Weather Forecasts at Sea

When you raise the question of the level of confidence in the forecast, I feel you illustrate exactly why it is good practice to watch the trend as forecasts are updated. By watching the trend develop and seeing if the forecast is changing or steady you begin to develop a level of confidence in the forecast.

Having said that, For a long time I have argued that weather models would help mariners a great deal if forecasts were issued with a “confidence level”. Hopefully the three hour forecast on the GFS model is very likely to be sound. But what about the 24 hour forecast and 48 hour etc?

Some companies (not Tote) require a weather class for some deck officers. I took the heavy weather avoidance class when I first started sailing C/M

One of the takeaways was to save the surface analysis and forecast charts and compare, for example, the 48 hr forecast to the surface analysis for the same valid time. In other words when the surface analysis comes in compare it to the 48 hr forecast from 2 days earlier.

The forecasts are actually quite good but the comparison will give a sense of what conditions makes them less trustworthy. Any forecast that I don’t understand or have doubts about I read the dissusion,

The other takeaway and the main point of the class is learn to use the upper air charts. Professional weather routers for shipping start with upper air, then surface air and last sea.

The book I use is Heavy Weather Avoidance and Route Design by Ma-Li Chen and Lee S. Chesneau. Some of it is tough sledding but once some key concepts are understood looking at the upper-air charts sheds a lot of light on what is happening on the surface.

I make heavy use of the weather / routing software but I also make sure the watch always keeps the most current text weather from the SAT-C posted on the bridge.

Finally I have access anytime to the weather routers even if I am not using routing on that trip. I’ve emailed them several times with questions and called them on the phone a couple times as well when I didn’t like my route.

In most of the vessels I have commanded apart from the offshore industry my vessel has been a voluntary recording ship and the deck officers send a weather report every 6 hours. The instruments, such as a precision aneroid barometer, have been supplied by the Met office or NOAA. In the case of the latter organisation we also carried out expendable bathythermograph drops and atmosphere sampling, both of these required little input from the watch officer and the report was sent automatically by a dedicated Sat-C.
My career at sea began in the Pacific well before any of the aids we now take for granted and accurate forecasts did not exist. The masters I sailed with relied on observation and experience to avoid hurricanes. It is my firm believe that having junior officers to be aware of the weather systems around them and reporting of it will serve them well as their career progresses.


There is no doubt that weather forecasting has changed beyond all recognition. Those of us from the UK will recall waiting up for the midnight shipping forecast and listening to “Sailing By” on Radio 4 long wave. Back them we trusted the Met Office as gospel.

Not we have computer modelling supplied with up to date data from thousands of sensors all ove the world… The forecasts are better, (indeed, right now I am looking at the weather for next weekend in preparation for a trip across the Thames Estuary) but I miss the romance of waiting for the forecast.

Better information is more easily available today. That has to used to meet the demand for increase schedule compliance with less risk of excess vessel motion, less vessel motion in general and so forth.

Mariners have to learn how to adapt to this situation, need to extract lessons learned from past experience and apply basic principles of seamanship to the new tools, to the new situation.

Instead of thinking how specific tasks were done “back in the day” need instead to think what were the principles behind those tasks and how the same principles can be applied today.

From one forecast model run to another, the best way to determine or to have confidence in the forecast is consistency.
The more consistent the forecast is from one run to the next and from one day to the next will tell you have much confidence you should have in the actual forecast.
The less consistent it is from one period tot eh next or one day to the next, should give one low confidence in the overall product.
That is one of the keys the forecaster uses and will usually remark about in the forecast discussion, if one is available.

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I carried a copy of the Mariner’s Weather Log magazine (winter 1995) that contained a detailed article on the use of the 500 mb chart by Lee Cheneau with me every work tour for the rest of my career. I got to meet Lee when I took the heavy weather avoidance course at MITAGS in 2000.

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I remember when getting the weather on the SSB was hardly worth the battery it used the forecasts were so far from reality. Now I can get a micromanaged weather and routing from Commanders Weather or even a buddy back home who knows how to read a weather chart.

I didn’t start paying attention till we got internet on board which made getting the charts easy.That’s when I bought the book and started paying attention. If I’d realized how useful the 500 mb charts were I’d have done it sooner.

Mariner’s Guide to the 500 – Millibar Chart

The professional mariner and ocean going sailor can use the 500 mb analyses and forecasts, in combination with surface pressure and wind and wave charts, to better understand and anticipate the workings of both the ocean and atmosphere. It first takes some basic knowledge of marine weather, not just surface weather charts but also 500 mb pattern recognition experience, to be able to make better educated and more self reliant decisions concerning upcoming weather.

Bowditch has Tropical Cyclone (hurricanes etc) basics but I found this to be very useful…

Mariner’s Guide For Hurricane Awareness In The North Atlantic Basin (PDF)

It’s the standard AFAIK

On our boat we get GRIB files from Saildocs ( It is just raw data from the GFS model so you have to take it with a grain of salt but you can specify spacing of wind barbs and other settings. It is good for low-bandwidth setups like we have on our sailing school boat. Just something to supplement your other forecasts with.

This raises a question - A container ship has a lot of “sail area”. It seems to me that if someone came up with sailboat style polars for it, a modified nav program that could combine the polars and the engine thrust could come up with fastest or least fuel used routing if fed with current GRIB files. Maybe this already exists???

There is a current thread on the weather routing software being used by Tote aboard the El Faro here - If You Don’t Trust the NWS You Sure Can’t Trust Applied Weather Technology -

The software basically models the ship’s behavior (speed, motion, fuel consumption) based on the ship’s characteristics (horsepower, displacement, draft, LOA) and environmental factors (current, wind, waves, swell). The software I’ve seen will on demand generate minimum time or minimum fuel routes within the sea height, wind speed limits.

I don’t know what type of files the routing company uses to download the weather data from the weather services but what is sent to the ship is a package of files, no GRIB files as far as I know,