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41 WEATHER FORECASTS AND THEIR INTERPRETATION
155. SOURCES OF WEATHER INFORMATION FOR MARINERS
The Meteorological Office provides special weather forecasts for commercial shipping and yachtsmen including regular radio transmissions from BBC national, regional and local stations and from coastal stations. The specific sea areas around the British Isles to which these marine forecasts refer are shown in fig.40-1, and it will be seen that each sea area is given a special name such as “Sole”, “Dogger”, “Wight”, etc. Larger maps or cards showing these sea forecast areas are available from Admiralty Chart Agents and are reprinted in Reed’s Nautical Almanac and many nautical textbooks. Mariners should always know the sea area in which they are located, as well as the names of all the adjacent areas.
The Principal national marine forecast of greatest value to all mariners, including yachtsmen, is the Shipping Forecast broadcast four times daily on BBC 1 Radio 4 on 1500 metres (200kHz) at *0033, 0555, 1355 and 1750 on every day of the week. This forecast contains:
a. Any gale warnings in force.
b. A General Synopsis of the weather situation.
c. A Forecast for the following 24 hrs for each sea forecast Area.
d. The latest weather reports from certain coastal stations and offshore light vessels.
Note that a report is a statement of what the weather is actually doing; a forecast is what the weather is expected to do.
In addition to these main Shipping Forecasts, there is an Inshore Waters Forecast broadcast on BBC Radio 4 at approximately 0020 (after the 0015 Shipping Forecast). For this, the Meteorological Office issues three forecasts, one for England and Wales, another for Northern Ireland, and one for Scotland. These forecasts are similar to the Shipping Forecast in form, but some indication of local coastal variations is also given when appropriate. Forecasts for Coastal Waters are included in the General Forecast on BBC Radio 3 (medium wave) at 0655 on weekdays and 0755 at weekends.
The amount of time devoted to weather information on local radio stations (BBC or commercial) varies considerably from one station to another, as does the quality of the service. Local radio stations which do give special forecasts for the mariner, however, include Radio Cleveland, Radio Newcastle and Radio Solent.
Plain language area shipping forecasts covering the 24 hours following the time of issue are broadcast from British Post Office Coast Radio Stations by radio-telephony (R/T) twice daily (for precise times consult the Admiralty List of Radio Signals or Reed’s Nautical Almanac). These forecasts are read first at conversation speed and then repeated at dictation speed, and cover the coastal areas adjacent to the transmitting station.
Of particular value to the mariner are the daily synoptic analysis (weather maps) for the general meteorological situation around the British Isles and Eastern North Atlantic available from the London Weather Centre, Gravesend Pilotage Office, Southampton Weather Centre, Liverpool Custom House and the Glasgow Weather Centre.
Forecasts may also be obtained by telephoning the nearest Meteorological Office although unless a person-to-person call is made to a Met. Officer, the forecast given is liable to be a tape—recorded one for local (land) purposes with insufficient detail for marine use. Both synoptic information and a forecast can be requested in these telephone calls and these are normally given free of charge (other than the cost of the call). If you do ring for information in this way, it is as well to avoid the hour when observations are being taken. Between 25 and 45 minutes past each hour is the most useful time to ring, as the forecaster will have had time to digest the substance of the message inherent in the observations which have been pouring off his teleprinters prior to that time. Also, forecasters like to talk to people who know what they want; therefore, try to be specific over the phone: have some knowledge of how the weather situation is shaping — if you know roughly where the pressure systems and fronts are located, the forecaster can give you much more precise information than otherwise.
Although the sources of weather information outlined above are of specific use to mariners, the General land area and Regional Forecasts broadcast throughout the day on BBC Radio 4 should not be neglected, because they provide confirmation of the Shipping Forecast and sometimes amplify it with additional information. A modification to the forecast can be detected as forecasts make use of more recent data. Perhaps of more importance, these land area forecasts include an ‘outlook’ giving an indication of the weather trend for the following two days. i.e. beyond the 24-hour period of the Shipping Forecast.
Although it is necessary to keep informed of the official forecasts for the area in which he intends to sail, of far greater importance to the navigator is the synopsis of the general weather situation, so that he is aware of the overall weather pattern and its probable movements. For this reason, if he can possibly get hold of the weather maps issued by the Weather Centres, he should certainly do so. If these are studied regularly during the days preceding an open-sea passage, they will give him the opportunity to see how the general weather trend is developing, and to watch the approach and build-up of low-pressure systems and fronts. Later, at sea, he will be in a better position to understand the broadcast reports and forecasts, and to interpret the weather from his own observations.
Immediate observations and weather maps are unfortunately reserved for professional meteorologists; the published weather maps are inevitably several hours old before they are received. More immediate weather maps may be seen briefly on TV, but the glimpses are usually too fleeting to make this a useful source of weather maps.
However, the general lie of the pressure systems can be gauged from TV charts, which may be a help. A longer—than—normal look at a weather chart on TV is possible at 7 p.m. on BBC 2 on weekdays and at 9.25 p.m. on BBC 1 on Fridays.
Gale Warnings are broadcast on BBC Radio 4 at 1500 metres (200 kHz.) as soon as possible after their issue by the Meteorological Office preceded by the words “Attention all Shipping”, and are then repeated at the following hour, usually just after a news summary. Every Shipping Forecast is prefaced by a summary of any gale warnings in force. a warning of a gale in a sea forecast area is issued if a gale is expected somewhere in that area, not necessarily over the whole, or even the majority of the area. For instance, a gale warning for sea area Fastnet (see fig. 31) may refer to local gales off the south-west coast of Ireland, but a yacht cruising in the same sea area near the Scilly Isles may not experience a wind stronger than Force 2 or 3. Warnings are issued if gales are expected at any time within the next 24 hours. A warning is even issued if the forecaster thinks that a gale might occur within the next l2 hours. If a gale might occur beyond 12 hours, the forecaster may use the words “perhaps gale 8 later” in the Shipping Forecast.
Gale warnings are only issued when winds of Force 8 or gusts of Force 9 or above are expected. The term “severe gale” implies a wind of Fore 9 or above, and the terms “imminent”, “soon”, and “later” indicate gales within 6 hours, between 6 and 12 hours, and more than 12 hours respectively, from the time of issue. An Amended Gale Warning is issued: (i) if a gale is expected to become more severe, or to develop earlier or appreciably later than was indicated in the original warning; (ii) if the direction is expected to change by more than 4 points (45°) from the range covered in the original warning; (iii) if the direction is expected to change from north of the east-west line to the south of it.
Visual Storm Signals have now been discontinued but are used to indicate that a gale is expected or is at present in an adjacent coastal sea area. These visual storm signals are illustrated in fig. 41-2 and a list of the Storm Signal Stations around the British Isles will be found in Reed’s Nautical Almanac. The signal is lowered when the wind is below gale force and it is anticipated that there will be a period of not less than six hours with winds of less than gale force. The signal is kept flying during a lull in the wind if a renewal of the gale is expected. The Night Signal is hoisted only at a limited number of stations, namely Southend, Fairlight, Brighton, Polruan, Arbroath, Rame Head (white lights), Workington (red lights), Newhaven (one red and one white light), Leith (one flashing red light), Portland Naval Base and Falmouth.
156. THE INTERPRETATION OF SHIPPING FORECASTS
The principal weakness of a forecast is its essential brevity and consequent generality. The meteorologist has little difficulty in issuing a detailed and accurate forecast for a particular location or a particular route. The shipping forecasts have to cover an area of over half a million square miles between Iceland, the German Bight and Finisterre, and the area ‘Wight‘ alone covers about 10,000 square miles. As a rule, many different sorts of weather are being experienced and can be expected, over this vast area and all this has to be compressed into a statement that can be read out in five minutes or less. These forecasts must be essentially general, if not sometimes actually vague.
For example, wind directions are referred to in terms such as “becoming westerly to north-westerly”, implying a range of over 45°, and of little use to an ocean racer beating to windward trying to decide on the more favourable tack for the next mark of the course, even though it describes accurately the wind direction over that particular area as a whole. In the same way, “becoming force four to six” is inadequate for the cruising yachtsman who is happy at sea in force four, but who may find it advisable to stay in the harbour in force six; again, this description of the wind strength will be true for the area generally.
Another difficulty for a weather forecaster is that the time period which many forecasters must cover is so long that certainty is impossible. Three hours ahead is just about possible for accurate forecasting of wind speed and direction, and the type of weather which will occur. This is then the usual period between the weather charts of the U.K., used internally at Met. Offices, although they may be drawn every hour when the situation is developing fast.
The major weather charts of the North Atlantic and Europe are drawn for midnight, 0600, 1200 and 1800 G.M.T. To explain the difficulties of issuing broadcast forecasts which are up-to-date, let the ‘synoptic hour‘ for which a major chart is to be drawn be called H-hour. Because of the overloading of communication channels, the observations are not in before H + 3 hours. These have then to be analysed and the H-hour chart is drawn; then the situation has to be projected forward in time and forecast charts prepared based on the senior forecaster’s review of the consensus of opinion on the likely course of events for the next 24 hours. All this takes time and the forecast chart for H + 24 is not ready for issue until H + 5½. On this the ‘outlooks’ which go with all the forecasts are based. So, while all the observations for the 0600 chart are made at that time, they are not on the charts and drawn up to fit isobars, pressure centres and fronts until several hours later, and it is this which leads to the forecasters being unwilling to issue a shipping forecast for 24 hours ahead before H + 7½ hours. The times of shipping forecasts, hence, tend to fit this pattern; for example, the shipping forecast broadcast at 1355 hrs. is based on the 0600 hours weather chart and is issued between 1300 and 1345, i.e. at the convenient H + 7½ hours. It is, of course, not just a question of convenience; the accuracy which can be achieved within the limits set is then as high as it can be.
These limitations of the shipping forecasts have to be accepted, but a good deal more can be extracted from them by applying a basic knowledge of Meteorology, coupled with shrewd observation of the local weather.
The forecasters use their words in a fairly precise manner and words which might be used as synonyms in ordinary speech have different shades of meaning in the forecasts. It is, therefore, important for navigators to understand the precise meaning of the various terms used in shipping forecasts.
The General Synopsis of the weather situation gives the positions at a stated time of all depressions, anti-cyclones, fronts and occlusions. It will also give their expected movements during the forecast period and the degree of activity associated with each. For instance, it will usually refer to the depth and intensity of depression. The depth of a depression means the barometric pressure at its centre; a deep depression is one with a central pressure of 990mbs. or lower, and a shallow depression is one with a central pressure over 1,010mbs. An intense depression is one in which the isobars are very close together (i.e. the pressure gradient is said to be steep, implying strong winds). A deep depression will normally be intense but not necessarily so; intensity depends upon the area covered by the depression. Depressions with weak pressure gradients are called feeble; they are often shallow, but again not always – even quite shallow depressions can have steep pressure gradients giving fresh or strong winds locally. The qualifications filling or deepening give a shrewd idea as to whether they are decaying or becoming more active.
Anti-cyclones with very high pressures at the centre are said to be intense. As an anticyclone must have a slack gradient near its centre, an intense one usually covers a very large area. Small anticyclones are usually described as weak. When the pressure at the centre of a High is increasing, it is said to be intensifying, when it is decreasing it is said to be giving way.
Fronts are usually described as vigorous, active or weak, terms which are fairly self-explanatory. The following terms are used in Shipping Forecasts to describe the speed of movement of pressure systems:-
Slowly = 15 kts.
Steadily = 25 kts.
Rather Quickly = 35 kts.
Rapidly = 35 to 45 kts.
Very rapidly = over 45 kts.
Forecasts usually begin with wind direction and strength. The direction of wind is the true bearing of the point of the compass from which it blows, e.g. W., NE., SSE, etc. Wind directions are given in the quadrantal system rather than by degrees. The suffix “-ly” (e.g. ‘easterly’) has not yet achieved an internationally-agreed degree of approximation but conventionally covers about a point on either side. In forecasting for large areas it is not usually possible to express wind directions to within three or four points without going into unacceptable length. Wind strength is usually given on the Beaufort Scale (see fig. 8 in this Chapter).
Precipitation is the forecaster’s expression for the results of the condensation of invisible water vapour into liquid or solid form at, or near, the Earth’s surface in other words, rain, hail, drizzle or snow. Forecasting terms used in describing precipitation are: –
Rain = precipitation in the form of water drops.
Drizzle = precipitation in which the water drops are very small. Visibility is normally poor in drizzle and occasionally it may occur in association with sea fog.
Hail = precipitation in the form of hard ice spheres.
Snow = precipitation in the form of soft ice crystals or flakes.
Periods of Rain or Periods of Prolonged Rain = rain lasting for several hours, or a lot of rain falling over an extended period, not usually heavy and at times akin to drizzle.
Shower = brief precipitation with more or less definite clearances between the falls. The rain is heavy for a short time, starting suddenly and dying away quickly in direct contrast to prolonged rain which usually starts lightly and increases in intensity.
Occasional = not continuous. The periods of precipitation are relatively short and occupy only a small fraction of the total time. Between the periods of precipitation the sky remains overcast or nearly so (if clearances are expected the term “showers” is used).
Intermittent = not continuous over a considerable period; but the periods of precipitation are of substantial duration and the sky remains overcast.
Thundery Rain = occasional or intermittent rain of varying intensity, but heavy at times. Usually, but not necessarily, accompanied by thunder.
Thundery Showers = showers of rain, hail, sleet or snow, usually heavy and accompanied by thunder.
Thunderstorn = thunder and lightning with or without precipitation which may be continuous over a considerable period, and heavy at times.
When it is expected to be dry, the following terms are used:-
Fine = no precipitation or thick fog; some sunshine by day.
Dry = no precipitation or thick fog.
Sunny = sunshine most of the time.
Sunny Periods = fairly continuous sunshine for an hour or two at a time, and in all, more sunshine than cloud.
Sunny Intervals = intermittent sunshine for rather less than half the period.
Bright = considerable diffuse sunshine and perhaps some direct sunshine.
Bright Periods = bright sky for more than half the time.
Bright Intervals = intermittent occurrences of a bright sky which are too brief to be termed bright periods.
Cloudy = cloud nearly or completely covering the sky and such as to reduce daylight to the extent that ‘bright’ is inappropriate.
Dull = a complete cloud cover so dark as to justify a stronger term than ‘cloudy’.
Visibility is defined as the maximum distance at which an object is distinguishable, i.e. not just vaguely visible, but clearly-recognisable as, say, a ship or land. The forecaster uses the following terms to express visibility:-
Very good or Excellent over 30 miles
Good 5 to 30 miles
Moderate 2 to 5 miles
Poor 1 to 2 miles
Mist or Haze* 0 to 1 mile
Fog 365 metres (400 yards) to 1,000 metres (1,110 yards)
Thick Fog Below 365 metres (400 yards).
*Mist is the term used when visibility is impaired by particles of condensed water vapour in the atmosphere, while Haze is the term used when visibility is impaired by particles of industrial smoke or dust in suspension in the atmosphere.
Further Outlook: Many weather forecasts include an outlook for a specified period of time beyond that of the forecast. They are necessary in very general terms and they normally convey some indication of their reliability.
157. RECORDING WEATHER FORECASTS
Forecasts are on the air at least every 6 hours and small—craft mariners should listen to all of them. Do not listen to just one forecast; as we stated at the beginning of this Manual, in small craft you cannot have too much weather information, because the weather can change very quickly and the situation can change drastically in the space of only a few hours.
The time allocated by the BBC for the transmission of a Shipping Forecast is, unfortunately, only five minutes, and a considerable amount of information has to be broadcast in this short period. In order to do this, the forecast has to be read at conversation speed and unless one is proficient at shorthand, and few yachtsmen are, it is impossible to take down the information direct. Even if a portable tape recorder is used, the information must eventually be written down for effective interpretation and use, and it is, therefore, necessary to resort to some abbreviated, but nevertheless understandable, form.
Special forms for writing down the Shipping Forecast and for constructing your own weather map are published by the Royal Yachting Association, the Royal Meteorological Society and the Southampton Weather Centre. Those who are proceeding beyond the Basic or Elementary stage of their Study will find a complete description of how to record a Shipping Forecast, the abbreviations which can be used, and how to construct their own weather map, using these special forms (shown later in this Study).
Even if you do not wish to draw your own weather map, it is important to write down at least the General Synopsis, the forecast for the sea areas in which you will be sailing, the forecast for the adjacent areas, the reports from coastal stations nearest to your area, and the outlook from the land area forecast. With this information, you can use it to decide whether or not to sail, whether to remain at sea or make for port, and the best strategy and tactics to get the most out of your sailing.
158. SECONDARY LOWS
Some years ago while racing somewhere a south of the Kerguelen Islands in the Southern Ocean, an area well known for cyclogenesis (where lows are born or develop), we went from running to beating in a short space of time. With sea water at about 6°C and 40 knots of wind, this was not a very pleasant experience.
In those days our forecasting was limited to occasional weatherfax pictures that depended on how clear a signal we could get on the SSB. The charts did not show any headwinds at all and we should have been barrelling along under spinnaker. What had caught us out was the rapid development of a secondary low that formed and deepened south of the Kerguelens. This slowed us up considerably, but secondaries are a feature that needs to be taken into account when sailing in mid or high latitudes.
In the days of the heavy IOR maxis the further south you dared to go, the stronger the wind and the faster you went, until ice and the cold made it unsafe, or you got on the wrong side of a low. All quite reasonable and by tracking the progress of lows it should be easy to stay on the right side. However, in this case what was not well forecast was the development of a secondary low. These form within the flow of another low. Initially they are of a higher central pressure, but they can deepen rapidly and produce some unpleasant weather. Although the lows nowadays will be reasonably well forecast, because of their rapid development it is easy to miss them until it’s too late.
159. LIMITED INFORMATION OFFSHORE.
Fast forward a few years and our forecasting models are very much better and freely available, but that said, the weather can still change rapidly and once you are offshore the information available is severely limited by your communications.
At the end of April we had a rather unusual snowfall in the UK, which made the news — unusual in that it was late in the year — but what caught my eye was that it came from a secondary low that was hardly noticeable on synoptic charts until it deepened and moved over the UK all in under 24 hours.
If you follow its development using GRIB files (see above) you can see initially there was a widening of the isobars to the north of Ireland. There was not a great deal to get excited about, although near Rockall a north-westerly of 25 knots was already blowing. If you scroll on six hours a new low has formed and there is circulation south of the Outer Hebrides. The interesting point about this is that from a synoptic chart you would have probably been expecting a north-westerly wind not an easterly.
Six hours later there is a tight little low over Tiree with gale force winds on the south-west side of the low — the side away from the parent low, which has moved away over Scandinavia. Twelve hours later the low has passed over southern Scotland dumping a load of snow before moving into the North Sea and away.
160. SPEED OF THE SECONDARY LOW
Although forecast, it was the speed that it deepened and became a significant feature that made the secondary low something worth watching. This is particularly important when offshore with restricted access to forecasts and. although there are limitations to GRIB files, they are extremely useful as you can scroll through in three-hour time steps, making it easier to see how systems are developing, rather than having to rely on synoptic charts with a 12- or 24-hour time step.
161. POINTERS FOR SECONDARY LOWS.
They develop within the existing circulation of a low pressure system. Fig. 41-3
The first pointer is the widening of the isobars, usually near a cold front or trough. There may be a small wave on the front. A secondary may also develop near the triple point where the cold front, warm front and occluded front meet.
The wind will increase on the side away from the parent low, but will also reduce between the parent low and the new secondary low.
There can be rapid development of the new low to a lower central pressure than the parent. This is most likely to happen when the secondary low is greater than 600 miles from the parent.
The low will rotate around the parent low.
On satellite pictures the first indication will be a comma cloud over the new low.
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