About this page

Background to the production of weather forecasts, particularly the Shipping Forecast, that may give users a better understanding of that service.

Cruising Association

On this page -


Introduction

Sailors have many misconceptions and misunderstandings about the weather and weather forecasting. I hope that my various writings on this site will help to resolve these. One problem common to many yachtsmen is that of knowing what are the most appropriate sources of weather information and what are the differences, if any, between the various forecasts that you can see and hear.

There are also greater expectations for precise forecasts than can reasonably be provided given the current state of the science and the communications available and appropriate for use on small vessels. Accuracy and usefulness are not synonymous. Weather forecasts are rarely accurate but they are mostly useful. If anyone tells you that there sea area or coastal forecasts are accurate ask the, first, how they define accuracy and, secondly, how the measure it. Let me know if you get a satisfactory answer!

Another misconception concerns the role of the UK Met Office (and, no doubt other European National Weather Services). The financial constraints of the modern day Met Office and why we pay for forecasts are too complex to be discussed on this page.

Such matters as the time allowed by the BBC for Shipping forecasts, the broadcast schedules, the length of the NAVTEX time slots are all beyond the control of the Met Office. More information could be provided on National Radio More use is now being made of the UK NAVTEX facility, with outlooks to 5 days, once a day on 518 kHz, Coastal forecasts are updated four times daily on VHF and are repeated on the secondary frequency of 490 kHz.

There could, in principle at least, be a dedicated VHF Marine Weather Channel. In the final analysis, it all comes down to cost and competition for resources. There are many demands on the public purse. There are many competing priorities. I suspect that there will be little improvement in broadcast services useful to leisure sailors unless the group as a whole provides the necessary finance.

A tabular summary of the many and various services, free and on repayment, available to sailors can be found on myforecast sources page on this site.


Background to the BBC Shipping Forecast


Meteorological observations from fixed observing stations, merchant ships, fixed buoys and weather balloons are distributed on a world-wide basis. For this purpose, the two main observing times are 00 and 12 UTC. The next two main hours are 06 and 18 UTC. Observations at these four times are exchanged globally. Data exchange, between western Europe countries occurs every hour. In addition, there are data at irregular times from peripatetic sources such as aircraft, drifting buoys and satellites. All these data are used by the numerical weather prediction models.

The Met Office computer model uses 00 and 12 UTC data to provide forecasts out to 6 days ahead. The 06 and 18 UTC data provide forecasts only to 48 hours. The difference is because many stations launch radio-sonde balloons at 00 and 12 but far fewer at 06 and 18 UTC. Every effort is made to ensure that all data, regardless of time and type, are used as effectively as

possible. The data are analysed using a 4-D "best fit" approach (in space and time).

Initial output from the computations using global data are available about 3-3½ hours after data time. When working in BST, this allows a very short time for the forecaster to complete the shipping forecast. To meet the 0520 and 1754 LT schedules on BBC Radio 4, he has to have the script, already in draft, to be finalised very quickly when he sees the latest model output. The BBC have tried to bring the 1754 forecast forward by an hour but the Met Office said that that was simply not possible if the most up do date information was to be used.

Approximate time scales for the production of the basic shipping forecasts are shown in the following table:

Data Time (UTC)

Text Leaves Met Office (LT)

Broadcast Time (LT)

0000

0445

0520

0600

1115

1201

1200

1715

1754

1800

2350

0048

NOTE These timings will probably have changed a little but the table will still give some idea of the timescales involved.

In addition to the predictions from the computer models, the forecasters have more recent data from the British Isles, the near Continent, data buoys, oil rigs, light vessels and islands. These are all used to fine tune the model output and are scrutinised as carefully as time allows, particularly with regard to the need for gale warnings.


Why are forecasts sometimes so indefinite?


First and foremost the weather itself is so indefinite; it is a truism to say that the weather does not know itself to within one Beaufort force. On Channel or North Sea crossings, for example, we can experience variability in the wind about which the forecaster knows very little. As an example, a westerly F4 up the Channel will occur with a 4 hPa difference in pressure between the south coast of England and northern France, say 1000 hPa over Southampton and 1004 over St Malo. A force 6 would occur with 999 hPa over Southampton and 1005 over St Malo - pressure differences of 0.1%. Relatively small errors in a forecast can make all he difference between a nice force 4, an exciting force 5/6 and a nail biting force 7/8 with gusts to 9!

Secondly, marine forecasts cover large sea areas or complicated stretches of coast. The wind can vary from force 3 outside the Dart to force 6 off Bolt

Head, in about 15 miles and 3 hours. Imagine the length and complexity of the forecast if all such detail was included for even 12 hours ahead from "Lands End to Lyme Regis including the Isles of Scilly".

It does not help anyone if the forecaster tries to be more definite than he really can be. The forecaster does not like using such words as "perhaps", "possibly", "locally" but it is all too often necessary if he is to convey the possibilities in a sensible manner, achievable in a script that is not so long that it becomes meaningless.

Another truism is that there is nothing really definite in the forecast except for the date. Even the date is subject to a certain amount of theological discussion!


What forecast to use?


It all depends upon what you are doing. For coastal passages I rely mainly on the MRCC VHF or NAVTEX (490 kHz) broadcasts. I keep a watchful eye on the shipping forecast. If possible I get a sight of some charts at the marina, free from the Internet, by paying up and using MetFax at home or at the local chandlers. I keep a listening watch on Channel 16 and on NAVTEX for any warnings.

Detailed local forecasts would be a boon. In practice, I do not think that it is possible to predict those very important local effects that are the cause of so much discussion in the bar later. The scale of such important geographical features as Start Point are well below the resolution of the finest scale models in operational use. It will be a matter of prudence to keep a listening watch for warnings from the Coastguard.

For longer crossings then the shipping forecast is my staple diet via Radio 4 and NAVTEX. Before departing I always have a good look at the extended range forecasts to 5 days. It is wise to look at these for several days prior to a Channel or North Sea crossing. Try to see if the forecasts are reasonably consistent from one day to the next. This can be done by accessing just the

charts on the Internet (free) or by paying for them on MetFax. Alternatively, GRIB files are useful as long as you remember that they will under forecast strong winds by about 20%. Marked variations from one day to the next imply uncertainty in the development of the atmosphere.

Whilst on passage I make sure that I hear every shipping forecast. I keep a listening watch on channel 16 for gale warnings; NAVTEX is good for this purpose. The four shipping forecasts a day on Radio 4 LW are invaluable for indications of changes in the weather situation. This is especially important when on a long passage, say down to Spain from the South Coast. Although the 5 day forecasts heard on departure may have been favourable, relatively small errors can mean that it may be wise to divert to Camaret or (my favourite) Audierne. On such passages, it is important to use the relevant NAVTEX broadcast. That from Météo France divide the UK area Biscay into four areas.

The use of weather forecasts in a very pragmatic fashion was the theme at a Weather Day held for the Hallberg Rassy Owners Association.


Finally!


There are many sources and forms of weather information available around the UK and Western Europe. Mostly, these come originally from the various National Met Services.. Costs are not large and there are a number of free sources of information. Find which services meet your need and which you can obtain the easiest. When listening to a forecast, do not rely on memory. Make

brief notes and transcribe them immediately afterwards, if only to save arguments with your crew!

Good luck and fair winds. If not fair winds then the least that I can wish you is foul weather well forecast.


A version of this page was originally published in Cruising, the quarterly magazine of the Cruising Association.


Return to Home Page