Southeastern Aust Seasonal Temperature Outlook: probabilities for Autumn 2009, issued 20th February 2009

Below average autumn temperatures favoured in Victoria and southeast South Australia

For southeast Australia the outlook for daytime and overnight temperatures in autumn (March to May) shows a moderate shift in the odds favouring cooler than normal conditions over Victoria and southeast South Australia. The outlook for daytime temperatures for the remainder of southeast Australia shows no strong shifts favouring either warmer or cooler conditions. However the outlook for overnight temperatures shows a slight chance of warmer temperatures in northeastern NSW.

For southeast Australia, the pattern of seasonal temperature odds is mostly a result of warm conditions in the Indian Ocean in January; the Pacific Ocean has had little contribution to this forecast.

The chances of above median maximum temperatures over March to May are below 40% for Victoria and southeast South Australia. This means for every ten years with ocean patterns like the current, about 6 would be expected to be cooler than average in terms of daytime temperatures averaged over March to May, while four would be expected to be warmer. Elsewhere in south east Australia the chances of exceeding the median maximum temperatures is between 40 and 50%.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During March to May, history shows this effect on maximum temperatures to be moderately consistent over all of Victoria and Tasmania, the South East districts of South Australia, and far southern, coastal and north eastern NSW. However, for the remainder of southeast Australia, including most of inland NSW, confidence is only weakly to very weakly consistent, thus the outlook for these areas needs to be used with caution (see background information).

The chances of exceeding median minimum temperatures are over 60% in northeastern NSW but grade down to 40% for southern South Australia, south western NSW and all of Victoria, and drop below 35% for south eastern South Australia and the western inland of Victoria. The remainder of southeast Australia, including Tasmania and northern South Australia, has about even chances of above or below average overnight temperatures.

History shows the oceans' effect on minimum temperatures from March to May to be moderately consistent over parts of north eastern NSW, western Victoria, and the South East districts of South Australia. Elsewhere the effect is only weakly or very weakly consistent, so for these areas the outlook needs to be used with caution (see background information).

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
 

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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 24th March 2009

Corresponding rainfall outlook

Maximum temperature departures from average for November 2008 to January 2009 - base period 1961-1990

Minimum temperature departures from average for November 2008 to January 2009 - 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/).