What and Why?
Back in October 2014 we at GWS decided to investigate the accuracy of 24-hour forecast temperatures that were being published by the Bureau of Meteorology (BoM) for Geelong.
As far as minimum and maximum temperatures are concerned, what is an acceptable level of error? If the forecast minimum is 14 degrees but the actual minimum is only 11 does anyone care? GWS believes that, if the level of forecasting skill is to keep pace with the financial input of taxpayers to the BoM, the general public deserves a more consistently accurate 24-hour temperature forecast (ie, made the afternoon before) hence this investigation.
So we set out to record all of the BoM’s daily 24-hour December temperature forecasts for Geelong, minimums and maximums, and to compare them with the actual temperatures recorded in round figures the next day at the Bureau’s station at Breakwater. All figures were taken from the BoM websites and published on our Facebook page. For interest, Geelong Weather Services (GWS) also made minimum and maximum predictions for each day.
Results for Minimum Temperatures
A commonly-accepted rule of thumb is that if a forecast temperature is within 3 degrees Celsius of the actual it is regarded as accurate. GWS disputes this for minimum temperatures owing to the lower range of likely figures and regards a standard of 2 degrees C as more acceptable. All recorded figures are rounded to the nearest whole number to facilitate comparison with the forecasts.
The graph below shows the results of daily minimum temperature forecasts. The BoM minimum forecasts exceeded the 2 degree tolerance figure on two occasions (1st and 18th), an accuracy level of 93%, which GWS believes is acceptable.
(For better resolution please click on each graph).
Results for Maximum Temperatures
If the rule of thumb of a 3 degree tolerance is applied to maximum forecasts, the results are less than satisfactory. The graph below shows the 3 degree tolerance was not reached on 4 occasions and on the 22nd was actually 6 degrees. Two forecasts were too high and two too low. Overall the accuracy rate within 3 degrees of the actual was 87%, which GWS believes is not commensurate with public expectations of a forecast made 24 hours in advance.
If you would like to make any polite comments, please do so on our Geelong Weather Services Facebook page.