How accurate are Australia’s weather forecasts?

Accurate weather predictions are something that Australians rely on every time they make plans for the week ahead, and while the forecast is most often accurate, when it misses the mark it can jeopardize our plans and lead to frustration. But is it possible to know when weather predictions are more likely to be inaccurate? This report aims to address this by examining how well predicted temperatures line up with those that were actually recorded.

The plot to the right shows the average difference between forecasted and actual temperatures in each state annually, based on all forecasts made 1 to 5 days before the actual date.

As you can see in the plot, weather forecasts are usually more accurate in the northern states. The southern states have less accurate forecasts because they are closer to Antarctica, where cold air and changing conditions often make the weather harder to predict.

Each chart plots the predicted temperature on the x-axis, and the actual temperature on the y-axis. The dotted diagonal line represents perfect accuracy - where the prediction perfectly matches the actual minimum temperature recorded. Deviation from this line shows a prediction error, the further the density contours are from the line, the less accurate the forecast was.

In these charts, we can see a few seasonal trends:

• Winter months, especially June and July show tight clusters on the diagonal, which means that the predictions were fairly accurate and reliable.

• Summer months show a wider spread and more deviation, meaning that their forecasts were more inaccurate - there was also more variability in the minimum temperature predictions.

• Spring and Autumn showed mixed results, with some months having more reliable forecasts than others.

Forecast accuracy appears to vary seasonally, likely due to changes in atmospheric stability and various other phenomena. Winter months, particularly June and July, appear to show high accuracy and consistency, possibly caused by stable weather patterns. In contrast, Summer forecasts are less reliable, with greater variability and deviation, likely caused by rapid shifts between inland heatwaves and cool Southern Ocean fronts. Spring and Autumn show mixed results, as transitional weather leads to less predictable conditions, especially at the edges of the seasons.

This chart is the same as the last slide, so we already know how it works. There are some interesting differences in the data from NSW when compared to Victoria, lets dive in.

In these charts, we can see the following seasonal trends:

• Unlike in Victoria, the winter months in NSW show a greater spread, indicating less accurate minimum temperature forecasts.

• Also unlike Victoria, summer forecasts in NSW are highly accurate.

• Spring and Autumn once again showed mixed results, with accuracy decreasing as the seasons transition toward Winter.

Based on this information, we can conclude that for a northern state like New South Wales, forecasts are more accurate in the Summer and less accurate in the Winter

There are a few possible explanations for this. One of which NSW’s complex topography, including the Blue Mountains and Snowy Mountains, can create micro climates, making winter forecasting more difficult. Another reason could be Victoria’s proximity to the antarctic results in high pressure systems making their way through the state, which may lead to a more predictable minimum temperature in the winter.

Once again the further from the line, the less accurate the prediction was. There are some major differences in the data shown here compared to the minimum temperature visualization, especially around the summer months. That being said, the general trends are the same as the observations for the winter months:

• In Winter, the predicted maximum temperatures are consistent, with most of the results tightly clustered around the dotted line. This suggests high accuracy and a low variability in maximum temperature, making Winter the most reliable season for the maximum temperature forecasts in Victoria.

• Summer months appear to perform better than minimum temperature forecasts, especially January and February, which show strong alignment with actual temperatures. However, December stands out as an exception, with greater variance and a noticeable drop in accuracy compared to both the other summer months and to Winter. While Summer’s overall performance is improved compared to minimum temperature forecasts, it still doesn’t match the consistency seen in Winter.

• When compared to the minimum temperature predictions, the maximum temperature predictions for the transitional seasons appear to be a lot more accurate, although as they transition away/toward summer and winter they appear to get less accurate.

The seasonal changes in forecast accuracy could be attributed to the influence of Southern Ocean weather systems, which frequently bring cold fronts that clash with hot, dry air masses from Australia’s interior. During Summer, Victoria is often ‘ping-ponged’ between intense heatwaves from the north and cool changes from the south, leading to rapid and unpredictable temperature swings. This dynamic makes it much harder to produce accurate forecasts, especially five days out.

In contrast, during Winter, the influence of heatwaves is greatly reduced, and the atmosphere is generally more stable. The absence of extreme swings between air masses means temperature patterns are easier to predict, resulting in more consistent and accurate forecasts.

In comparison to the results from the minimum temperature forecast for NSW, the results here look remarkably similar. Lets analyse:

• In comparison to the results in Victoria, the Summer forecasts were a lot less varied - but had a similar accuracy. However when compared to the minimum temperatures for NSW they do not differ all that much. For example, the min and max temperature forecasting for December both feature a bimodal distribution.

• The winter forecasts once again are less accurate than Victoria, but show remarkably similar variance to the minimum temperature, although slighly less accurate.

• Once again the transitional seasons feature a wide range of reslts in variance and accuracy, with no real changes in the appearance of the between the minimum forecast and the maximum other than the temperature being predicted itself.

The results here are likely a result of geographical factors, as New South Wales sits on the Eastern coast of Australia, it is not exposed to the cold that hits Victoria from the Southern Ocean - resulting in it being easier to predict the weather in the Summer. Additionally, the presence of mountain ranges such as the Blue and Snowy mountains result in a slightly more difficult to predict Winter, as they create micro climates.