The Offshore Voyaging Reference Site

Weather Analysis—Hardware and Software

This is a new version of a chapter I wrote back in 2017, part of an ongoing update of our entire Weather Reception and Analysis Online Book. The rewrite of this chapter was so extensive that it made sense to republish it, rather than just add it to our Recently Updated area.

Also, we rewrote chapter one, so you may want to read that now to understand why all this matters.

In the first chapter, I introduced the concept of strategic weather analysis as an addition to the tactical version that most cruisers do.

In this chapter, and the next three, I’m going to get down into the details of analyzing weather on a day-to-day basis so we can plan our cruises as much as two weeks out, and to evaluate the risks of a forecast or GRIB being wrong in a bad way.

But first, let’s look at the gear and software.

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Jonathan Schwartz

The LuckGrib module for IridiumGo! downloads is excellent. It is far and away the best compression scheme I have seen for grib files. Very efficient and it auto-tailors the fidelity of the downloads based on your target file sizes while showing you exactly what it is doing and giving you an option to override the suggested settings. It is also notably faster downloading data than PredictWind when you compare the file throughput bit for bit over IridiumGo! We use both daily at sea and remote cruising as they have different advantages and disadvantages as tools. Windy is still our preferred tool for keeping a general eye on the weather when coastal cruising.

Nick Dawbarn and are two completely different groups (confusingly) and both offer a pro subscription. For my money seems like the better option, but there are features in each that are unique and useful (eg I’d love a home screen app for, but only seem to have one). Just don’t get caught out paying for pro on one and expecting the full feature set on the other.

Robert Newman

Have you looked at Ventusky? I haven’t looked at it in depth but it seems to have merit.

Seth Winnick

I second Jonathan’s observations on LuckGrib’s relatively blazing speed for grib downloads via IridiumGo! The files come through in a fraction of the time for PredictWind downloads. I have not found the PredictWind weather routing function to be helpful, perhaps due to lack of practice, but I have had good success with LuckGrib weather routing on passages between the Chesapeake and New England/Maine. Granted, these are not long or distant voyages, but over a two to three day period, I find the boat’s progress stays close to the weather route-predicted position. I use a conservative set of polars, and dial them back for short-handed sailing to about 90 percent for daylight and 80 to 85% at night. I also regenerate a weather route at least daily when updating the gribs. (Actually, I generate a variety of routes and decide which I think best mirrors conditions.) Overall, the module is a useful tool in figuring out how to apply the grib data.

Dick Stevenson

Hi John and all,
I generally think it is wise, when using routing features of various programs (and for-hire routers), especially when using them for working out weather windows for departure, to work your own route and weather forecast first and then use the outside generated route as a check on your judgments and as a back-up: paying particular attention to discrepancies. Generating your own route gives a feel for the conditions that are likely to occur that relying on an outside generated route is unable to do. And searching weather conditions well beyond the route allows one to see possible occurrences not forecast: such as, how will my weather change if that low track a few degrees higher in a few days?
Not only can you generate a better feel for the coming weather, but you pay attention to your boat and its performance in a far more intimate way: much different than relying on polars: again, they can be used to notice discrepancies and give a base line.
Then when the weather picture changes from forecast, you are in a far better position to respond to changes in an informed way. This is especially true if new outside generated routing information is unavailable when on passage.
My best, Dick Stevenson, s/v Alchemy

Michael Jack

Hey, John. Thanks for updating this and making me finally bite the bullet on getting organised here. A couple of questions:

  • You say “In addition to a way to download and view GRIBs, we also need a way to get and view weather maps and forecasts available as PDFs, graphics and HTML pages from the internet.” You then say that Saildocs gives you all that. But as far as I can see, Saildocs doesn’t download any of the above. I just turns everything into text. What am I missing?
  • I decided to use XyGrib since I am on a PC (will never convert to Apple). XyGrib downloads most of the Gribs from within the software. The ones that it does not, I can get from Saildocs (such as current). But, I saw a David Burch video where he is using OpenCPN to load a surface pressure Analysis on top of a Global Model to do a comparison (seems sensible to me). Unfortunately, he doesn’t say where he got that Analysis from as a grib and I can’t see that on offer with Saildocs or anyone else. Any ideas? Btw, I couldn’t get OpenCPN to work. Seemed very buggy to me.
Michael Jack

Thanks, John. Can’t wait.

Michael Jack

Oh my, I don’t feel good about that, John. Its quite like proving your dad wrong (although our age differences are quite that vast) 😉 Not to preempt your next section, but the only thing I found that might be images are their pre-canned images show in the “Index” email. Having said that, I haven’t successfully got one of them yet as I always get a message back with “XXXX is not a valid command or document code”. I am also getting quite varied success on subscribing to things. Some come regularly and some don’t (I always assume PEBKAC but I have tried many times). Looking forward to seeing how you go in the next article.

Simon Robinson

If it helps, I have used saildocs to email a single periodic pdf, such as:

sub time=08:00 interval=12 days=45

so you can for eg get a full surface analysis daily – albeit sizewise that’s a long long wait on an Iridium Go!

There’s also services like this:
which for a fee will capture and email a screengrab of any page at timed intervals. Blit’s focus is really the testing and support of websites, but it also works okay for us.

William Murdoch

I don’t use Saildocs. I am a Winlink user. I have received weather fax over Winlink via HF radio. I just tried Settings, Winlink Catalogue Requests, WX_FAX, PJE188.TIF Gulf of Mex & Trop Atl Wave/Swell 48 hr Fcst Wefax. I got the file in my mail in just a few minutes and opened it in Windows Photo Viewer. I’m surprised that the same thing is not available over Saildocs.

Dick Stevenson

Hi William and others,
I also used Winlink to get weather data in just the same manner, but it should be noted that Winlink is an Amateur (Ham) Radio  program that is not available without a Ham radio license (fairly easy to get) and some way to receive the weather product: usually a SSB radio.
My best, Dick Stevenson, KC2HKW, s/v Alchemy 

Douglas MacDonald

Can anyone recommend a decent service provider for the unlimited Iridium Go plan? The company I was using has been acquired and their service is now terrible. I am based in the United States. Ideally they would also offer email through the Iridium plan

Thanks in advance,

Douglas MacDonald

William Murdoch

I’m not a user of Saildocs, but this seems to be a way to get a tif file of a rfax chart via Saildocs.

Go to and click on Gulf of Mexico, Condensed Version.

Hover over 24 hr Wind Wave Forecast, TIF to get its URL and remember PYEE10.TIF .

Send an email to moc.scodlias@yreuq with any subject and the message, send PYEE10.TIF .

In a couple of minutes you will get a reply email with a 11.3 kb attachment of the rfax chart.

You might want to make up a list of the files that you want and test them.  I don’t know if it works for all the products.

Terence Thatcher

A great synopsis, thank you. When my son Emerson and I sailed to Polynesia (pre-pandemic) we had an Iridium sat-phone with external antenna and used something called an Optimizer to create a hotspot. We used XGate as our email server and to download Predict Wind. All using a computer running Windows. We thought of using UUPlus, but at the time, they only were set up to use an older Windows program, so it was not compatible with our laptop. I am generally computer illiterate, so Emerson handled the set-up. At the time, also, I think you were less enthusiastic about the Go! Has that changed? What is its advantage over what we did? ( I plan to go offshore again, so am thinking about options I can understand, even if Emerson is not there. (E.g., our system kept turning off if the sat signal dropped for more than 5 or 10 seconds. He fixed that mid-voyage. It was all in the Southern latitudes and I understand there are more satellites .) So, for a person like me, who needs it simple and pretty stupid-proof, what would you suggest?

Peter Kleim

I used LuckGrib regularly during my Atlantic crossing last year and in Europe this summer and and can confirm that the download speed via Iridium Go is excellent! Way faster then PredictWind. However, PredictWind improved in the last years: for instance superimposing fronts on the Gribs. That’s a very fine service.
I wish LuckGrib or PredictWind could also handle (man made) OPC surface charts – then I would have everything in one program to compare.

Peter Kleim

Good news, albeit pricey indeed! However, downloading surface charts is one thing, showing it georeferenced (with position of boat) is another thing. That would be really needed to have all weather on one platform in one work flow. Will that be possible?

Andy Schell

I’ve evolved a lot of our weather analysis from reading John’s stuff. Quick notes –

1. You can do this well on an iPad once you’re familiar with the device;
2. LuckGRIB works SO GREAT downloading directly from Iridium Go!, we do this daily offshore;
3. xGate is a great day email software for iPad that we’ve used for 10+ years.

So we do everything sat related on the iPad from weather analysis to email to our onboard checklists and it works 🙂



Hi John.
Since reading your article, I have acquired LuckGrib and have been using it on an iPad and a MacLaptop. The GUI menus and function keys are unfortunately different between the two. While the basic results are the same. Biggest issue was in how to define the area for the weather grib so that you get usable data formats. It does not work well if the area of data is only 2 days when you are expecting 15.

Testing the Grib file data displayed on the LuckGrib GUI with the NOAA available GFS charts was a help.

I have been working with a friend in the Canaries who is returning to the Caribbean. We were looking for an opportunity to set sail. Once the structure LuckGrib was discovered, generating repeatable weather routes was accomplished and I became a bit more confident about the results generated. I am still working with both iPAD and Laptop and will hope to smooth out the process of updating the weather data on a given route.

It is very evident that one should spend a couple of weeks pre-departure working with the software before untying the lines. I have had the benefit of fast internet data downloads using both my office and my iPhone as a hot spot. I will be exploring the Iridium system in the new year.

I would like to get your thoughts on how you set up the various grib download parameters for LuckGrib (LG). Why you chose one source over the other. The LG programer suggested the value is to compare different data sources. I have done this with Windy yet the data selected to drive windy is preprogrammed and not variable. This means you can get GFS or ECMWF source data and compare different models for divergence or congruence to help establish a level of confidence in the report.

Thanks for the weather resources shared.


What I was curious about are the parameters for the grib files from the various data resources that you find most useful. Checking all of the boxes for all data options puts too much data on the screen as to obfuscate important data from nice information. I can go through pushing each of the buttons one at a time, or I thought “ask to tap” a bit of your experience with the tool.

Brian Russell

We still have our Iridium Go and Unlimited package, and glad of it. Our vaunted Starlink decided two days out from Norfolk recently to park itself in a sulk. Conditions were very rough and apparently the Starlink doesn’t like being tossed about and will simply hibernate until calm waters of an anchorage are reached. Upon reaching Guadeloupe a few days ago it started right up. Plus, who needs to look at the news whilst on passage? Regarding evaluating models: We now just look at GFS and ECMWF when offshore. How to decide which is the more accurate? Look out the window! And look at your logbook, which should show the wind trends for the past day. If GFS is predicting 18-21 but you’re seeing 25-28, then obviously it’s not today’s best model. Overall, on transatlantics and north-south transects we have found ECMWF to be the reliable model about 75% of the time. Look at the Gusts, not the Wind. Add a few knots. Look at the Rain to visualize fronts. We use PredictWind and it has slowly gotten better. I still wish It would show H and L in the highs and lows. Their weather routing is useful, just don’t be a slave to it. We re-route from current position twice a day. It can help you to make decisions about gaining easting versus making rhumbline course, etc. Your polars are vital. I’m trying our PW’s DataHub which records our conditions data and incorporates into a customized polar, especially useful as we are a super-custom boat not listed in the polars database. I have tuned the generic polar to reflect our actual speeds in various conditions.
We also still pay a weather router when on passages of 4 or more days. There are features like Waves and Trailing Lows that are difficult to distinguish on GRIBS , at least for us amateurs. A weather router gives us a professional human interface between the raw data (gribs) and us. We recommend Chris Parker at Marine Weather Center for western Atlantic /Caribbean and East to West Transats or Commanders Weather for northern routes or transats.

Brian on Helacious
Currently Pigeon Island, Guadeloupe