Cooler autumn days favoured in northern parts of SE Australia

Southeastern Aust Seasonal Temperature Outlook: probabilities for Autumn 2012, issued 22nd February 2012

Cooler autumn days favoured in northern parts of SE Australia

The autumn 2012 temperature outlook for southeast Australia shows the following:

  • cooler days are more likely over the northeast half of NSW and northeastern SA
  • warmer nights are more likely over NSW, eastern SA and much of Victoria
  • At this time of year, the outlook model has low skill in maximum temperature for most of southeast Australia; we advise caution using this output.

    This outlook is mostly due to warmer than normal waters over the Indian Ocean; cooler Pacific waters associated with La Niña have had a lesser impact.

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

    The chance that the average autumn maximum temperature will exceed the long-term median maximum temperature is between 30 and 40% over the northeast half of NSW, and northeastern SA. In other words, the chance of below normal temperatures is between 60 and 70%. This means that for every ten years with ocean patterns like the ones currently observed, autumn would be expected to be cooler than average in about six or seven years over this area, with a warmer autumn in the other three or four years. However, confidence in the outlook is low in southern Australia during autumn, so this outlook must be used with caution.

    Over remaining parts of both NSW and SA, together with Victoria and Tasmania, the chances of a warmer or cooler autumn are roughly equal.

    Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During the autumn period, history shows this effect on maximum temperatures to be moderately consistent across Victoria and Tasmania, but weakly to very weakly consistent in NSW and SA (see background information).

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

    The chance that the average minimum temperature for autumn will exceed the long-term median minimum temperature is between 60 and 75% across NSW, western Victoria and eastern SA, with strongest odds in northern NSW. Across most of SA, Tasmania, and eastern Victoria the chances of a warmer or cooler season are roughly equal (see map).

    History shows the oceans' effect on minimum temperatures during the autumn period to be weakly to moderately consistent across most of southeastern Australia.

     

    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 21st March 2012

    Corresponding rainfall outlook

    Maximum temperature departures from average for November 2011 to January 2012 - base period 1961-1990

    Minimum temperature departures from average for November 2011 to January 2012 - 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.

    • 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)

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