under the Display heading is really 10 different controls. The controls are activated  by the choosing one of the items in the selection menu below.

Many of the entries are repeated, so let's first have a closer look at the the most important one, the Wind. Entries on the same line are connected.

  • Units refers to the choice of units for the selected record.
For wind speed the choices are.

  • Barbed Arrows. Display the traditional barbed arrows, showing wind direction and wind speed. Range refers to the distance between displayed arrows ( not working 3.3.1117). Default Color / Controlled Colors  Default is dark brown while "Controled" varies from blue for very light winds to red for very strong winds
  • Always visible This option only exists for "Wind" and "Pressure". When ticked the object is shown regardless of what is ticked in the "Settings Control". The idea is to be able to Show wind and/or pressure at the same time as other options. Use with Rainfall, for example, to help identifying fronts.
  • Display Isotachs shows contour lines connecting points with the same wind speed. Spacing controls which wind speed Isotachs will be shown. Use knots for units and 10 for spacing and isotach contours will be shown at 10 kts, 20kts and so on.
  • OverlayMap  Depending on your system capability,if you use "Accelerated Graphics (OpenGL)" (see General Options/Display/Advanced ), the overlay will be visible at all scales. But if you don't, the overlay will only be visible at small scales. A small yellow pop up at the bottom left of the screen will inform you. Zoom out to a scale of 1:13 millions, or smaller to see them.
  • Colors is another selection menu containing pre-set color schemes. The idea is to use the scheme with the same name as the control, but this is just a hint, nothing more.
  • Numbers shows the wind speed in small square boxes. Minimum Spacing refers to the spacing between these boxes.
  • Particle Map is a kind of animation of the wind field. It is created by letting  "particles", or dots, move in the wind direction and leave a trail. The trail is colored after the wind strength, and each particle has a limited "life span". The Density slider creates particle trails that are shorter but closer together with increased value, which is the same as  further to the right.  The particle Map below clearly shows a convergence zone with stronger winds (red). This is probably a cold front.  Performance Warning: Note that high density Particle Maps on Low Powered Machines and Large Files may slow your machine to a crawl. First test on small files.
  • Overlay Transparency works as expected.

All the controls for Wind Gust, Rainfall and Cloud Cover, have similar entries; only the units differs.

The pressure display has no overlays, only the options of isobars and numbers.
Overlay Transparency is redundant.

Waves and Current have a more or less similar display. A Particle Map is available for currents. See the ddescripion above for wind Particle Maps. Once again different units.
There is a choice between three different display arrows shown in two sizes.
If wave height, direction and wave period is present in the grib file it will be displayed in the Display Control like this:

Air Temperature and Sea Temperature looks the same.

Convective Available Potential Energy (CAPE) has these settings

Altitude and Relative Humidity have a simple one choice entry.

The new Grib-file Request Button

  • Grib files can be requested directly from the plugin. The request is in the form of an email to SailDocs or ZyGrib. The requested grib file is also delivered via email.
  • The area for which data is requested defaults  to the area visible on the screen, but the request area can be selected by other means as well. More below.
  • It's possible to request gribs from 4 different Grib prediction models when using Saildocs GFS, COAMPS, RTOFS and indirectly WW3.  When you request "Waves" in GFS, Saildocs merge wavedata from WW3 into the delivered grib.
  • The ZyGrib option can only deliver GFS gribs, which is the "standard" model for gribs.
  • RTOFS gribs only contains current and water temperature data.
  • COAMPS delivers wind and pressure with a higher resolution than GFS
  • The minimum time between grib records are 3h for GFS and 6h for COAMPS & RTOFS.
  • GFS can be requested for 8 days ahead. An extended GFS request up to 16 days ahead is possible. This warning will  pop up.

  • The same value for COAMPS is 3 days and RTOFS 6 days.

There are two layouts of this dialog, depending on wether it is a request to SaiDocs or to ZyGrib. Some alternatives are only available from SailDocs, and some are only available from zyGrib. Alternatives not available are grayed out.

The SailDoc Request looks like this.

Compose the request by picking parameters and data. Not all data are available for all choices of parameters. For example, in the picture above "Wind Gusts" and "Current" are grayed out, as the are not available with GFS from SailDocs.

When selecting "Moving Grib" the dialog expands and makes i possible to choose a speed and course. The selected grib forecast area will move, using these values, for each grib interval. The idea behind this is to minimize the download  while still covering a longer passage.
Moving grib files are incompatible with interpolation, so if a "moving file" is detected a warning is displayed. Interpolation, if set is deactivated, but only for moving file, the settings as such, are not modified.

Resolution  The choices are 0.5, 1.0 or 2.0. A recent upgrade to gribs makes it possible to download even 0.25° resolution. This is not yet an alternative in the plugin and requires editing the email manually before sending. In the picture above ...|0.5,0.5|... should instead look like ...|0.25,0.25|...

The default geographical coverage of the requested grib file is the area you can see on the screen. A manual Selection Mode is also available.
When the Area Selection -> Manual Selection box is ticked the dialog expands

Choose the limiting Latitude and Longitude for the grib-file manually or press "Start graphic Sel."  button.

Press the left mouse-button and draw a rectangle around the required area.

When selecting "Pressure Altitude" and the GFS forecast model, the dialog expands and makes i possible to choose forecasts for different altitude levels. The SailDocs dialog only supports the 500 mb altitude, while zyGrib suports all options.
When you are ready press  "Send".
This message will show in the "Mail" window instead of the "send" request:
Your request is ready. An email is prepared in your email environment.
You have just to verify and send it...
Save or Cancel to finish...or Continue...
As stated, a mail is composed for you and ready to send with your normal mailing program.
This is the standard way of getting a new grib in Windows and Linux. (Mac ??)

There is an advanced alternative for  Mac OSX, Linux (and BSD). Install and configure a mail server. Instructions on the Mail Servers page. This will enable automatic transmission of a grib request to SailDocs.
After installing and configuring a mail server, there is one more step.
Open "opencpn.conf" and ad the line below. As always, do not run OpenCPN when editing opencpn.conf.
Setting "SendMailMethod" to "0" restores the default.
To get a grib, just press "send", and wait for a return mail.

The zyGrib Request looks like this.


More to know

  • ZyGrib is a free and open-source software (FOSS) dedicated grib viewer. Grib files can be downloaded by ZyGrib and then opened in OpenCPN.
  • QtVlm a FOSS weather routing program, combines a grib file with boat polar data, and produces an optimized route. More here Weather Routing. This is a very good tool to use in routing decisions, but has the same limitations as all grib-data.
  Go to Table Of Contents