Friday, February 19, 2021

GRIB School

This document is an index to the several articles and videos Starpath has available on the subject of GRIB files and their use in marine weather analysis and routing.  You can skim through this to go directly to individual topics.

What is a GRIB File?

Use of numerical weather model forecasts by individual mariners is an ubiquitous component of modern marine weather. Almost any navigation program these days connected to a wireless source on land or at sea can press a button to generate the latest wind, waves, and currents forecast across the chart with forecasts extending out a week or more.

The data come to us in a digitized format called GRIB, standing for gridded binary. It is a vector product, meaning it is all numbers and symbols, but when rendered in an appropriate software program ("GRIB viewer app") it can appear as a graphic map of the isobars, wind vectors, rain distribution, and other parameters, laid out on a Lat-Lon grid. 

The GRIB standard was developed by the World Meteorological Organization (WMO) specifically as a way to transfer weather data in an efficient, standardized manner amongst meteorologists and mariners. The grid is a Lat-Lon lattice of points, with digital values (binary data) presented only at those specific points. The distance between points on the grid is the resolution of the file. This can vary from as large as 1º (60 nmi)  intervals to as small as 1.3 km (0.7 nmi), which is about the finest step size available outside of the laboratory.

A GRIB weather map at 0.5º resolution over a 10º x 10º area including wind and pressure will be about 50 kb in size. File size increases very roughly proportional to the number of parameters, but increases as the square of the resolution and area covered. 

Several apps let the viewer request the latest grib forecast from a specific model from within the app, and these generally estimate the file size as you define what you wish to request.

Terminology: We often hear or say something like, "Have you downloaded the Grib?," or "What does the Grib tell us," and so on. Usually this is not ambiguous, but we should keep in mind that "Grib" is not a thing; it is a format.  It is like saying "What does the PDF tell us? The term Grib alone does not tell us at all what we are looking at. This could be a wind forecast from the GFS model or a wave forecast from the WW3 model. Use of the word Grib in such discussions requires a very clear understanding of the context.

The following links contain information related to use of GRIB files

Background on GRIB files: Numerical weather models and computed parameters

Includes topics below plus a comparison with other forms of marine weather data. 

Traditional Weather Products

Numerical Weather Prediction

Values of GRIB Formatted Forecasts

Grib2 Weather Parameters

Categories of Digital Forecasts in GRIB Format

Basic Properties of Selected Models

Sources of Model Data in GRIB Format


Applications of Weather Data in GRIB Format 

Each topic has a short discussion along with video illustrations.

1. Global model weather forecasts for ocean navigation

2. Global ocean model forecasts of wind waves and swells

3. Ocean model for currents and water temperature

4. Regional model forecasts for inland and coastal sailing

5. Overlay model forecast winds on weather map and cloud photo images

6. Probabilistic forecasts from ensemble forecasts and model blends

7. View sea ice coverage from RTOFS in LuckGrib

8. Compute optimal sailing route 

9. View ASCAT scatterometer near-live satellite wind measurements


Introduction to using GRIB files with XyGrib

A few basic tips with a video example.

Introduction to using GRIB files with qtVlm

We have a couple videos on this, but the article is not done yet.

Optimum Weather Routing with qtVlm

A short discussion with video example

Optimum Weather Routing with OpenCPN

A short discussion with video example

Playlists of Related Videos on Use of GRIB Data








Tuesday, February 16, 2021

Optimum Weather Routing with qtVlm

This note on qtVlm routing is part of our sequence of articles on the Applications of Weather Data in Grib Format. We have similar demos of routing with other applications.

qtVlm is a free, donation-supported navigation and weather software program for both Mac and PC computers, as well as mobile devices.  It is an internationally popular product with versions in multiple languages. It is the leading program worldwide for virtual tracking and taking part in ocean races presented online.  It is a versatile navigation program as well as weather resource. See also notes in the Starpath Glossary.

We cover charting aspects elsewhere; we have a video on using grib files in qtVlm; here we just look at the app for routing computations, which is of course tied to its display of grib formatted numerical weather and ocean predictions.  The program includes extensive functionality, which means there are numerous nested menus, some of which are interrelated. In short, there is a learning curve, as is with any such versatile program. So we start by just focusing on what we need to create a basic route, and then we will come back to the numerous ways it offers to fine tune and optimize the results further.

We will list the steps here and then add a video demo of the process.

Step 1. When you download and install the app, be sure to load the "maps," which in this case will be the high-res base maps. 

Step 2. The default display shows a daylight terminator which is very handy when underway or when planning a voyage, but for training and practice if you find it distracting it can be toggled off at menu View/Show-Hide/Night Zones.  

Step 3. Load a Grib forecast (review earlier notes on gribs) that will cover the range of the route you want to compute, and long enough to let the vessel finish with its known polar data.  In this example, we want to do summertime ocean routing using archived wind data from July, 201o that we have stored on the computer. Thus the steps are: menu Grib/Grib Slot 1/open and navigate to the file to load it into slot 1.

Doing these historic runs, you will get a warning that the grib data is old, and consequently it will not show on the screen. Click the clock icon in the middle of the menu bar and set the grib time to match the first forecast loaded. Using live data this will not likely arise, as the newest live data is some 4 or 5 hr old at best.

The second from the right magnifier icon (with a square inside) will center the view on the active grib data.

If you do not see the grib data, but you do see elevation and rivers on the land, then you have an overlay turned on that could be hiding the wind data. Click the menu chart icon "O" (online charts)  to toggle this on and off.

Step 4. Load the polar diagram you wish to use. It can accept files in the .pol format or the .csv format, with the separators being semicolons. See related polar format discussion.  To load the polar use menu Boat/Boat Settings.  Note we are not using menu Boat/Polars. That will be used to study the polar once it is loaded.  Once in Boat/Boat Settings, add a name and or model of your boat then open the Polars tab below it.  Navigate to your polar and import it. We can leave the other settings in default choices.

Step 5. Check the polar. Menu Boat/Polars/Wind polar analysis. Check all TWS (true wind speeds) to see if it looks as expected. Then go back to just one wind, and notice that you can click on the curve to read the data. We are not getting into this now, but if you want to change anything in the polar you can do it in Menu Boat/Polars/Wind polar editor. 

Step 6. Set start and end points. At the desired start point, right click within the grib area and make a mark. Give it a name and click OK. We can leave all defaults as is.  Do the same with the destination point.

Step 7.  Right click anywhere on the chart and chose Create a routing.

Give the route a name 
Unclick routing from boat
Confirm that start and finish are the points you want. The heading says "Finish and start points," but you want to have start first (top) and finish second (below it).

Turn on Keep Starting Date and type in the initial time of the grib forecast installed (default is month, day, year). Later we can use other start times. Select the month, type it in and just keep typing. 

Rest leave default and click OK, and you should see the isochrones being computed from the start point headed to the finish point. Convert to Route at the bottom should be set.  (If anything is not right, then when done just say OK, then Cancel, and from menu Routings select delete routing, and do it again.)

Step 8. Optimize the routing. When done, it presents the ETA, duration, and computation time. Click OK. Then we get the opportunity to convert this routing to a route.  The former is the result of an optimization using isochrones, the latter is a sequence of waypoints (POI). Unless we feel we need to study the solution in more depth, then the logical next step is create the route. 

We have the choice of Optimum or Maximum conversion. The process removes excess isochrone solution points along the route, for example, if three points are on a straight line, then you can remove the middle one. Optimum does that level of simplification. Maximum does more by readdressing the route decision between each POI left in the Optimum pass. Leaving the proposed setting of Maximum is best, but it might take a few more passes up and down the route to converge on the best.  Click OK and watch it optimize.

Once this is done, maybe with the offer to do more, then the routing will be moved from the routings list (menu Routings) to the routes list (menu Routes) and we can then look at details.

Step 9. In menu Routes/Edit Route  select your route. Logbook shows the conditions at each POI along the route. Histogram is an interesting way to look a plot of various parameters along the route. Statistics summarizes a few parameters of the whole route.   To export the route as GPX file, use menu Routes/Export route.

Below is a video sample of an optimum route computation.

Routing example with no special conditions [17m:33s].

qtVlm has many options and restrictions that can be placed on the computation. Like most other apps, you can define boundaries to block the route from certain areas or passes, but unlike other apps (except Expedition), qtVlm can route around a course of marks, or through specific gateways, which adds a layer of versatility to the solution.  This is accomplished by optimizing along a pathway.  In qtVlm, a pathway is a series of waypoints, what might be called a route in other programs. Routings, routes, and pathways have distinctions in qtVlm. 


Users Manual:

Forum details:

Many videos in YouTube in French.

Monday, February 15, 2021

Optimum Weather Routing with OpenCPN

This note on OpenCPN routing is part of our sequence of articles on the Applications of Weather Data in Grib Format. We have similar demos of routing with other applications.

Optimum vessel routing across forecasted wind, seas, and currents patterns based on the known vessel performance in various conditions is the primary goal of modern marine weather technology. We can do this manually with graphic weather maps we obtain at sea by radiofax or email, and we can do it semi-manually using numerical weather model forecasts we receive in grib format by satcom or HF-radio. 

We can also now do this fully automatically using dedicated software that perform the optimum routing computations for us. In principle, if all input is correct, this would yield the best result and indeed be the optimum route. Experienced mariners know, however, that the wind forecasts are rarely right in all details, and that special conditions such as relative angles of wind and swells can notably affect the performance of the vessel relative to what is expected in its polar diagram of vessel speeds for various wind speeds and angles. Correcting for waves is even more problematic, and ocean currents remain elusive, even with the best ocean models to guide us. In short, the process is not going to be plug and play. Success will still depend on individual's knowledge of weather and their ability to evaluate not just the quality of the forecast, but also the quality of the computed solution.

All optimum routing apps require several inputs to be set correctly or the computation will not work. The various apps differ in how easy it is to insure this is done right and in the detail of the error messages given to help. In any event, for every app that does these computations, it is helpful to have a cheat sheet to follow until the process is mastered in your favorite app.

Furthermore, beyond the basic requirements to compute the route, there is a large layer of adjustments and scaling factors available to optimize the route. Although the apps have some overlap in the most basic corrections available, they do differ notably in the full range of such corrections.

But before we tackle these in future articles, we look at the basics of running a route computation in several apps, as in this example with OpenCPN. We also  have a couple videos showing the process, but these assume we have the basic details in place beforehand. Here is a summary of the basic steps followed by a video illustration.

OpenCPN Weather Routing Plugin
Mac and PC versions seem to behave the same.

(1) Polar diagram files can be any of three file types (.csv, .pol, or .txt), but they must all be in the same basic structure, shown below. (Note: sometimes Windows hides file extensions. If so, open File Explorer and under "View" turn on file extensions.)

Above is the CSV (comma separated values) format. You can use semicolons as well as commas for this. These would be in neat columns when viewed in a spreadsheet, which is one way to create the file, although the WeatherRouting plugin has a very convenient creator and editor for polar files—we do not need the separate plugin called Polar_pi.

If you replace the commas with tab you get the POL format. Example below.

Replace the comma in the CSV with a space and you get the TXT format.

These three are all really the same. True Wind Angles down the first column; True Wind Speed headers along the first row; then subsequent rows are the vessel speeds at the indicated TWA and TWS. The first element of row 1 is sometimes just "twa,"  but this does not matter. Sometimes the polar includes an inserted first line being the name of the sailboat model.

(2) Polars must be stored in the right place.

Mac:  HD\Users\username\Library\Preferences\opencpn\plugins\weather_routing\polars

PC:    C:\ProgramData\opencpn\plugins\weather_routing\polars

(3) Save only one polar per boat for now.  Later you can mix polars for various conditions (high wind, low wind, upwind, downwind, big seas, crossing seas).

(4) Be sure the polar covers all wind speeds along the route. If wind picks up to 25 and highest polar wind is 20, it might not run. Right click any place along the general route area on the grib file and choose "Weather Table" to get a digital meteogram of wind vs time at that location.

(5) Be sure the starting and destination points are within the wind field. Setting up all digitally within the plugin, these points are not shown on the chart as you set them, so typos may not be apparent. Recall S Lat and W Lon are negative values. See notes below on better ways to set up the route.

(6) Be sure the forecast duration is long enough to get to destination with the given polar. Else it might just fail and not say why. Try a quick test with shorter route. There are ways to fake this with extending last winds or using climatic winds, but you then get fake results.

If you get the message "polar failed" and it does not compute, then the cause is most likely in the list above.

Set up procedure
after weather route plugin is installed and enabled.

Step 1.  Drop a mark at the desired start point, then right click to properties, and give it a name; likewise with the end point.

Step 2. Open the WeatherRouting plugin, and with this open, right click the start point (be sure it highlights) and choose Weather Route Position. This will add it to the positions side of routing window. (You can do this at any point on the chart, it does not have to be a mark, but in that case you will not get a graphic confirmation of the location.)  Do the same thing with the end point, to get it into the positions list.

Step 3.  Check the positions list to make sure they are both there. If needed, edit the configuration window and the positions window from the main menu bar controls. Click anywhere in the routing window to show the menu. Note: you might have to try both right and left clicks to get an item highlighted.

Step 4.  Load a wind grib file that covers both start and end points, and be sure it extends far enough in time to get the boat to the end with the polar(s) chosen. You can also load waves and currents, as covered later. 

Step 5.  Set the forecast to the first of the sequence for now. This can be changed later.

Step 6. Viewing the routing panel (Positions and Configurations), use the position dropdown to load your start and end points. These must be in the positions panel to show up in the dropdown. 

Step 7. In the Start panel click Grib Time, and check is consistent with grib forecast showing.

Step 8.  In the Boat panel click Edit. You will initially see samples provided with the program. Do a select all and remove them, then press Add, and navigate to your polar file, which must be stored in the proper place explained below—it should default to that folder. Then "Save as Boat" and give it a name. You might also open the polar to confirm the maximum wind speed included, because the computation cannot proceed through higher winds.

Step 9.  Check in the Data Source Panel that we are using Grib data. Close the polar window and check the path in the Boat Panel to be sure you are using the one you saved. If not, click the three dot menu beside it to assign the right boat. Then close the configuration window. We can leave the rest of the settings on default to get started.

Step 10.  Leave Last Valid unchecked and Climatology disabled.  We will treat inadequate forecast duration another way.  Close config window.  Return to the main Weather Routing window, select the route we are working on, say two Hail Marys, and press Compute.


Once a route is computed, the parameters showing in the configuration window are chosen in the menu View/Settings. You must click the weather routing window to show the menu.

The menu View/Plot offers an insightful way to look over the route, both as computed (Current Route) or as you look over alternatives (Cursor Route).

Completed weather routes are not automatically saved when closing the program. The Export button at the bottom of the config window copies the route to the Routes Manager list. It arrives there as a track, but can be converted to a route. Either can be exported as a GPX file.

Here is a video illustration of the creation of an optimum route:


Side Note: If you make a route on the chart, say C to D, then right click the route and choose Weather Analysis, it will both load these two points into the positions panel, and it will create what looks like a normal configuration in the configurations panel, but when you open it, you will see the start and end are filled in but grayed out. 

When you then compute that one, it will look at the course C to D, then look at the wind speed and direction at the start time from the grib, figure the true wind angle from that, then look into the polar to find the expected boat speed, and use that as the average speed for the whole course and give you that speed and the time it takes at that speed. That is not an optimum route; no isochrones were computed.  It is just a rough estimate of the time assuming the wind does not change. This result is roughly what qtVlm calls the VBVMG (very best VMG) route, but it uses only one step to get there.

To go from that set up to a real optimum route computation, just delete that configuration, make a new one, add C and D to it, and run as above. Then it will crank out the isochrones.


Wednesday, February 10, 2021

Applications of Weather Data in GRIB Format

We have several articles and videos on getting started with GRIB files cited at the end of this note. Here we take a look at various applications of this powerful technology. It would be too much to cover in any useful manner in one place, so we present this index to specific topics with linked video demos. We initially focus on a couple free viewers that we use in our online course, and then at the end show how these analyses can be expanded upon with several outstanding commercial products.

1. Global model weather forecasts for ocean navigation

This was the original application of the data, which was spearheaded by Saildocs using Airmail for HF radio communications and the PC program ViewFax for viewing the grib data. ViewFax remains a versatile way to view and obtain the files, along with other marine weather data.

We started our introduction to grib files using the grib viewer app XyGrib, so we will show samples from it whenever possible. We also look at GRIB files in the nav app OpenCPN that we use in both the coastal nav and marine weather courses. Our goal is then to also include qtVlm which is a powerful program that does both navigation, grib viewing, and grib file sourcing. All three apps have both a PC and a Mac version. We have an introductory article on XyGrib that includes a short video on loading a global model

The sample below looks at more details of using a global forecast of an ocean route.

We set up a global model display to look like the OPC maps. By comparing surface and 500 mb maps we get an insight into the dependability of a forecast [13m:32s].

The same files we download from XyGrib or by email request from Saildocs, can also be shown in  OpenCPN—recall that OpenCPN has a way to view the files, but does not directly download the files, although it does assist with preparing an email request for some of the saildocs files. See models available at Saildocs. For the most part, XyGrib offers a more informative way to display most GRIB files than does OpenCPN, but OpenCPN has a super convenient way to load georeferenced weather map images, upon which we can overlay the GRIB model forecasts for improved insight into both the model forecast and the graphic map image from a weather agency such as OPC. This process is illustrated in Section 5 below. 

There are  some 14 global weather models internationally, about half of which offer free access to their data in grib format. A couple of the popular ones are:


NAVGEM (US Navy, see also)

ARPEGE (France)

ICON (Germany)

GDPS (Canada)

For historical analysis we can use re-analyzed model forecasts. These global data are available every 6 hr from ECMWF and GFS. When you use this data starting at say18z, July 10, 1990 for 10 days, you can get one map every 6 hr, but they are not forecasts, but the equivalent of surface analyses valid at those synoptic times. It is an excellent way to study past races or voyages.

Here is another example of viewing grib files in a free app with both Mac and PC versions called qtVlm.

Loading and viewing grib files in qtVlm [11m:35s].

2. Global ocean model forecasts of wind waves and swells

We can access the US WW3 model in almost any navigation  and grib viewer apps. The model has an extensive set of parameters (see definitions in the Background article.)  The German weather service, Deutsche Wetterdienst (DWD),  also have a high-quality ocean program with a global wave model (GWAM), European wave model (EWAM) and a higher resolution coastal wave model (CWAM).  XyGrib will download WW3, GWAN, and EWAM. Only WW3 is available from Saildocs. (All are available from Expedition or LuckGrib.)

Below we compare two global wave models in XyGrib. Recall the grib2 wave parameters listed below, which shows that both models use the same parameters, but they have different abbreviations for them.

Sample page from Panoply showing content of two grib files from XyGrib

Here I have loaded both files into Panoply, which not only displays the data graphically but also shows exactly what is included. We have videos on the use of Panoply.

A video comparison of WW3 and GWAN in XyGrib with notes on wave display [12m:13s].

3. Ocean model for currents and water temperature

The most popular ocean model for global currents is the US RTOFS model. It is available from Saildocs.  Other global ocean current models include the US Navy models HYCOM and NCOM, the latter of which is a regional model, generally offering the best current data when available. 

XyGrib v1.2.6 does not offer ocean current data download, but it does have a currents parameter to be shown if the data are obtained elsewhere. To get RTOFS grib file, send this in the body of a mail to

send RTOFS:42.1N,24.2N,34.3W,3.4W|0.08,0.08|0,6..144|CUR,WTMP

Change the Lat Lon box to what you want. This is for every 6h out to h144 (6 days). It provides current direction and speed as well as sea surface temperature (called WTM). The 0.08º (4.8 nmi, 8.9 km) is the resolution. 

The model is run once a day, completing at about 1700Z. Each run starts with a 24 hr hindcast and produces ocean surface forecasts starting at 12z the previous day for every 3h to h48, then every 6h till h48, then every 12h to h192 (8 days).

A video display of RTOFS data  in XyGrib [7m:43s].

4. Regional model forecasts for inland and coastal sailing

The main data sources for US regional models are HRRR, and the CONUS or Regional versions of NBM, NAM, and NDFD. In other parts of the world, the best regional data will typically be a WRF model run by a local university, institute, or commercial company. Samples can be seen at OpenSkiron and within XyGrib for Europe, and at Expedition Marine for other parts of the world, especially near AU and NZ.

HRRR is the only hi-res US dataset  that is readily available to free GRIB viewers, because we can get it from Saildocs. You might note that NAM and NDFD are also available from Saildocs, but you will discover that the NAM they offer is only 12-km resolution (we need the 3 km version) and the NDFD they offer is only the oceanic version (10 km). XyGrib also has only 12 km data for their version of NAM CONUS.  See our note on NDFD: Oceanic, CONUS, and Regional.

The HRRR model (High Resolution Rapid Refresh) is a 3-km model, updated every hour. The model is updated every hour with forecasts that go out 18h.  The runs that occur at the synoptic times are special in that they then go out 48h, with hourly forecasts to 18h, then 3h forecasts to 48h. We can get these extended HRRR forecasts from saildocs.  (The 2.5-km NAM data goes out for 2.5 days, but we need special sources such as LuckGrib to get these data.) 

For now, we can practice with regional forecasting using HRRR data in US waters (inland and coastal) or any of several hi-res models available from within XyGrib for European inland and coastal waters.  For HRRR data we can use this request to

send HRRR:47.9N,48.5N,123.4W,122.3W|0.03,0.03|0,1..18|WIND,PRMSL,REFC

It is important to use the decimals on the position, as these can be very large files at 3-km resolution. Even this area maybe more than we want.  Modify the Lat Lon coordinates for your location. For minimum files omit the PRMSL and REFC

A video demo of displaying HRRR data from Saildocs in the XyGrib viewer [9m:45s].

5. Overlay model forecast winds on weather map and cloud photo images

A major contribution that OpenCPN makes to weather work at sea is its ability to show selected model wind forecasts in grib format overlaid onto official, up-to-date weather maps made by the OPC or their counterparts around the world (UK Met, DWD, BOM, and others). Once the grib file is in hand, this overlay can be accomplished  in a few seconds.  It is a powerful way to evaluate the model forecasts. The procedure is implemented with the weatherfax plugin, which can be downloaded from within the program. We have several videos on the use of this tool and a note on how to add your own files to the list. Here is another example.

Overlay of GFS winds and pressure on an OPC map and on a GOES West satellite image [7m:40s].

The program qtVlm also has a sophisticated means of importing and displaying georeferenced weather maps and other related images. They are not shipped with the program, but they are very easy to import and save so that each run brings the latest maps. Here is a video of the process using qtVlm. Expedition also has a similar functionality.

Loading and viewing weather map images in qtVlm [11m:48s].

6. Probabilistic forecasts from ensemble forecasts and model blends

We get now to the modern aspect of using grib format of model forecasts, namely the use of standard deviations in the forecasts and probabilistic wind forecasts. In the past we would say "There will always be a forecast, and they are not marked good or bad," but that is no longer true—providing we have access to all the newest data.

Ensemble models, such as the US GEFS and the Canadian GEPS, compute 20 or so solutions (see Background notes) at each synoptic time and then provide us with an average of those solutions, along with the standard deviation of the set, which is in practice an uncertainty level. A wind forecast of 12 kts ± 2 kts is obviously much better forecast than 12 kts ± 10 kts.

The National Blend of Models (NBM CONUS) also includes similar standard deviation parameters. On the other hand, the NBM Oceanic domain includes actual wind probabilities deduced from the very many sources they take into account. 

This dataset includes these probabilistic winds: P25 is the wind at the high end of the lowest 25% of all wind solutions at that point; P75 is wind at the low end of the highest 25% of all wind solutions; and the mean wind called P50 is the average of all solutions between P25 and P75. The P50 wind represents the middle 50% of all wind solutions for that point. Likewise there is a P10 and P90. A P90 wind speed of 20 kts means that 10% of all solutions at that point were higher than 20 kts, and so on. Or you could say, there is only a 10% chance that the wind will be greater than 20 kts at that point and time. This is precisely the type of forecasting we need when we have crucial routing decisions to make.  See Evaluating a Weather Forecast.

These data are presented in standard grib2 format, but they are special parameters and not many apps have adapted to them yet. To my knowledge the only readily available source and display of this full range of data is the app called LuckGrib which is app for Mac computers and for iOS tablets and phones. Each app is a one time fee of $25 in their respective app stores; there is no charge for data downloads. The data can be obtained directly from NOAA and the viewed in a free program like Panoply, but this is very time consuming process.  LuckGrib has a very convenient interface and certainly state of the art file handling and graphic display. Below is a brief demo of these probabilistic forecasting features.  I am sure that more apps will adapt to these parameters once the utility of the data becomes better understood and tested. Several apps show the ensemble runs but not yet the standard deviations.

A quick look at GFS compared to GEFS standard deviations and NBM Oceanic probabilistic wind forecasts [12m:38s].

7. View sea ice coverage from RTOFS in LuckGrib

If you plan to travel to high latitudes, RTOFS as of this year also provides sea ice coverage. We can obtain and view this new ice data in LuckGrib—Saildocs has RTOFS currents but not yet the ice data. See the notes above on RTOFS model runs.

Looking at several northern waterways for sea ice coverage with LuckGrib [3m:44s].

8. Compute optimal sailing route

We come now to what is in one sense the ultimate goal of working with model forecasts in GRIB format, namely using that data along with the known performance of our boat in various wind conditions (polar diagrams) to compute the mathematically optimum route across the forecasted wind field, taking into account forecasted ocean currents and waves as they might apply. There is a section of Modern Marine Weather devoted to this topic that highlights the precautions we must keep in mind when relying on such results. It is crucial to go over those points before using such data, computed yourself or provided by third party services. Recall the joke of being told when asked for directions that "you cannot get there from here." For a sailor who does this routing incorrectly from the beginning (often traced to inadequate polar data), this might no longer be a joke!

There are several apps that provide optimum routing. The historic leader in this field has been the PC navigation program Expedition, which not only provides the GRIB files and does the routing computations, but has state of the art procedures for collecting and analyzing performance data so the best polar diagrams can be made. It also has many other custom features, which makes the investment worth it for the majority of racing sailors, worldwide. Cruising or day sailors may not need the full power of that app. There are several books on the use of Expedition

A short overview of optimum routing using Expedition [18m: 23m].

Another sophisticated routing solution is the add-on routing option to LuckGrib for Mac or iOS devices. That link also includes a detailed discussion of the routing process. The key to any good routing solution is the ability it offers to study the proposed route to learn why it was presented as the "optimum" and now sensitive that solution is to other factors. Luckgrib has several unique approaches to this function.

A short overview of optimum routing using LuckGrib [12m: 34m].

All good routing apps include numerous filters to be applied, such as avoid winds greater than a specified speed, or waves bigger than a specified height, motor if the wind speed is below something, or sailing speed below something, or scale the polar boat speeds up or down by a factor, or scale the model winds by some factor, or scale the effect of current by some factor, add so many seconds for every tack or jibe, don't sail closer upwind than some value or downwind by some value, and so on. There are many, depending on the app.  

We do not get into any of these details here, but leave those to our textbook and online courses, and to the several other books and online seminars on the topic. The job at hand is just to show a few solutions to see what they look like, and to note a couple options for practicing with this on your own. 

Also to consider is the Time Zero app, which is another popular commercial app for Windows. It  evolved from the collaboration of Nobeltec and the MaxSea app, which was one of the pioneers in optimum weather routing.  

As for open source or free products,  both OpenCPN and qtVlm have both Mac and PC routing functions. Below is an article and video demo on the OpenCPN version. 

A video demo of OpenCPN routing procedure [22m:55s]. Details are presented in a separate article.

Optimum routing is a key part of qtVlm, which is a popular app for following along and taking part in ocean races online. It is a free program for both Mac and PC, with grib file sourcing as well as routing. Even though this is a free program, its routing function is among the most sophisticated of the offerings. 

A video demo of qtVlm routing procedure [17m:33s]. Details are presented in a separate article.

Optimum sailboat routing in local coastal or inland waters is the exciting frontier of this technology. This must be done with the regional models, with special adjustments of the routing app routines. We will add notes on this process shortly.

9. View ASCAT scatterometer near-live satellite wind measurements

This, it turns out, is not a new technology. The Ocens company in Seattle had a grant to distribute the QuikSCAT data in GRIB format during the last few years of that program, which ended in 2009. The European program ASCAT has replaced that now, and these new data are available in graphic format and in GRIB format.  To my knowledge the only two packaged apps that can both download and display this data are Expedition and LuckGrib. Without these apps, you can download the files yourself and view them in Panoply. We have a video on how to do that. The latest data files are not large as they only include the latest passes. ASCAT wind is the truth meter for any forecast.

We do not have a video demo of this powerful tool because at the moment (early Feb, 2021) the data are not available. There is some technical snag going on. This is the first time we have seen this in years. We should note, however, that the GRIBed ASCAT data, since its inception nearly 15 years ago, has always been labeled "experimental," which distinguishes it from "operational."  The latter are intended to be guaranteed; the former are specifically not.  I will update this when it gets fixed.

We have ways to get this data onto your boat at sea by email request, but before rejuvenating that technology we will wait and hope that the NWS gets this sorted out. 


That concludes this brief overview of GRIB file applications. Please stand by the announcement of a new short online course we will offer on this topic with hands on practical details. 

Related topics:  The series of articles we have on related topics are in our blog index for Jan and Feb of 2021.

Saturday, January 30, 2021

Introduction to using GRIB files with XyGrib

A key aspect of modern weather work is the use of numerical weather model forecasts in Grib format. This should not be treated as a sole source of information, but it is extremely valuable once you have evaluated the forecast. Methods of doing that, along with an overview of models and available weather are covered in Background on GRIB Files and Numerical Weather Forecasts.

Here we jump straight into the process by looking at a couple options for downloading and viewing the files.


We start with XyGrib because it is a free, easy to use Grib Viewer app that has both PC and Mac versions. It also has the advantage of providing very convenient GRIB file selection and download from right within the app, including a wide range of models, and several unique European sources. 

Get the app from in the  Downloads section. Choose the online installer for Mac or PC.  Install in the normal manner, and open the app.  For the Mac install, you will need to open System Preferences / Security and give the app permission to be installed. During the installation process, be sure to check the option to install high-resolution maps. We need these for regional forecast work. Here is a short video on getting started. We will later add some with more details.

A review of the steps to download a file and  a couple tips

Step 1. Default location of downloaded Grib files on a PC will be c:/users/username/xygrib/grib. This is similar in the Mac, but it is a hidden file, namely /.xygrib/grib. To see the contents of either one, use the folder icon in the menu bar, next to the quit button. The contents can be edited from within that window, or separate folders could be set up for different voyages or projects.

Step 2. Draw a box with the cross-hair tool in the main menu icon, then press the globe icon to select a model and other options, and download. 

Step 3. After downloading, confirm the file storage location, which will display the data. If not already showing on the left, turn on the View Data panel  ctrl+V (PC) or cmd +V (Mac). As you move the cursor over the map you will see the values showing in the panel. You can pull the panel off to the side. Once taken out, get it back in by restarting the program.

 XyGrib Start up tips

• In the window selection of window position, N and E are +, amd S and W are – numbers. 

• On the menu/Weather map the top parts are closing what uses color overlay, and that overlay covers land and water.  For inland work, maybe save that color for REFC or rain 

• For a bigger view of time and date, click the slider bar at the bottom

Note: the menu /File/New instance creates a new window of the program duplicating what was in the last instance. This can be valuable in some circumstances, ie for comparing different  models or times  side by side, but troublesome in others if you forget more than one is open.

• To update a file, use ctrl+d or cmd+d (Mac). This will fetch the existing file area and the same info that was left in the set up window. Note it will give the file marked with the UTC of the download time, but you will not know from the name if the file is same or different data for the region.

•  In menu Options/Units: set speeds to kts and coordinates to ddº' (note use of European comma for a decimal point.) One arc minute (1.0') is one nautical mile, so we can think through the extent of 12.2' more easily than 12' 24".)

• To see true GRIB file resolution, use menu/Weather Map/Weather Options/Wind arrows on GRIB grid, then zoom in to see where actual data points are located. You can then measure the distance between grid points (tips of the wind arrows) with the the tool from menu/Earth/Great Circle Distance. Then shut this off when done. The true grid point display is awkward for  normal use, so it then can be shut off for a scaled interpolation display.

• Caution, when you include higher altitude data, such as 500 mb or 850 mb surface, you do not get to choose parameters, so you are enhancing file size. There are several that get loaded: height, wind speed (the two main ones we need) plus you get RH, air temp, and theta-e temp, which is not a parameter we use in our weather course—it is a measure of stability, but we now have forecasts of simulated weather radar (composite reflectivity, REFC) which is a much easier and dependable way to forecast squalls.

• Models not yet in XyGrib but expected in the next build are HRRR (regional US weather) and RTOFS (global ocean currents and SST).

• The NAM CONUS available in XyGrib is only 12 km, not the 2.5 km version that would be preferred for regional route optimization. XyGrib does have the Arome data at 2.75 km that can be used for practice in Europe. The HRRR when available is 3 km which will serve US needs for regional work.  (Note that the challenge the programmers face here is offering the hi-res data can overwhelm your delivery system if the user is not careful. Saildocs also limits NAM to 12 km.  File size goes up as the square of the resolution.)

• Practice using right click on a data set to create a Meteotable of the data, which is similar to the meteogram seen in other programs.

• Practice downloading computed skew-T data from GFS model. We discuss these diagrams in our textbook, but we do not include them in our online weather course. This is a nice feature of XyGrib. Expedition is the only other grib viewer app I know of that includes these. Not many mariners use these underway, because we have the forecasts that are based on these, but they do provide a picture of the vertical structure of the atmosphere. 

We have a playlist of videos on this type of diagram, but there are many more, and many live sources online including those with real data. Soundings are measured at 00z and 12z, and their input to the models generally makes the analyses and forecasts made at those times somewhat better than the 06 and 18z data.

Wednesday, January 27, 2021

Updating Internet File Source for OpenCPN WeatherFax Plugin

The echart nav program OpenCPN has tons of very useful plugins. Starting with about version 5.2, this large list of plugins is incorporated right into the main program under the Options tab. At that location you can choose to install a plugin or update one already installed. There is also an option to update the master list of plugins.

A notable plugin is the one called weatherfax_pi, whose primary task is to allow OpenCPN to collect and display live wefax broadcasted weather maps received over the HF radio, which it does well. But a huge bonus of that plugin is its ability to display georeferenced graphic images of weather maps or other related images such as cloud pictures, ASCAT wind data, wave data images, or SST and current data images. Essentially any image can be loaded and then manually georeferenced by defining the coordinates of two points on the image.

But better still, the program includes a vast set of weather maps that are already georeferenced and directly linked to their sources so that the latest images can be loaded with a button click. The source information is stored on your computer (Mac or PC) in a file called WeatherFaxInternetRetrieval.xml. The georeferencing information is stored in a file called CoordinateSets.xml.

Some years ago, the Ocean Prediction Center and other NOAA/NWS departments changed their websites and when they did so, it broke the links in WeatherFaxInternetRetrieval.xml so the US maps would not load any longer.  At that point we made a corrected file and a video on how to install it, which was then incorporated into the program.

That file was then further updated, and the latest ones online as of today are dated 8/22/20 at 17:31. On the Mac the dates are the same and the times are 17:39.

More to the point if we want to update these ourselves, the location of these files on your computer has changed. The present correct locations are given below. You may want to search your computer for these files and remove the old ones from other locations before updating.

We have made a new update to illustrate the process. We made a new set of files that add 3 new Gulf Stream current maps produced by the US Navy. A sample of the new maps is shown at the end of this note.

You can download the new files from



Weatherfax_plugin_readme.txt   (explains what we did)

Here is where you will find the latest ones (if you have the latest edition of the program and plugin) that can to be replaced with these new ones.... Note these are the same except for adding the Navy files.

PC computers:  


Mac computers:  

Mac HD\Users\username\Library\Application Support\OpenCPN\Contents\SharedSupport\plugins\weatherfax_pi\data\

In those locations, find the two files and rename to filename.xml.old then move in the two new ones and check that you get the US maps as well as the Navy Gulf Stream ones we added. Note if the files you find are not dated as noted then you have older or newer ones than we know about.

Once you have downloaded an image and displayed it, the image file is stored for later use in these folders:

PC computers:


Mac computers:

Mac HD\Users\username\Library\Preferences\opencpn\plugins\weatherfax\

Here are samples of using these new Navy image maps (considered the best source) together with the RTOFS currents forecast obtained from Saildocs.  See

RTOFS current data as GRIB file

US Navy Gulf Stream of this region

RTOFS GRIB current arrows overlaid on Navy image

Wednesday, January 20, 2021

Background on GRIB files: Numerical weather models and computed parameters

Beside model forecasts in GRIB format, there are several traditional formats of marine weather forecasts available to us for analysis and route planning. These were available before model forecasts became popular for general use, and they remain the primary resources we must rely upon. 

Traditional Weather Products

(1) Voice synopsis and forecasts over VHF radio for coastal and inland sailing, and over HF radio for ocean and offshore sailing. Zones covered, times, and frequencies are in Modern Marine Weather.

(2) Text version of the identical voice reports. We can get the text forecasts from NAVTEX radio receivers, radiofax receivers, or by email request to NWS or various 3rd party services.

(3) Graphic weather maps of the latest analysis at each of the synoptic times of 00z, 06z, 12,z 18z (z is an abbreviation for zulu, which indicates UTC), along with graphic forecasts for 24h, 48h, 72h, and 96h. We obtain these via radiofax, internet download, or by email request to NWS or various 3rd party services.

(4) We also have satellite cloud images that we can obtain from the same sources: radiofax, internet download, or by email request to NWS or various 3rd party services.

The above products are created by meteorologists at the NWS and OPC, and each is typically signed with the name of the author. To create these forecasts, the meteorologists use multiple resources including live or archived observations from many sources, at many altitudes, along with their own historical knowledge and records of how the surface weather behaved in various circumstances in the past. 

Numerical Weather Prediction

The human knowledge and intelligence that creates those weather products is also guided by numerical weather models of the entire atmosphere, which are effectively artificial intelligence. These are vast computer programs running on giant computers (some of the largest in the world) that can create a snapshot of any aspect of weather, at any altitude, at any location on earth, and they can make forecasts of things to come, as well.

The human meteorologist of yore might note from their knowledge of an area, that the wind speed at Point A will be 12 times the pressure difference in millibars between Point B and Point C... whenever the pressure at B was higher than C; but when C is higher, the factor is 10, and the wind blows the other way. Then they might know that if the cloud cover over the past 12 hours was more than 80%, then the wind at A would be 15% less, and so on. In short, they had a prescription for making the forecast of one parameter based on what they know about other parameters.

Weather models (numerical weather prediction) take such prescriptions to a whole new level; they are essentially elaborate mathematical equations that predict any weather parameter such as wind, rain, pressure, fog and hundreds more.  The wind speed at point A is then a complex function of just about everything! Not just the horizontal pressure gradient on the surface, but also the vertical gradient, and the humidity of the air, and certainly on the shape, texture, and temperature of nearby land, and more. And this equation has to encompass the whole global atmosphere, because what happens at Point A an hour or a day from now can be strongly dependent of the structure of the atmosphere hundreds of miles away and several miles straight up.

The various global weather models around the world are running night and day solving these equations based on the physics of fluids (air), along with various (physical and mathematical) approximations required to solve such complex interactions.

Every 6 hours, at the synoptic times, the models are fed thousands of inputs of global observations from buoys, lighthouses, airports, ship reports, land stations, balloon measurements, aircraft measurements, along with a vast array of data from satellite measurements, such as ASCAT ocean surface winds. These data seed the models, but the goal is not to make the forecasts match any specific data set, but rather to create a more dependable forecast overall.  

This process, called data assimilation, is just as complex as the models themselves, in that the data must undergo extensive consistency checks before it is allowed in, and it must be entered in ways that do not disrupt the main computations, which are so interdependent. Refer to Modern Marine Weather for more details.

Once synoptic-time observations are assimilated, it takes 3 or 4 hours for the global analysis map (valid at that synoptic time) to emerge. This then becomes the basis for the set of forecasts at later times. This  first file in the string of forecasts is called h0, which is effectively the surface analysis at that synoptic time. The 48-hr forecast would be h48, and so on.  Different global models have different forecast intervals and durations, but the h0 forecast will always correspond to the most recent synoptic time. 

The valid time of h0 is often referred to as the "run time" of the model, but this is a bit misleading. The model is running continuously,  and it will not be ready to account for the latest synoptic observations for several hours after that synoptic time. 

As soon as a forecast from any of the global models around the world is completed it is uploaded to a private international network that all national weather services around the world have access to. Thus each nation's meteorologists have access not to just their own nation's models, but to all of them. The meteorologists can then compare all these model forecasts with their own experience and local data sources to form their own best analysis and forecast for the areas they are responsible for.

When the OPC produces a graphic ocean surface analysis map or one of their forecast maps, they are using their human intelligence to evaluate the various solutions from artificial intelligence (the models), along with their own experience, and their own interpretation of the observations that went into the model solutions. The models do not try to fit specific observations in the data assimilation process, but meteorologists making their own map of what they think is right can use the data as they see fit. Many at the OPC know specific ship captain's reputations over years, and can believe their reports, especially if they get supporting evidence from the ASCAT wind data. I have seen quite a few cases where it appears that the OPC have taken a model forecast of an isobar and pulled it over to align with a ship report. 

The ongoing revolutions in practical marine weather over the past 10 years or so are (1) these numerical models are getting better and better every year, and (2) for quite a few years now, mariners can also directly access the raw model forecasts—they are not the private domain of the professionals any more. Individual mariners can now access the model forecasts in the form of digital weather maps that are distributed in a GRIB (gridded binary) vector format that can be displayed in navigation programs much as we display the ENC vector echarts. There are also numerous stand alone programs called GRIB viewers that do not show navigation charts, but do show the GRIB weather maps from the model outputs.

Figure 1. OPC graphic map on the left; corresponding GFS grib file on the right, viewed in LuckGrib

The ready availability of model forecasts has pros and cons. The cons are simply that when we use the model forecasts direct from the computers, we are looking at unvetted data, and it is known for certain that the models have weaknesses, and in some crucial cases they are wrong. The mainstay US global model called Global Forecast System (GFS), which we cover below, usually underestimates the wind speed in tropical storms, often seriously. It is also not dependable near coastlines. When we use GFS forecasts alone, without the filter of professional vetting, we run the risk of being led astray. Also the model output does not show such features as frontal lines, which the OPC fills in manually based in part on cloud images and ship reports.

The pros, on the other hand, are many, and certainly in the long run outweigh the cons, especially when we are aware of the limits of model forecasts and account for them. We cover these in Modern Marine Weather and we have procedures on how to evaluate a model forecast before we depend on it.

Values of GRIB Formatted Forecasts

Once we take those precautions into account, the pros of model forecasts include:

Ubiquitous, easy access 
Essentially every navigation program has the ability to download and display weather data from one or several models. Because of this, mariners who might not have taken the trouble to access proper ocean and coastal weather in the past, now have this data at their fingertips.

Digital format
Just roll the cursor over a wind arrow on the model forecast to get digital wind speed, direction, pressure, rain, and so on.

Versatile, informative displays of weather parameters 
With digital data we can employ such features as meteogram plots of say wind speed or direction vs. time at a specific point;  we can color code the values of any parameters for a quick overview of the distribution; or we can display custom defined contours outlining ranges of values; and so on.

[meteogram and color code]

Interpolated forecasts at any time 
Standard NWS/OPC products are valid at the fixed synoptic times every 6 hours, with forecasts only at specific times, such as 12z 48 hr from an earlier 12z surface analysis. GRIB formatted model forecast maps, on the other hand, can be interpolated for any time at all. We can show how a map evolves in one hour steps, or we can ask to see the map at any specific time within its forecast range. With OPC maps we are interpolating between 6-hour intervals, but the models actually compute forecasts for much smaller intervals, down to one hour steps for many, and at least every 3 hours for most, and indeed some have sub-hourly forecast intervals.

Consistent region coverage for analyses and forecasts 
The OPC forecast maps vary in the Lat-Lon region covered depending on the forecast time. Surface analysis covers one region, the 24h forecast a different one, and the 48h and longer use still a different one.  When we digitally request a model forecast in GRIB format, we specify the region we want, and all forecasts are for the same region. The h0 or first map of any sequence will always correspond to the most recent synoptic time and thus represent a surface analysis.

More available parameters 
With standard OPC maps we get forecasts  for wind, pressure, sea state, and in the tropics we can also get tropical storm tracks. With model forecasts we can get very many  more parameters. The GFS model, for example, has over 300 parameters. Beside rain, cloud cover, we can also get simulated weather radar forecasts that are useful for squall forecasting, plus air temp and relative humidity, and many more. We can also request data for the boundary layer conditions at 850 mb. With OPC maps we get surface weather and winds at 500-mb level, although weather at other levels is available in aviation products.

More than just model output 
ASCAT satellite near-live ocean wind measurements are available in GRIB format. At present Expedition,  LuckGrib, and Panoply are the only programs I know that can display the ASCAT GRIB data. The NWS also makes a set of GRIB files that are the digitized versions of their own forecasts, which is not the direct output of any one model. This dataset is called the National Digital Forecast Database (NDFD).

Waves and currents 
The standard GRIB viewers we use for weather data can now also be used for ocean model products such as detailed wave data from the WW3 model and ocean currents and SST from the RTOFS model.

Longer term forecasts 
OPC forecasts go out 96 hour (4 days) created once a day. This limit is largely do to the fact that this is about the reasonable limit to dependable forecasting for ocean weather. At modern standards, 3 days (72 hr) is typically very good forecasting, but at and beyond 96 hr becomes increasingly questionable. But that does not mean that the longer term forecasts are certain to be wrong, and not useful. As noted below, we can use these as a guide to what happens, and we do have ways now to assign some level of dependability to them using computed standard deviations (from the ensemble models) and specific probabilities (from the National Blend of Models). 

Optimum weather routing 
Another crucial value of the digital GRIB forecasts is we need data in this format in order to carry out computations of optimum routes for sailing vessels. This is a key component of the future of weather routing and the model data for wind, currents, and waves are the basis for it. The longer the forecast the better for this purpose, but we need procedures that emphasize the first few days of routing, with ways to update the route daily if not more often.

Grib2 Weather Parameters

The full list of Grib2 parameters and categories contains way more than we use at sea, but you may find some of interest for land based interests.  The primary ones we care about for marine weather are:

Atmosphere (GFS and others)

Pressure reduced to sea level (PRMSL)

Wind at 10m (as E-W and N-S components, or as scalar speed  WIND)

Wind gust at 10m

Rain, accumulated or rate (APCP, PRATE)

Cloud cover, usually total in % (TCDC), but low, medium, and high are available.

Temperature at 2m (TMP)

Relative humidity at 2m (RH %)

Dew point at 2m (DPT)

Convection, several parameters used by some (CAPE, CIN, LI, others) See Table 3.4-3 in 

Modern Marine Weather 

Simulated radar, composite radar reflectivity (REFC), good squall indicator. See 

Modern Marine Weather 

Visibility (VIZ)

500 mb, height of the surface and wind speed

850 mb, height of the surface and wind speed

Ensemble forecasts (GEFS and others)

Wind and pressure from individual components

Mean values of all components

Control value used 

Standard deviations for wind and pressure

 National Blend of Models

Probabilistic winds (10%, 25%, 50%, 75%, 90%) Oceanic domain

Standard deviations for wind and pressure, and other parameters. CONUS domain

Currents (RTOFS, OSCAR)

Current speed and direction

Sea surface temperature (SST)

Sea State (WW3 and others) Note these terms; they are sometimes mixed up online.

Significant Wave Height of wind weaves (WVHGT)

Significant Wave Height of Swell Waves (SWELL)

Significant Wave Height of the Combined Seas (HTSGW)  

Direction Wind Waves come from (WVDIR) 

Direction of Swell Waves come from(SWDIR)  

Mean Period of Wind Waves (WVPER) 

Mean Period of Swell Waves (SWPER)  

Primary Wave, Mean Period (PERPW)

Primary Wave, Direction it comes from (DIRPW)

Wind Speed and Direction... is offered in WW3, but best to get this from GFS. There is no special value of this wind data... except it comes with the waves, which is a virtue if your viewer app will not let you show two grib files at once.) 

Other global wave programs offer the same parameters, but can have different abbreviations for them. 


Categories of Digital Forecasts in GRIB Format

Numerical weather prediction (NWP) models use essentially the same parameters defined by the grib2 standard, but they compute them using quite different physical and mathematical approximations. It is fair to ask if we really benefit from having so many different models, but they each argue they are unique in one way or the other. We consider here just an example or two from the most basic categories.

Global Models
One distinction or category is global model vs regional model. Global means just that, it covers the whole earth. The premier US version is the Global Forecasting System (GFS), which in its latest version could well be the best available worldwide in the most circumstances. Other global models include the UK Met version (UKMET), EU version (ECMWF), Canadian (GDPS), and a few others. The US Navy also runs a global model (NOGAPS), independent of the NOAA products. Properties of the GFS are listed below.

Regional Models
Regional models start with a global model base and then fine tune it with higher resolution, smaller  forecast steps, and more frequent update times, using more thorough accounting for the effects of shape, texture, and temperature of adjacent terrain. They generally cover much smaller regions, and do not extend very far out in time. Notable US regional models include North American Mesoscale (NAM) and the High Resolution Rapid Refresh (HRRR). Properties of both are listed below.

The weather research and forecasting (WRF) is a regional model run by multiple universities and institutes around the world. They tend to focus on their own local regions, but often indeed provide the best forecasts for those regions.

It is fundamentally important that we choose the right model for the task at hand. We cannot rely on global models like the GFS to give us dependable forecasts for inland waters. We must use a regional model for that.

Ensemble Models
Another distinction between model forecasts is whether they are deterministic or probabilistic. Deterministic models compute (determine) specific parameter values, at a specific points, at specific times. The GFS and regional models mentioned are all deterministic. In contrast to that are ensemble models that run the same computation many times with slight variations in each run and then average the results. The spread in the results (standard deviations) is a measure of the probability of the answer being right.

Each run varies some element of the computation that could impact the results. This usually involves varying some of the input data (which has a statistical uncertainty)  and varying some physical or mathematical approximations in the solutions, which work better in some cases than in others. The US ensemble model is called the Global Ensemble Forecast System (GEFS). This model has several outputs we can view:

Control run. This is the base model with no perturbations. Usually same as a GFS run.

Mean run. This is the working output we use to represent this model; it is the average of all the individual runs. This mean run will also have a parameter showing a standard deviation (SD) for wind and one for pressure. Generally we see the SD being low for the first few days, and then grow larger, reflecting an increasing uncertainty in the results. In some cases, even in the early forecasts, we see small SD in one part of a forecast region, but larger in another, which is our alert to be careful in those larger SD regions. Use of SD and probabilistic winds in general are discussed in Evaluating a Weather Forecast.

Member runs. These are the 20 or so individual runs, each with slightly different initial conditions. Depending on our GRIB viewer, we can look at these individually or all at the same time. When doing optimum routing computations, these member runs show the extremes of what the best route might be.

Model Blends
The National Blend of Models (NBM) is a combination of many models, including some international products. It is available in a near global version as well as a Continental US (CONUS) version and several regional products (AK, HI, etc).

NBM CONUS includes standard deviations, which  makes it one of the best for regional models where there is coverage. This combined with the HRRR model are the workhorses for inland and near coastal weather. This dataset includes standard deviation.

NBM Oceanic is a near global product that includes probabilistic forecasts. This is likely the best long term model (>4 days). For shorter periods, the GFS should be consulted, but still keeping in mind the probabilistic winds of this forecast. This dataset includes these probabilistic winds: P25 is the wind at the high end of the lowest 25% of all wind solutions at that point; P75 is wind at the low end of the highest 25% of all wind solutions; and the mean wind called P50 is the average of all solutions between P25 and P75. The P50 wind represents the middle 50% of all wind solutions for that point.  See Evaluating a Weather Forecast.

National Digital Forecast Database (NDFD)
The NDFD is a unique GRIB formatted datasource and potentially the most important way to access US forecasts for local or ocean waters, but it is not yet fully implemented. It is not a model and not a blend of models, but rather it is the digitized versions of the human intelligence discussed above that makes the US forecasts, which are of course aided by all the model outputs. Since this dataset includes input from the local forecast offices there is sometimes a coordination conflict that impacts the GRIB formatting. Also, although the OPC does create their forecasts in a digital format, they do not offer this data to the public so far. See this note for further information on the NDFD.

Basic Properties of Selected Models

From our textbook Modern Marine Weather

Sources of Model Data in GRIB Format

A primary source for GRIB forecasts is the NOMADS (NOAA Operational Model Archive and Distribution System) site. 

If we do have to come to this site because our favorite provider does not have what we want, then note there are three options: grib filter, https, and gds. The "gds" is  for unix based custom software that most mariners will not have access to. The "https" does link to GRIB files, but they might not have a .grb or .grb2 extension. They will still load in some GRIB viewers but these are huge files (hundreds of MB) that cover the whole earth including all parameters and for one time only. In short, not a practical source for most mariners. 

The "grib filter" option lets users choose the lat-lon box and the parameters desired, but each file is for one forecast-time only. There is also a learning curve to applying the filters. We have examples online, but this is still not a convenient method.

What we want ideally is a nav program or GRIB viewer where we can, with a few button clicks, specify exactly what we want, and then the program goes and fetches this for us and displays the results. Most nav programs include some level of this GRIB file request. State of the art nav programs like Expedition have the most options; dedicated GRIB viewers such as LuckGrib for Mac and iOS have even larger sources of data, but these viewers do not show nautical charts and are intended for weather analysis with primary chart navigation being done in a different app.

A compromise solution is a program like the free OpenCPN chart navigation program (Mac and PC), which can display GRIB files, but you must obtain them from other sources. A common external source is Saildocs, who provide basic model data by email request including global and some regional models. See Modern Marine Weather for details. OpenCPN has an automated way to create a Saildocs request.

In our weather course we recommend starting with XyGrib. This is a combined GRIB viewer and data source that is free, with both a PC and Mac version. The folks that produce and support XyGrib also provide WRF model data for several locations in Europe as well as other global data.

The free program for Mac and PC called qtVlm is also an excellent resource. It incorporates accessing GRIB files from both XyGrib and from Saildocs. It is a full navigation program as well as GRIB viewer.