National Seasonal Temperature Outlook: probabilities for Autumn 2010, issued 24th February 2010
The national outlook for mean maximum temperatures over autumn (March to May), shows a mixed pattern of odds. A moderate to strong tendency in the odds favours above average values in a broad region covering much of northeast and northern Australia, and in the west of WA. In contrast, cooler than average daytime temperatures are indicated in parts of southeastern Australia.
The pattern of seasonal temperature odds across Australia is due to recent higher than average temperatures in both the Pacific (El Niño) and Indian Oceans. The Pacific pattern has had the greater influence.
The chance that the average March-May maximum temperature will exceed the long-term median maximum temperature, is between 60 and 80% northeast of a line from Derby in northern WA to the northern NSW coast, and between 60 and 70% in western WA (see map). This means that for every ten years with ocean patterns like the current, about six to eight autumns would be expected to be warmer than average over these areas, while about two to four would be cooler.
In contrast, there is a 60 to 65% chance of cooler than normal days averaged over autumn across western Victoria and the southeast of SA.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During autumn, history shows this effect on maximum temperatures to be moderately consistent over Victoria, Tasmania, the far north and south of WA, and much of Queensland and the NT. Elsewhere the effect is weak (see background information), so this outlook should be used with caution in western WA.
The average minimum temperature for March to May is favoured to be above the long-term median minimum temperature over WA, the NT, Queensland, and the northern parts of both SA and NSW (see map). The chances of increased overnight warmth (averaged over autumn) are between 60 and 80% in these areas, with the highest probabilities being located in the far north. In remaining areas of southeast and southern Australia, the chances of above average autumn minima are between 40 and 60%, so warmer or cooler than normal nights are about as equally likely.
History shows the oceans' effect on minimum temperatures during autumn to be moderately consistent over most of the country.
Click on the maps above for larger versions of the maps. Use the reload/refresh button to ensure the latest forecast maps are displayed.
The following climate meteorologists in the National Climate Centre can be contacted about this outlook: Grant Beard on (03) 9669 4527, Karl Braganza on (03) 9669 4344, Elise Chandler on (03) 9669 4748.
Regional commentary is available from the Climate Services Sections in the Bureau's Regional Offices:
|Queensland -||(07) 3239 8660|
|New South Wales -||(02) 9296 1555|
|Victoria -||(03) 9669 4949|
|Tasmania -||(03) 6221 2043|
|South Australia -||(08) 8366 2664|
|Western Australia -||(08) 9263 2222|
|The Northern Territory -||(08) 8920 3813|
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 23rd March 2010
The Bureau's seasonal outlooks are general statements about the probability or risk of wetter or drier than average weather over a three-month period. The outlooks are based on the statistics of chance (the odds) taken from Australian rainfall/temperatures and sea surface temperature records for the tropical Pacific and Indian Oceans. They are not, however, categorical predictions about future rainfall, and they are not about rainfall within individual months of the three-month outlook period. The temperature outlooks are for the average maximum and minimum temperatures for the entire three-month outlook period. Information about whether individual days or weeks may be unusually hot or cold, is unavailable.
This outlook is a summary. More detail is available from the contact people or from SILO (Seasonal Climate Outlook Products).
Probability outlooks should not be used as if they were categorical forecasts. More on probabilities is contained in the booklet The Seasonal Climate Outlook - What it is and how to use it, available from the National Climate Centre. These outlooks should be used as a tool in risk management and decision making. The benefits accrue from long-term use, say over 10 years. At any given time, the probabilities may seem inaccurate, but taken over several years, the advantages of taking account of the risks should outweigh the disadvantages. For more information on the use of probabilities, farmers could contact their local departments of agriculture or primary industry.
Model Consistency and Outlook Confidence: Strong consistency means that tests of the model on historical data show a high correlation between the most likely outlook category (above/below median) and the verifying observation (above/below median). In this situation relatively high confidence can be placed in the outlook probabilities. Low consistency means the historical relationship, and therefore outlook confidence, is weak. In the places and seasons where the outlooks are most skilful, the category of the eventual outcome (above or below median) is consistent with the category favoured in the outlook about 75% of the time. In the least skilful areas, the outlooks perform no better than random chance or guessing. The rainfall outlooks perform best in eastern and northern Australia between July and January, but are less useful in autumn and in the west of the continent. The skill at predicting seasonal maximum temperature peaks in early winter and drops off marginally during the second half of the year. The lowest point in skill occurs in early autumn. The skill at predicting seasonal minimum temperature peaks in late autumn and again in mid-spring. There are also two distinct periods when the skill is lowest - namely late summer and mid-winter. However, it must always be remembered that the outlooks are statements of chance or risk. For example, if you were told there was a 50:50 chance of a horse winning a race but it ran second, the original assessment of a 50:50 chance could still have been correct.
The Southern Oscillation Index (SOI) is calculated using the barometric pressure difference between Tahiti and Darwin. The SOI is one indicator of the stage of El Niño or La Niña events in the tropical Pacific Ocean. It is best considered in conjunction with sea-surface temperatures, which form the basis of the outlooks. A moderate to strongly negative SOI (persistently below −10) is usually characteristic of El Niño, which is often associated with below average rainfall over eastern Australia, and a weaker than normal monsoon in the north. A moderate to strongly positive SOI (persistently above +10) is usually characteristic of La Niña, which is often associated with above average rainfall over parts of tropical and eastern Australia, and an earlier than normal start to the northern monsoon season. The Australian impacts of 25 El Niño events since 1900 are summarized on the Bureau's web site (El Niño - Detailed Australian Analysis).
© Australian Government, Bureau of Meteorology