EXPERT REACTION: What’s up with the latest weather forecasting?

The term ‘unpredictable’ is frequently confused with ‘variable’. The concept of weather predictability must also be considered in the light of the ‘natural variability’ in a given location.

The dynamics of the atmosphere dictate that the atmosphere becomes more variable as one moves away from the Equator and towards the poles. For example, the maximum temperature on 80% of days in Darwin in January lies in the 5oC range of 29-34oC (with an average of 32oC). By contrast, Melbourne is exposed to the wild storms which regularly cross the Southern from west to east. A consequence of this is that on 80% of days the range of Melbourne maximum temperatures is 16oC (range of 20-36oC, average of 27oC). Hence if one uses the average temperature in Darwin to ‘predict’ tomorrow’s max most of the time there would be an error of less than the 2oC. Following a similar method for Melbourne most of the time there would be an error of less than a much greater 8oC.

It follows that the concepts of ‘accuracy’, ‘skill’ and ‘predictability’ have different meanings. The skill of a forecast must be seen in terms of its accuracy relative to the background variability.

A significant proportion of atmospheric variability is tied up with the presence of moisture. With global warming the atmosphere is able to hold more water vapour, which in turn can mean cyclones and atmospheric fronts are more energetic. This leads to increased variability and more extremes. Hence the ‘prediction’ task becomes more difficult.

It should be pointed out that the BOM (along with all other weather centres in the world) base their forecasts on the fundamental physics governing atmospheric and climatic phenomena. These fundamental laws are encoded into computer models of the atmosphere. With increasing computer speed and memory these models, representing our best understanding of the atmosphere, are advancing. So even though the atmosphere is becoming more variable there is, overall, a progressive and demonstrable improvement in forecast accuracy.

It is important to remember that recent events tend to assume a greater importance in our minds than those of the past. As such the recollection of the impact of past events tend to diminish with time.

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