Southeastern Aust Seasonal Temperature Outlook: probabilities for Summer 2010/2011, issued 23rd November 2010
The December to February outlook for maximum temperatures across southeastern Australia is favouring warmer than average temperatures for Victoria, Tasmania, southern New South Wales, and the southern half of South Australia, while cooler than average conditions are favoured for northeastern New South Wales.
The pattern of seasonal temperature odds across Australia is a result of warm conditions in the Indian Ocean, as well as cool conditions in the equatorial Pacific Ocean associated with the current La Niña.
The chance that the daytime temperatures will be warmer than the long-term median maximum temperature for the December to February period is greater than 60% over southern New South Wales and in the south of the South Australian pastoral areas, with the odds strengthening to greater than 75% over all of Victoria and South Australian agricultural areas. This means that out of every ten years with ocean patterns like the current, six to eight years would be expected to be warmer than average for this summer.
Over northeastern New South Wales, the odds are less than 40%, meaning that for every ten years with ocean patterns like the current, only about three to four years are expected to have warmer than average days during this period. This outlook for cooler daytime temperatures in this area is consistent with the increased likelihood of rainfall across this area.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During December to February, history shows the influence on maximum temperatures to be moderately to strongly consistent over New South Wales, Victoria, Tasmania and South Australia, with the exception of far northeastern South Australia where it is only weakly consistent (see background information).
The average minimum temperature for December to February is favoured to be above the long-term median minimum temperature across all of Victoria, southern NSW, SA agricultural areas and most of Tasmania with odds of 60 to 70%. The outlook is neutral for South Australian pastoral areas, of the remainder of New South Wales and parts of west and southwestern Tasmania.
History shows the oceans' effect on minimum temperatures during December to February to be moderately consistent for almost all of Victoria, Tasmania and South Australia. The summer influence is also strongly consistent for northeastern New South Wales, but only weakly consistent for south and southwestern New South Wales.
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More information on this outlook is available Monday to Friday from 9.00am to 5.00pm local time by contacting the Bureau's Climate Services sections in Queensland, NSW, SA, Victoria and Tasmania at the following numbers:
|Sydney -||(02) 9296 1555|
|Adelaide -||(08) 8366 2664|
|Melbourne -||(03) 9669 4949|
|Hobart -||(03) 6221 2043|
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