How to interpret a Shipping Forecast

In the UK, the Shipping Forecast is currently provided by the UK Met Office and is regularly broadcast on BBC Radio 4 on behalf of the British Maritime & Coastguard Agency.

It is broadcast at 0048 hrs on FM and LW (including reports from coastal stations, an inshore waters forecast and brief UK weather outlook). The forecast is broadcast also at 0520hrs (FM & LW), 1201 hrs (normally LW only) and at 1754hrs (LW on weekdays and FM and LW at weekends).

The broadcast format is limited to 370 words and has a very strict format. This is so as to simplify and standardise the delivery of what is a lot of detailed information. The format is as follows;

  • Time and Date of the active forecast being read
  • List Gale Warnings current around the British Isles
  • General Synopsis
  • Area Forecasts
  • Inshore Waters Forecast

The sea areas around the British Isles and the Western Coast of Spain, Portugal and Ireland are included in the forecast. This has relevance as in the UK most prevailing weather systems arrive having passed through the North Atlantic.

Making sense of the Information

The time the forecast was issued is critical as all other times and estimates relate to it. The time of the forecast may be different to the time of the broadcast. Listen!

Listening to what is happening in the South West and Western Approaches to the Channel is likely to give you a good idea of what to expect tomorrow South of the Solent, dependent on the track of the low and high pressure systems and their speed. So listening to the synopsis and the surrounding areas will give you a good idea of what’s to come.

Make notes as you go. Ideally you will use an easily identifiable shorthand that all can read but if you have your own make sure you ‘translate’ the relevant information and log it for their information.

A typical forecast for several sea areas might be;

"Humber, Thames. Southeast veering southwest 4 or 5, occasionally 6 later. Thundery showers. Moderate or good, occasionally poor."

What this is telling us is that the sea areas Humber and Thames (basically a large part of the western North Sea from Hull to the Dover Straits) is forecast to experience a SW wind direction blowing Force 4 or 5, sometimes F6 later. ‘Later’ is specifically at at time over 12 hours from the time the forecast was ISSUED. 

Force 4 or 5 might seem to be of little worry, even to most yachtsmen, especially if they are heading North up the West coast of the UK in the lee of the mainland on a broad reach. In fact, if you keep listening, the Inshore Waters Forecast for your area might well reflect that rationale and wind strengths and sea state might be significantly less as a result.

However, the fact that a F6 is forecast later should make you think about your plan, especially with thundery showers and occasionally poor visibility forecast.  

A Beaufort Wind Strength Force 4 is an average of about 15 knots. That is pleasant sailing for most. Even a Force 5 from abaft the beam is usually nothing to worry about. Using a quick ready-reckoner, we can soon work out that a F5 is likely to deliver about 20 knots (The Force - in this case 5 - minus 1 = 4 x 5 = 20kts). This works well as a mean wind strength conversion from the Beaufort Scale to knots.

A Force 6, coupled with veering winds (moving clockwise), which are likely to put the wind on the beam or forward of the beam when sailing, potentially increasing the sea state (especially if blowing down the coast against a flooding tide) is a different animal altogether. And the visibility is dropping too. Thundery showers, poor visibility, wind veering and increasing. Sounds like a front could be blowing through to me.. Let’s keep an eye on the barometer and consider a change of course. What was our nearest port of refuge in our passage plan? How about we set a course for that sheltered port and enjoy some pub grub tonight?

To clarify the terminology further, see our Beaufort Scale article for easy reference;

Further definitions used in the shipping forecast:

VISIBILITY

Good = >5 n miles
Moderate = 2 - 5 n miles
Poor = 1,000 metres - 2 n miles
Very Poor = Less than 1,000 metres

TIMING

Imminent = Within 6 hours of issue
Soon = 6 - 12 hours of issue
Later = >12 hours from issue

SPEED OF PRESSURE SYSTEMS

Slowly = Moving less than 15 kts
Steadily = 15 - 25 kts
Rather Quickly = 25 - 35 kts
Rapidly = 35 - 45 kts
Very Rapidly = >45 kts

WIND

Direction = The direction from which the wind is blowing
Becoming Cyclonic = Indicates a significant change in wind direction across the path of a depression
Veering = Wind direction moving clockwise
Backing = Wind direction moving anti clockwise

AIR PRESSURE TENDENCY

Rising (or falling) more slowly = Pressure rising (or falling) at a progressively slower rate through the preceding three hours
Rising (or falling) slowly = Pressure change of 0.1 to 1.5 hPa in the preceding three hours
Rising (or falling) = Pressure change of 1.6 to 3.5 hPa in the preceding three hours
Rising (or falling) quickly = Pressure change of 3.6 to 6.0 hPa in the preceding three hours
Rising (or falling) very rapidly = Pressure change of more than 6.0 hPa in the preceding three hours
Now rising (or falling) = Pressure has been falling (rising) or steady in the preceding three hours, but at the time of observation was definitely rising (falling)

Understanding the information provided is one thing. Being able to interpret it and make a safe plan as a result takes knowledge and experience.

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