What this page is about
Comments on the Assessment of Forecast Accuracy published by the UK Marine and Coastguard Agency in response to criticisms in editorials, articles and letters to the editor.of the (UK) Practical Boat owner.
In a letter published in the April, 2010 PBO, the MCA has given some figures for “accuracy” of forecasts. They state that over a period of one year -
- The “hit rate” for gale warnings (for sea areas) was 92%
- The “hit rate” for coastal strong wind warnings 96%.
- Within a typical 24 hour period forecast, 94% of winds were within one Beaufort force of prediction.
Leaving aside any remark about lies, damned lies etc, what do these figures mean?
- Assessment of gale and strong wind warnings should be reasonably accurate. It is an objective assessment that a gale or strong wind has or has not occurred.
- The figures on accuracy of the 24 hour forecasts must be much more questionable. I doubt that any assessment possible can deal satisfactorily with the major problem of the need to express an inevitably complex wind pattern in a few words. Further, no assessment possible will cope with topographical effects such as headlands and sea breezes nor localised, random events such as showers and thunderstorms. Currently, these can only be predicted in general terms and not in detail by any operational system.
- From the MCA letter, I assume that the assessment has been taken objectively from the Numerical Weather Prediction (NWP) output using the Met Office 4 km (~2 nautical miles) grid length model for the United Kingdom. Undoubtedly this will give good results as the Met Office is a world leader in NWP.
Remembering that even this model can, at best describe weather on a 10 mile scale and that small weather details have short life-times and cannot be predicted in detail, sailors have three choices.
- To use forecasts unthinkingly, as some seem to do – and then complain. I hope that few take that approach.
- To recognise forecasts as broad guidance for use with experience, care and nous.
- To supplement MCA broadcasts using more detailed (meso-scale) forecasts available free from such as Passageweather.com and Windfinder.com or on repayment from Theyr.com, Predictwind. com or Clearpointweather.com. I doubt that any is consistently better than any other.
The two latter options used together will give both the best results and a good steer on what can and cannot be predicted. From reading the yachting press, from discussions on forums and from letters to the RYA, it seems that some sailors have lost touch with the realities of weather and its predictability. We have moved a long way since we had to draw our own charts from the shipping forecast. However, using weather information effectively still needs intelligent thought.
I am sure that the majority of sensible sailors recognise that fact.
Seekers after truth might like to idle a few minutes away by browsing my pages on using and understanding weather forecast
See my page on Limitations of Forecast Accuracy.