Using the Weather Forecast

The following originated as notes for a Hallberg Rassy Owners Association Weather Day. The emphasis on this course was very much on a practical and pragmatic approach to using what must be a very imperfect tool.  The reasons for the imperfections are discussed on other pages of this site, click  here or  here . The ideas developed may be of interest and, I hope, use to others.

Introduction

Using Forecasts

Assessing Forecasts

Limitations of Forecasts

Weather Forecasting

  
Current Numerical Weather Predictions are better than ever before; forecasting has improved more or less steadily over the past 30 years. That is not to say that everything is perfect. Far from it. However, the longer period forecasts from 3 to 5 or even 6 days ahead especially, have shown a quantum leap from the time when I last worked as a forecaster, back in 1977. Some background to the forecast were written in  an article for the Cruising Association .

There is a mistaken impression, and enshrined in the memories of some sailors, that the local old salt is a fount of meteorological wisdom. This is a travesty of the truth. Single observer forecasting is limited in what can be achieved e.g. falling barometer, backing wind, increasing cirrus are good indications that a warm front is approaching - BUT how fast will it come? How far will the wind back? How strong will it become? When will it veer? And so on. What the local expert can do, is to know just how the weather behaves in his particular area in a given weather situation, how the sea breeze behaves, whether it is prone to showers and so on.

It follows that the intelligent user cannot beat the professional in predicting the overall weather pattern. However, he can improve on the forecast using local knowledge and his own observation etc. For the cruising yachtsman, most important  is the use of the forecasts in a logical and structured manner as an aid to decision making both at sea and before setting sail. This is primarily to keep out of trouble but, secondly, as an aid to efficient passage making - the two are, of course, linked

It should be rare to experience strong winds on passages (of up to 24 hours) of more than one force above that expected on the basis of the forecasts. At the lighter end of the scale, the picture may not be so rosy but then there is less danger and the engine is always there to get you out of trouble or to get you into harbour before the bar closes if the wind falls unexpectedly light and contrary.

A summary table of  Marine Meteorological Services is available on another page of this site.

Using Forecasts

 
The big question for the sailor, usually with little formal training in meteorology, is how to use this very important but imprecise service.  The notes below are ideas based largely on my own experience of many thousands of  cruising miles. Obviously, they are written in the light of my professional meteorological background, especially my ten years in mid career as a Senior Forecaster with the UK Met Office.
1.    Day sailing - off your mooring and back
Tools to use:- The Shipping Forecast, local radio, Marinecall/Coastguard VHF, SMS are all useful in indicating whether it is go or no go. Look for changes in the last three or four Shipping Forecasts. Is there a trend?  Are there inconsistencies implying uncertainty?

Most important will be your own local knowledge of how the wind is likely to vary locally due to the effects of topography.   Good starting points will be found in books by David Houghton and Alan Watts. Both are good observers of weather and well able to relate what they see to local topography.

 Think carefully about -

  • Sea breezes

  • Effects of headlands, cliffs, straits and channels.

Forecasters can already build these effects into their NWP models although not yet on the very small scale of, say, Torbay. Just how this very detailed information will be made available to the sailor has yet to be determined.

 
2.    Coastal and Cross Channel Passage making  

Tools to use :-  Forecasts out to 5 or 6 days ahead  from the Internet, Marinecall 5 day planners, DWD RTTY, GRIB Files, UK NAVTEX. Shorter period - up to 24 hours forecasts - from Radio 4 (or other national radio), NAVTEX, Marinecall/MetFax (or the local equivalent), MRCC/MRSC VHF Inshore Waters forecasts,

About 5 or 6 days before, start watching the outlook forecasts from MetFax at a small cost or use the Internet. Look at the charts and, perhaps, the worded forecasts. Look at the DWD 5 day forecasts and try the US GRIB service. Ask yourself the following questions -
 

  • Does it look as though the trip is going to be possible in terms of wind direction and strength?

  • Are the forecasts fairly consistent from one day to the next? (Consistency is a reasonable indication that the atmosphere is in a predictable mood.). Look for trends in the forecasts.

  • As your departure date approaches, are there any obvious (or not obvious) potential problems?

  • What about wind direction? Might you have to be beating and how will that affect your ETA?

  • In particular, what if the wind is a little stronger than forecast? Is it still within your limits? 

  • What does the longer period look like? Does it look settled? Or does it look like being changeable? If so, then your plans for the return could all too easily go awry.

  • You will really have to have keep a weather eye open for problems. Will you be able to delay a day or so or are you going to have to come back by ferry?

  • There really is little point in popping across to Cherbourg for the pre Christmas wine run if you are going to have strong Northerly winds for the following week.

3.    For longer passages, lasting around 3 to 5 days  

The same initial approach - keep listening and watching all the forecasts that are available (and relevant). Try to see how reliable the weather situation is. Do not go, as some people seem to, just on the basis of one good Shipping forecast that is only valid for the next 24 hours. Take a long hard look at the longer period. Take your time. If you want further advice, then you might like to speak to a Met Office forecaster on MetCall or to a private consultant at WCSMarine. Having got what seems like a good weather window then on passage:-

  • If you have HF/ SSB radio and RadioFax then keep taking the charts. Watch for changes in the patterns. Is the wind still favourable? Is the direction tightening up on you? Try, also the DWD RTTY service.

  • Without HF/SSB, Radio 4  may well be your only source of data. But this only gives 24 hour forecasts.  Listen carefully for changes in the forecasts. Keep checking that the forecasts are reasonably in agreement with the forecasts that you started out with. Watch out for that little insidious "perhaps later...".

  • Keep an eye on the NAVTEX  forecasts which usually include a very brief outlook that warns of severe weather, however, do remember that

it is orientated towards SOLAS ships and may not make much of that nasty little thundery low that can blow your electrics skywards.  UK NAVTEX gives a 5 day outlook which is rather more useful, but this is only once a day.

Returning from abroad, much the same applies.

  • Remember that you may have difficulty in accessing Marinecall from an overseas PTT or mobile phone.  Marinecall Club can be useful in this respect. Or use a BT phone card that connects direct into the BT system - a little costly but well worth it. 

  • Marinas in both France and Spain quite often have the longer period forecasts on display (but, just check that they are the latest issues). The UK Met Office does not have a monopoly.

  • Use the Internet. Text forecasts are usually cheap to download even over a mobile phone.

What to think about when assessing a forecast for a day or more
The following are uncertainties that occur in forecasts. They are not usually questions that the yachtsman can begin to answer, but they do point to the uncertainties even with the current state of the science. REMEMBER - the  computer predictions give the best answers possible and not the best possible answers.

LOWS

  • Where/when will next one form? The low approaching the day after tomorrow may not even exist today.

  • How fast will it come? [Up to 60 knots NB, 30 knots = 720 miles per day, 60 knots = 1440 miles per day. An error in the speed of 10%?]

  • How rapidly will it deepen?

  • When and how much will it turn away - usually to the left (north)?

  • Will a breakaway form? - [Usually at the point of occlusion]

  • In the showery air stream behind a cold front - variable wind and weather conditions with showers. Gusts, thunderstorms, squalls. Such details will not be predicted accurately.

HIGHS

  • When will they collapse - not usually a 24 hour forecasting problem but may be embarrassing if you are committed to get back home in three days time!  And even more embarrassing in the middle of Biscay!!

  • How quickly will they form?

  • And, if you are near the coast, then:-

  • Minor shifts in position or shape may lead to marked wind and weather effects e.g. fog near the East coast, showers or no showers.

  • Wind strength  and direction variations due to LOCAL EFFECTS

  • Sea breeze effects.

What you will not hear in the forecast

  
There are always uncertainties, even over the open sea. These can be due to minor variations in the pressure pattern (unpredictable, certainly to the sailor)

Then there are local effects, sea breezes, headlands, straits, gusts, down draughts. David Houghton's book Weather at Sea or Alan Watts Wind and Sailing Boats are both good reading.

Remember the limitations of the phraseology. In the shipping forecasts "West" means 45 degrees either side. The Beaufort scale already implies a range of wind, but the weather does not know itself to within one force.

Nothing is certain in the forecast, except for the date. Even then, you have to keep the theologians out of the discussion!

Best Advice of All

Plan for the worst, hope for the best. The worst may not happen.


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© Frank Singleton, February 2000

Revised  November 2005