Southeastern Aust Seasonal Temperature Outlook: probabilities for Summer 2009/2010, issued 24th November 2009
For southeastern Australia, the outlook for mean summer (December-February) temperatures favours warmer conditions in far northern NSW, while cooler than normal nights are indicated in parts of Victoria and SA.
The pattern of seasonal temperature odds across southeastern Australia is a result of recent warm conditions in both the Pacific (El Niño) and Indian Oceans. A contrasting influence is apparent over southern SA, Victoria and Tasmania: The Pacific pattern promotes below average maxima, while the Indian Ocean pattern shifts the odds in favour of warmer conditions. The combined result is a neutral pattern in these areas.
The chance that the average December-February maximum temperature will exceed the long-term median maximum temperature, is around 60% in the northern and northeast border districts of NSW (see map). This means that for every ten years with ocean patterns like the current, about six summers would be expected to be warmer than average over these northern parts of southeastern Australia, while about four would be cooler.
The 45 to 60% chances across the remainder of southeast Australia indicate that neither warmer or cooler daytime temperatures are particularly favoured.
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 moderate to strongly consistent over most of NSW, northeastern and southern SA, western and eastern Victoria and most of Tasmania. However, over northwestern and inland SA and parts of southwest NSW and inland Victoria the influence is only weakly to very weakly consistent. In these areas where outlook confidence is not high, caution should be used when interpreting these outlooks (see background information).
The average minimum temperature for summer is favoured to be above the long-term median minimum temperature over the northern parts of both SA and NSW (see map). The chances of increased overnight warmth (averaged over Dec-Feb) are between 60 and 70% in these areas. In contrast, a small region covering western Victoria and southeast SA has an increased likelihood of cooler than normal nights averaged over summer.
History shows the oceans' effect on minimum temperatures during summer to be moderately to strongly consistent over the northern half of SA, northern half of NSW and much of Tasmania. However, over most of Victoria, and the southern halves of both NSW and SA, the influence is only weakly to very weakly consistent.
Click on the map above for a larger version of the map. Use the reload/refresh button to ensure the latest fo recast map is 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 21st December 2009
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