Count re-started 5 October 2003
What Forecast to Use?Day Sailing, Coastal Passages or Blue Water? |
| This page has been produced at the suggestion of the OCC and the CYCC. It is intended to be a quick guide to what forecasts can be used for different types of sailing. On this page I give a one stop listing of forecasts that are appropriate. There are, of course, many other forecasts not mentioned here. See my Sources of Forecasts and other pages on this site. Some of the more experienced sailors will be able to use and interpret synoptic charts but many will not feel sufficiently | competent. In any case, it is always useful to see different versions of the same forecast. I have not given any reference to the use or reception of synoptic charts here. For sources see my Chart list and Radio Fax pages. Although primarily concerned with and based on experience in Western Europe and the Mediterranean, Similar services will be found elsewhere. |
Day Sailing - Local experience or forecasts? |
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| Forecasts most used are in worded form, received by voice or text. They will not and cannot be expected to give the variation in weather detail, especially wind, that a sailor will experience even over a few miles. This is a consequence of limitations in computer modelling of the atmosphere and predictability. The smaller the size of weather feature, the shorter time ahead can it be forecast, but there is a lower limit of what can be meaningfully and usefully predicted.. For example, if detail at 1 km (½ NM) scale could be observed, it is unlikely that it would still be there by the time a user obtained the analysis information, and certainly not by the time of the 6-hour forecast. Sailors may well find very short period forecasts useful, if not necessarily accurate. Accuracy and usefulness are not synonymous. For short period sailing in a limited area, the main input will be, or should be, personal experience of what happens or can happen locally, used in conjunction with the most appropriate Inshore Waters forecast, . For example, sailors out of |
the Dart always bear in mind what might happen around Start Point. They never know what will happen and no forecast system yet envisaged will give that level of detail. They are just on the look out for the need to put in a reef at short notice. Books by David Houghton and Alan Watts deal admirably with local effects. Such knowledge is (or should be) part of dinghy courses. The thinking sailor, especially the racing sailor will, in any case, make sure that he knows about the behaviour of wind around coasts where he sails. He will be able to translate that knowledge to other areas. The Met Office is developing weather prediction techniques to handle small scale features over a few hours ahead. But, for wind, the expected minimum resolution is likely to be between 5 and 10 miles. To put that into context, in my old UK waters, from the Dart to Salcombe is about 15 miles and Torbay is roughly 5 miles square. |
Day Sailing Tools |
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| VHF MRCC broadcasts give Inshore waters forecasts and, usually, those for neighbouring sea areas. National and Local Radio varies from country to country. BBC Radio 4 Shipping Forecast is one of the best, being rewritten every 4 times a day. In and near the Baltic, Stockholm Radio and Norddeutscher Rundfunk are particularly good. NAVTEX 518 and 490 kHz texts on 518 kHz are in English and are usually the same as National Radio. The UK is the only country to use 490 kHz for Inshore Waters forecasts. Marinecall/MetFax and similar services from other countries are on a premium rate or similar telephone service. Texts will sometimes (eg the UK) be more detailed than those broadcast by MRCCs. SMS services are available on a mobile phone for the UK from the Met Office and from Met Marine Ltd for the English Channel, Atlantic France, Mediterranean Spain and France, Greece and Turkey. Météo France has a very detailed set of services some free and some on repayment via its website. Météo Direct has a free service online for a 24 hour forecast at specific |
coastal locations and the nearby seas. It is rather too slow to be useful over a mobile phone link. BBC Coastal Forecasts around the UK and Channel Isles are issued twice a day, covering 24 hours in 6-hour periods. The texts seem to be generated directly from the model output with no human input. There are 24 areas covered around the UK and the Channel Isles. Lengths of coast to which the forecasts refer are thus, on average about 2/3 of those covered by the Inshore Waters NOTE - automated forecast services on the Internet or using SMS and MMS are usually direct computer products. Some claim to give location specific forecasts. These should be interpreted as being for an area of around 15 - 20 miles around the location mentioned. In general they will be inferior to forecasts that embody human experience and interpretation over the short term. But, like the BBC forecast referred to above, they may be found useful over the short term, say up to 6 hours ahead. A fairly complete listing of online texts of GMDSS forecasts will be found on my GMDSS page See also pages on Mediterranean, Baltic and Icelandic forecast services. For details of actual weather ie measurements made by automatic means and/or a human being, rather than forecasts, see my Actuals page. |
Coastal Cruising - Planning ahead. |
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| There are two aspects to using weather information for coastal cruising. The first is to use forecasts in order to be in the right place at the right time and to avoid being in an inhospitable port or anchorage in weather that you do not want to go out in. The second is the decision on the day, whether to go or not. The first demands a look at the weather for the next few days - up to 5 days is sensible. For this is it worthwhile looking at forecasts from more than one independent source, such as the UK, US and German National weather services and the ECMWF; also, from the same source on successive days.
Modern forecasting relies on Numerical Weather Prediction. This takes all the many, complex, interacting physics of the atmosphere and models them mathematically. However, the models cannot and will not ever be exact. There is much very necessary approximation and estimation of all the physics. How these are dealt with differs slightly in the models used by the major countries. Also observational data, over the oceans particularly, are never enough to
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define the atmosphere exactly. How the initial data are processed and interpreted differs between the various centres.
As a result, the answers of the computations can and do differ. Where they are more or less the same, then the implication is that the predictions at that time are not sensitive to the different treatments and that the forecasts are along the right lines. Similarly, if the output from one centre is consistent from day to day, it again implies that that particular forecast is not sensitive to different data inputs. Consistency means that you can plan with a fair degree of confidence. Inconsistency means the converse. The value of using several forecasts in this way cannot be over-stressed. Most forecasts for a few days ahead are straight from computer output with no human interpretation or amendment. For shorter period forecast, up to 24 hours, there is considerable benefit in having input from a human forecaster who can use intelligence and experience to interpret the computer output. |
Coastal Cruising Tools. |
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| For decisions on the day, use the day sailing forecasts listed above. For planning ahead ie not only "Can I go?", but, also, "Should I go?" US GRIB Files A free service available worldwide that gives wind forecasts in vector form at one degree lat/lon intervals for any area of your choosing for up to 15 days ahead. A sensible limit is about 5 or 6 days.. DWD RTTY For areas from the south of Greenland, the Baltic, Western Europe, the Mediterranean and into the Black Sea. For details, click here and for schedules, here. VHF MRCC broadcasts from the UK and French MRCCs, in particular, sometimes include outlooks for a few (up to 5) days ahead. UK NAVTEX 3 - 5 day outlooks on NAVTEX 518 kHz. The UK, alone, gives an extended outlook, for three days beyond the 24 hour outlook . |
UK NAVTEX 3 - 5 day outlooks on 490 kHz there is a brief national outlook out to 5 days for Inshore waters. Italian NAVTEX has a two day outlook expressed as wind and sea state at 12 hour intervals for 48 hours. Nordsee and Ostsee 5 day forecasts on line from the DWD Marinecall/MetFax Planning forecasts. are very good premium rated telephone services from the UK and similarly so from other countries. Météo France has a comprehensive set of services.. These are "must haves" for longer crossings around Western Europe, eg 3 to 5 day crossings of Biscay or the North Sea. Météo Consult has some free services of coastal and sea area 24 hour forecasts around the whole of France and further afield. |
Blue Water Sailing - Climatology and warnings. |
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| Blue Water Sailors plan on the basis of climatology, and, when they are on passage, there is little that they can do to avoid bad weather. Unless, that is, they are super yachts capable of 15 to 20 knots or more. A vessel doing 110 miles or so a day cannot be weather-routed usefully; the boat is too slow and storm systems are too large, coupled with the inherent uncertainty in predicting the track of a storm. A 5 day forecast is, very obviously, useful to help decide on which day to leave |
port. After that, these is not a great deal of use although the more knowledgeable blue water sailors study forecasts and charts while on passage in order to seek an optimum route. How effective they are is uncertain. Experience with weather routing ships was that it was very difficult to provide useful advice to a ship doing 10 knots. A better service might help, if only psychologically, but, that is a long way from storm avoidance. |
Blue Water tools. |
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| For decisions on which day to depart, use the appropriate forecasts from the Coastal section above. INMARSAT-C provides the GMDSS service covering the globe. For texts online, see the WMO site. As an alternative source of these forecasts, see also HF/SSB broadcasts in voice from Monaco 3AC and Radio France Inter for the Mediterranean and the Eastern Atlantic, also US HF/SSB for the Western Atlantic. Email via Marine HF or HAM can be used to access the US GRIB files forecasts. For HAM use, Winlink provides many services. The U.S. Coast Guard SITOR broadcasts, sometimes called Long Range NAVTEX, are prepared co-operatively by the Marine Prediction Center, Tropical Prediction Center and Honolulu Forecast Office. Offshore and coastal forecasts are available in areas such as Alaska. The International Ice Patrol also |
broadcasts from Boston sharing the same transmitters.. Transmission range is dependent upon operating frequency, time of day and atmospheric conditions and can vary from only short distances to several thousand miles. RadioFax is slow but this is not necessarily important on a long passage.. Schedules for Royal Navy and DWD transmissions can be found here and elsewhere in the world using the NOAA Guide to world-wide radio-fax broadcasts. I am sometimes asked about routing from onshore services, on repayment or by the redoubtable Herb on the Atlantic. Unless they have a direct line to the Almighty, such services can only use forecast output from the major National Met services although they can look at several such so as to give a consensus view, In that sense they are useful but whether they are more so than the informed on-board use of GRIB coded forecasts and/or RadioFax charts will depend on the individual sailor. The comments above regarding the difficulty in giving useful routing advice to a slow moving yacht still holds whoever is doing the interpretation. |
Additions and Corrections |
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| This page is not intended to be exhaustive and should be taken as a starter page. Serious omissions and any corrections received will be implemented as soon as possible. | |
© Frank Singleton, October 2005, minor revision February 2006