Weather Analysis, A Step-By-Step Guide—Part 1, Tactical

Reading Time: 10 minutes
Sailing in these conditions (Near-Gale Force 7) is way more fun when the wind is aft of the beam. Good weather analysis can help make that happen.

In the last two chapters I introduced the concept of strategic weather analysis, in addition to the tactical that most cruisers do, and then detailed the hardware and software I use.

In the next three chapters I'm going to detail exactly how I go about downloading the information I need and analyzing it on a daily basis—a step-by-step guide to both tactical and strategic weather analysis.

So imagine we are in a remote anchorage on the Quebec Lower North Shore of the Gulf of St. Lawrence—easy for me, since I recently was—or, alternatively, several hundred miles at sea on an ocean passage (the steps are the same although the goals of the analysis a little different), and it's time for our daily look at the weather.

I usually do mine first thing, but any time will do, although I do suggest getting into a routine of doing this at the same time each day.

Let's do it:

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Robert Muir

Great info John! You should make this a “free” chapter. If this doesn’t whet their whistle for the rest of the information linked in it, nothing will.

Dave DeWolfe

This is excellent! I agree with Robert that this should be a ‘free’ chapter …. because I want my candidates for Yachtmaster exams to be on top of passage planning. I recommend to them all that they buy an AAC membership, but I doubt that all of them do.


Good information? From my little point of view the information spread and shared on AAC (including most comments) is invaluable. I have to say, since registering my account here I have upgraded from a lousy to a less lousy sailor, and the reactions of my crews show it. Actually I have been able to implement a lot of Johns (and others) thoughts and ideas on charter boats as well, and it has been helping tremendously.

Steve Payment

Great overview. I like how you give a step by step process to creat your own forecast then compare to the pros. Doing it daily also helps learn common weather patterns. Thanks!

Marc Dacey

It’s like splicing or sextants, really: Practice makes (near) perfect.

Emile Cantin

Hi John,
Nice writeup of your process, and I especially appreciate that you provided links to all your information sources that you actually used. I’m further up the St-Lawrence (a bit upstream from Québec City), so pretty much all of them are directly relevant to me.

Dick Stevenson

Hi John,
Excellent series.
I would want to underline your suggestion that writing it down helps weather/passage/cruising planning in multiple ways. I do not know any data that puts me in a trance like state that leads to forgetfulness faster than weather data.
I will write down the days/dates in 6 hr intervals out 4-5 days or so or longer leaving lots of room. I then, for each time, put down the forecast winds/direction etc for that time from each source I am using. Starting days ahead of time, I then end up with a sense of how the pattern might be evolving, how different forecast sources are seeing the weather etc.
I find that without writing all the information down in this spread sheet/crib sheet manner, I forget and get it all jumbled up.
As you said, it sounds like a pain, but after you reap the rewards of being able to think clearly about upcoming weather and plan accordingly, you may find it quite satisfying. Fitting in well with weather is immensely gratifying. Getting on the wrong side of what Mother Nature puts out can ruin one’s day.
My best, Dick Stevenson, s/v Alchemy


Each does weather analysis and route planning a tad different to suit their own perceived needs, time, and budget. As mentioned above it is worthwhile to write the plan down.

What is harder to find is information of how beneficial the effort in developing the plan actually was. To get a “feel” for other’s actual results versus their plan, I monitored the passage logs (to include SSB updates of their actual conditions) of different real-time bloggers that used an assortment of techniques and paid professional forecasts as they traversed from Figi to New Zealand.

In almost all cases the weather analysis and route planning did not mirror the actual conditions experienced. Depending upon selected departure times as little as one day, the conditions experienced varied widely from too little wind to gut busters.

It might be beneficial to substantiate weather planning techniques with actual real data results to validate the worthiness of the planning effort. It would be valuable if an objective analysis was provided as to what matched the plan, to what facets didn’t, and speculation as to reason why? Sometimes, the why will just be ceded to the whims and unpredictability of Mother Nature. Anyone willing to give it a shot?

Regardless, the old advice of have fun and expect the best, but be prepared for the worst, remains pretty sound and validated.

Marc Dacey

I am reminded of how Herb Hilgenberg used to first ask for the local conditions (given in a strict order so he could transcribe them efficiently) on the SSB. He would then give you back “personalized” weather routing based on your desired course but using the macro views described here with data points given by boats within a couple of hundred miles. He was surprisingly accurate with this approach and we were able to handle 30 knot breezes when 40-50 miles away, race boats were dealing with 50 knot gales that caused a couple of them to have to retire. The point being that if you can develop these skills and have the means to contact other boats (or to see their weather data), you can refine your own forecasting to take into account “micro” factors as well as general tendencies. I’d rather have a steady 30 knots in some respects than 18 gusting 36, and I would certainly like to know if the second case was in store.


Hi John,

I admit I read this with a bit of trepidation, because I almost always learn something new on here and then have to change how I think about things! However, I was pleasantly surprised that the methods of weather forecasting I’ve developed on my own eerily mimic what you’ve described above. So I feel very good about that! You put it into words very clearly too. Looking forward to the next one.



Your explanation makes sense to me. But it presumes a certain level of comfort and ability with this data to start off with. Can you point me to the resources that would bring me to the level of ability that you describe in your caption on the second picture, “…shows that we will have a weak low and frontal passage in a couple of days…” All the rest of this is pointless for me if I don’t know how to read the data. Not complaining, just aware of how far behind I am in my meteorological ability. Will definitely keep this process in mind once I get better.


Hi John,

Thanks again for a very good article.
Just for the completeness sake (since this site is the best true-bluewater resource on the net…) – one possible subject for a forthcoming article could be what kind of procedure to use when sailing outside NOAA, EU and Australian forecast areas on the southern seas – or indian ocean west of 90E.
There we seem to pretty much fall back to gribs and whatever else we can get, since it appears that SA, Chile, India etc. services only produce “streamline charts” (with ~1.1M file size), not eg. +24h synoptics.
Additional challenge there is sparse wx station and ship population, that could complicate proper seeding of the GFS etc. models. So, extra vigilance out there shouldn’t be exessive.

As it seems, on “odd waters”, the only way to complement gribs and boat’s own realtime weather observations is:
– ship wx reports (sparse),
– sat images (preferably received directly since file sizes)
– and ASCAT wind measurement gribs.
It’s just that none of them seem to come easy…
– ship reports could be extracted from , but they don’t go to chart without some kind of script.
– sat images direct is trickier now that analog APT service is suspended. Direct LRPT chart receiving is doable but geeky, and gear doesn’t come free.

Just a thought 🙂
Cheers, JCF

Craig McPheeters


One of the weather courses I took was by by a former meteorologist and he described the system where soundings were taken of the atmosphere – where measurements are taken via a balloon raising a radiosonde to record a column of temperature/pressure/wind/moisture/etc. I was taught that these measurements are then used in the initialization of the weather models. Given this understanding it makes sense that weather models would be less accurate in areas there are fewer ground stations.

However, modern weather data assimilation systems take into account many types of input. GFS was updated by NOAA this summer, the current version is now ‘GFS-2017’. Part of the notice that was sent out contained a link to a page with more information. All weather models need an approximation of the atmosphere, the initial conditions, for the computer simulation to run and produce forecasts. For GFS, the initial conditions system is called GDAS – global data assimilation system. As part of the upgrade there are many improvements to GDAS, incorporating new data into GFS. For example, one of the points that is called out in the notice is:

“- Extend Regional Advanced Television Infrared Observation
Satellite (TIROS) Operational Vertical Sounder (ATOVS)
Retransmission Services (RARS) and Direct Broadcast Network
(DBNet) capability.”

There are lots of lines like that, where you barely understand what they are saying but its suggestive of a very interesting capability – measurements independent of ground stations and so available across large areas of the world.

It turns out that modern weather data assimilation systems and the weather models accept initial conditions from a wide variety of sources. This includes satellites and can also include commercial aircraft, for example. The US HRRR regional model accepts high resolution radar imagery.

So – the initial conditions are probably much better in areas where there are many land stations, but, depending on the weather model, the model may be accepting data from a wide variety of satellite and other sources, so even in areas over the open ocean or other sparse areas there will be some guidance in the initial conditions.

Craig/s.v. Luckness/LuckGrib.

Emile Cantin

I couldn’t resist automating the download of the NOAA weather maps (and rotating the first one):

This script should work pretty well when you’ve got a proper internet connection.

The list for the other oceans is here: You can adapt the script to download the proper files for your location.

Carlos A. Mendoza

Thanks for the great articles John.

Just in case it helps someone, I thought I would emphasize the fact that if you do use the NOAA FTP server via email, “Attachments are received in UUencoded form”

If you are using the native mail app on a Mac, which does NOT support that protocol, you will not be able to “see” the associated TIF files without using external UUdecode software to decode the email.

I spent quite a bit of time thinking I was doing something wrong in my request email. Luckily, I decided to take a look at the “Raw Message Source” & realized the file data was in fact there, but UUencoded.

That said, Google’s Gmail does automatically decode the attached files, so I started using my Gmail account to retrieve said files.

Again, hope this helps.

Glen Doyon

Sorry if I missed it, but what commands do you type to obtain the weather map (big picture) forecast? I.e. Not the GRIB commands. Thanks g


John, comin’ at ya again. Do you have instructions for retrieving GRIB weather files using cell phones? You mention that it’s easier, but all I see is a Sailmail query or UUPlus using standard email, but you said that they were not necessary. The cellular steps may be here, but I have yet to find them.


Never mind! If I read it correctly in a later chapter, all I need do is copy and paste your instruction line into an email to moc.scodlias@yreuq and since I’m currently using a PC at home, download Viewfax and start practicing. Of course, I will need to figure out how to ask for weather data for the areas that I plan on sailing in. Hopefully, that’s here somewhere too! Ok, so I’m a little dense, but I went back and looked at your request line and viola, there were two lat and two long parameters that obviously define your target area. So, I just change those to mirror my intended cruising area. I get it.


Good morning, I’ve been downloading GRIB files and analyzing per your steps and producing my own forecasts, but I’m not seeing the Gust parameter on my viewer ViewPlus, even though I sent the same string that you did to saildocs for GRIB files. Should the Gust parameter show up in the popup box wherever I place the cursor on the graphic? All I see are wind and wave. Thanks

Andre Langevin

USCG is conducting a survey probably to gauge the value/usage of old fashion radio fax and HF. We as a community of high seas traveller should raise our opinions.

The United States Coast Guard and the National Weather Service are conducting a joint survey of the maritime public in order gain a better understanding of the current use of Marine Weather Information, and to determine the level of interest in potential future products and dissemination methods.

The maritime public is invited to participate in the survey, which can be accessed using the following link: