Southeastern Aust Seasonal Temperature Outlook: probabilities for October to December 2008, issued 25th September 2008

Warmer conditions indicated for most of southeastern Australia

The national outlook for temperatures averaged over the December quarter (October to December) indicates a strong shift in the odds towards warmer than normal maximum temperatures and a moderate shift in the odds towards warmer than normal minimum temperatures over most of southeastern Australia.

The pattern of seasonal temperature odds across southeastern Australia reflects a strong contribution from the Indian Ocean where continued warmth exists in the central and southeastern Indian Ocean, especially off the west coast of Western Australia. Although there has been some warming in recent months in the equatorial Pacific, it is generally in a neutral state, and not contributing to any significant shifts in the odds for the temperature outlook in southeastern Australia.

Averaged over October to December, the chance of exceeding the median maximum temperature is mainly between 60 and 75% for much of New South Wales and South Australia. The chances of above average maximum temperatures increases to 75 to 85% over southeastern parts of South Australia, southern NSW, Victoria and Tasmania (see map). This means for every 10 years with ocean patterns like the current, about six to eight years are expected to be warmer than average during the day, while about two to four are expected to be cooler.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During October to December, history shows this effect on maximum temperatures to be moderately consistent over much of northern and western NSW, most of SA and the northern half of both Victoria and Tasmania. However, over a large part of eastern NSW, southern Victoria and southern Tasmania confidence is only weakly consistent, so this outlook needs to be used with caution in these areas (see background information).

There is a moderate shift in the odds towards warmer than average October to December minimum temperatures, with nights favoured to be slightly warmer than normal across much of southeastern Australia (see map). The chances of increased overnight warmth (averaged over the coming three months) are around 60% for NSW, Victoria and Tasmania. The odds increase to 65% over much of South Australia, particularly the NW corner. This means for every ten years with ocean patterns like the current, about six to seven would be expected to be warmer than average in terms of overnight temperatures averaged over October to December, while three to four would be expected to be cooler.

History shows the oceans' effect on minimum temperatures from October to December to be moderately consistent over most of SA, NSW and northwestern and southern Tasmania. However, over most of Victoria, most of southeastern NSW, southeastern SA and central Tasmania the effect is generally only weakly consistent, so this outlook needs to be used with caution in these areas.

probability of exceeding median maximum temperature - click on the image for a larger version of the map
probability of exceeding median minimum temperature - click on the image for a larger version of the map
 

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 28th October 2008

Corresponding rainfall outlook

Maximum temperature departures from average for June to August 2008 - base period 1961-1990

Minimum temperature departures from average for June to August 2008 - base period 1961-1990

 

Background Information

  • 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 (www.bom.gov.au/silo/products/SClimate.shtml).

  • 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 23 El Niño events since 1900 are summarized on the Bureau's web site (www.bom.gov.au/climate/enso/).