Sunday, January 1, 2017

Short Survey of Marine Weather

These three environmental sources surveys cover key resources available to mariners who have not taken our Marine Weather Course, which covers each of these in depth, along with the background needed to use them efficiently. Our goal here is to be sure that you do not leave one of our other courses without at least knowing what the basic tools are and how to find them. 

All three surveys (weather, waves, and currents) take advantage of viewing forecasts or actual data in GRIB format using dedicated GRIB viewing software or a navigation program with that functionality. A discussion of those programs and sources of GRIB formatted data are in the Weather Survey, so that one is needed to understand the Currents Survey and Waves Survey.

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Marine weather is essentially all about the wind. Wind determines our progress, power or sail, and wind makes the waves, the ultimate threat to vessels at sea—a cubic yard of water weighs a ton. We typically only care about things like temperature, precipitation, clouds, etc, to the extent they help us understand or forecast the wind.

We have a couple articles online including another weather overview as well. See in particular the weather checklist.  This survey here is a more generic sources survey.

Long-term Planning

For long term route planning around the wind, the primary source of climatic wind data is COGOW. This essentially replaces the pilot charts or any other wind atlas for climatic wind prediction, though we still look to the pilot charts for other crucial planning data. This is a must-know source of wind data. 

If we need more specific historic data this can be found in what was recently called the National Climatic Data Center (NCDC), but is now part of the National Centers for Environmental Information (NCEI). You can get almost any past weather data from this site.

Short-term Planning and Underway

For short term planning—do we leave today, tomorrow, or next week—and for decisions underway (a crucial part of marine weather), our main source of data are the various Centers of NOAA and the NWS. It is difficult to keep track of their organizational structure, which is as fluid as the wind and sea they cover. Here are the key ones and where to find them at the moment (they are in the process of changing again).

These three above provide analysis, forecasts, and observations. Again, it is fundamental to know these three agencies and the many products they provide.

View weather maps available by satphone or HF radio directly from the HF Fax Schedules

We are also dependent on two other complex entities that produce key products we need for weather work underway.

National Center for Environmental Prediction (NCEP), and in particular the Environmental Modeling Center. NCEP runs the numerical weather prediction models that we rely on underway for wind forecasts. We can go their link above and see graphic images of the model computations, but the main products we get from this agency are delivered by other means to us in the form of digital GRIB data, discussed below.

For US inland and coastal waters mariners are increasingly using the digitized NOAA forecasts known as the National Digital Forecast Database (NDFD). These forecasts are based on a compilation of the best forecasting available from the NWS along with the National Blend of Models to come up with a NWS forecast in digital format. This program is implemented by the Meteorological Development Laboratory, but we get the fruits of their work from another agency outlet in GRIB format.

GRIB Viewers

The model forecasts and the NDFD data are viewed underway in GRIB format. This is a special vector format that provides extensive forecasts in small files that are easy to download by satphone or HF radio underway. There are many notes on this in the overviews cited above, but the best way to get involved is to install one of the programs and start using them. There is a free program for Mac or PC called zyGrib, which is one good way to start.  If you use a Mac, then there is a must have $20 app called LuckGrib, which is the state of the art GRIB viewer and source of GRIB files.

For global winds, the best starting point is the GFS model, which is available to mariners in any GRIB viewer and also on essentially any navigation program. A free nav program called OpenCPN is a good way to start if you have not used electronic chart navigation in the past. Later there are commercial products that one might want to consider based on personal preference and goals.  Expedition, for example, is the choice of competitive racing sailors.

You can also request GRIB files by email from We have extensive notes on this starting at Weather by Satphone and included links.

Of the many articles we have online about weather, I point out this one if you sail in hurricane or cyclone zones.  Tropical cyclone alerts by email request.  This is crucial as these text alerts are the primary source for weather near tropical storms.  The official weather maps are not useful for this, and the GFS model data are not dependable for these systems—on the other hand, those with NDFD data will get more useful results in GRIB format, but the text alerts from NHC remain the primary source.

( More generally you can get any forecast by email request as noted here: Zone Forecasts by Email.)

To show how a user might put together their own set of resources, we have made these custom examples:

Weather for the US West Coast to Hawaii route.

Weather for the Pacific NW and Inside Passage to Alaska.

Weather for Puget Sound and connecting waters (the Salish Sea)

Modern marine weather is a broad topic these days, incorporating all we know traditionally from wind, sea, and sky, along with the latest technologies which are vast and evolving. 

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For those who do not have time for our full Marine Weather Course, we have available our textbook Modern Marine Weather  and our Weather Workbook for study on your own.

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