What this page is about

GRIB forecasts for 7 days are shown against what actually happened.

Related pages


Meteorologist continually assess their numerical weather forecasts, especially when they have made changes to their forecast programs and analysis techniques. Although useful to them, measures such as the Root mean Square Error of surface pressure mean little to us sailors. Model performance for wind forecasts can be given as Root Mean Square Vector Error but, again, these mean little to us.

The PDF files below are from talks that I have given mainly during winter of 2010/2011. Although they are a small sample, they will give sailors a good idea of what forecasts can and, importantly, cannot do.

How these were selected

In lectures to yacht clubs, I usually show examples of GRIB forecasts and what actually happened. The selection is fairly random as my main criterion for selecting a sequence is that there should be some significant changes in the wind.

Click on the area name to see the forecasts and the wind patterns as analysed by the computer at the verifying times. These are pdf files copied from PowerPoint. Page down each to see the sequences and how they worked out. Make your own judgement as to potential use.

Area of Forecast


Western Mediterranean -- January 2010 -- 950 kb

This was before the enhancement to the GFS in mid 2010.

Central English Channel -- September 2010 -- 850 kb

Days 1 to 5 were good, Day 6 was poor, Day 7 rather better

The Baltic -- February 2011 -- 2.6 mb

Days 6 and 7 were poor

Waters around Scotland -- February 2011 -- 1.7 mb

Pretty good in general. Not misleading apart from Day 7

English Channel and Biscay -- February 2011 -- 1.5 mb

Day 1 was not very good, but winds were light and variable. Otherwise good. Days 6 and 7 not misleading

Eastern Mediterranean -- February 2011 -- 1.9 mb

Despite some rather light abd variable winds, a pretty goos shot

Southern North Sea, E Channel and SW Approaches -- November 2011 -- 2.3 mb

This is after the last major enhancement of th GFS has had time to settle down. Good to about 6 days but a timing error at day 5.

English Channel and SW Approaches -- January/February 2012 -- 1.3 mb

Poor on Day 3, but otherwise good guidance until Day 6. Poor thereafter.

English Channel and SW Approaches -- February 2012 -- 1.5 mb

Poor in south on Day 7. Poor day 8. Good otherwise.

You can make up your own mind about the usefulness of these GRIB products. I suggest that using them as planning tools would rarely be misleaading to 5 days ahead.


See my page on Limitations of Forecast Accuracy.

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