Guest Post by Charles Duncan
An update to previous analysis and also this time looking at temperature.
A few days ago I posted an analysis of the Met Office’s rainfall forecasts. I realised after the event that I had used actuals from England and Wales, not the UK, so here’s the revised version. This time we are comparing apples with apples!
Using the 3-month outlooks[i], published monthly by the Met Office, I looked at the most likely rainfall and temperature for the first month of each; in other words their forecasts in a window of one to six weeks out.
Rainfall is very variable, and therefore harder to predict, than temperature, which varies little year on year. The Met Office’s forecast seems to be based on previous years (1910 – 2011), with a correlation coefficient between the two of 0.77. In the event the actuals were, of course, much higher than either previous averages or the forecast:
By contrast, a scatter plot of forecast vs actual temperatures yields an impressive correlation coefficient (R2) of 0.95:
In the graph below, the average is the mean temperature from 1910 to 2011, with the standard deviation shown as error bars. Also plotted are the Met Office’s forecast and the 2012 actual temperatures:
The correlation coefficient between the historical average and the forecast is 0.985, and between the mean and the actual is 0.94. This suggests the forecast for 2012, whilst accurate, was not significantly better than just taking the average of previous years’ temperatures.
However it is not as simple as that; the average is affected by the post 1970 warm period. The average of the whole data set is 8.5°C, but that from 1997 is 9.25°C. Furthermore 2011 was a warm year (average 9.64°C).
Generally temperature varies little from year to year; the standard deviation of year-on-year changes is just 0.35°C. The Met Office’s forecast for 2012 of 8.8°C was not only more that 2 standard deviations from 2011, but surprisingly accurate; 2012 turned out to have an average of 8.7°C.
The forecasts and actuals for 2012 suggests the Met Office are reasonably accurate at predicting UK temperatures for the next month, but woeful at predicting rainfall.