Southeastern Aust Seasonal Temperature Outlook: probabilities for Summer 2011/2012, issued 22nd November 2011
The southeast Australian temperature outlook averaged over December 2011 to February 2012 shows the following:
A persistently warm Indian Ocean is the main driver behind this outlook, cool conditions in the tropical Pacific Ocean associated with the La Niña are also contributing.
The chance that the average December to February maximum temperature will exceed the long-term median maximum temperature is between 60 and 80% over Tasmania, Victoria, and southern parts of both NSW and SA. Odds rise to above 80% across most of Victoria and southern SA (see map). This means that for every ten years with ocean patterns like the ones currently observed, about six to eight December to February periods would be expected to be warmer than average in these areas, with about two to four being cooler.
In contrast, there is a 25 to 40% chance of warmer than normal days over northeastern NSW. In other words, there is a 60 to 75% chance of cooler days over this region. Across northern SA and central NSW the chances of a warmer or cooler summer are roughly equal.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During the December to February period, history shows this effect on maximum temperatures to be moderately consistent across most of southeastern Australia, except for western Tasmania and northwestern SA where it is only weakly consistent (see background information).
There is no strong shift in the odds towards warmer or cooler minimum temperatures across southeastern Australia during summer (see map). In other words, the chance that the average summer minimum temperature will exceed the long-term median minimum temperature is roughly 50%.
History shows the oceans' effect on minimum temperatures during the December to February period to be moderately consistent over most of southeastern Australia, but only weakly to very weakly consistent in sotuhern NSW and southeastern SA.
Click on the maps above for larger versions of the maps. Use the reload/refresh button to ensure the latest forecast maps are displayed.
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 20th December 2011
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. 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