National Seasonal Temperature Outlook: probabilities for Summer 2010/2011, issued 23rd November 2010
The national outlook for maximum and minimum temperatures averaged over summer (December-February) favours warmer daytime and night time temperatures over southern NSW, southern SA and all of Victoria and Tasmania. Warmer night time temperatures are favoured for southwestern WA, while cooler daytime and night time temperatures are favoured in parts of Queensland and central Australia.
The pattern of seasonal temperature odds across Australia is a result of recent warm conditions in the Indian Ocean as well as cool conditions in the equatorial Pacific Ocean associated with the current La Niña event.
The chance that the average December to February maximum temperature will exceed the long-term median maximum temperature is above 60% for southern NSW and SA as well as all of Victoria and Tasmania. The odds are particularly strong over large parts of Victoria and southeastern SA where they exceed 80%. This means that for every ten years with ocean patterns like the ones currently observed, about six to eight years would be expected to be warmer than average during this summer, with about two to four being cooler.
In contrast, the outlook favours cooler daytime temperatures in the southern half of Queensland, northeastern NSW and central Australia, where the odds are between 25 to 40%.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During summer, history shows this effect on maximum temperatures to be moderately consistent across much of the eastern half of the country and western WA, but generally weakly or very weakly consistent elsewhere (see background information).
The average minimum temperature for December to February is favoured to be above the long-term median minimum temperature in northern Queensland, parts of southeast and southern Australia, as well as western and southern WA. Cooler night time temperatures are favoured across most of NT and northern WA.
History shows the oceans' effect on minimum temperatures during summer to be moderately consistent over much of the country, with the exception of parts of southeast Australia and southern NT.
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The following climate meteorologists in the National Climate Centre can be contacted about this outlook: Agata Imielska on (02) 9296 1539, Andrew Watkins on (03) 9669 4360, Lynette Bettio on (03) 9669 4165.
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 17th December 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.
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 past El Niño events since 1900 are summarized on the Bureau's web site (El Niño - Detailed Australian Analysis), and past La Niña events (La Niña - Detailed Australian Analysis)
© Australian Government, Bureau of Meteorology