Cooler nights expected in southern parts of SE Australia

For southeastern Australia, the outlook for minimum temperatures averaged over the June quarter (April to June) shows a pattern of strongly contrasting probabilities. Moderate to strong shifts in the odds favour below normal overnight temperatures over the southern parts of the region, while warmer than normal temperatures are indicated in northeast NSW. Although not as strong, the outlook pattern for maximum temperatures is reversed, with warmer days favoured in parts of SA and Victoria and cooler days more likely in northeast NSW.

The pattern of seasonal temperature odds across southeast Australia is mostly a result of warm conditions in the Indian Ocean in February, with the Pacific Ocean having very little effect at all. The chance of exceeding the median minimum temperature for the April to June period is between 20 and 40% across southern SA, southern NSW, Victoria and Tasmania, which translates as a 60 to 80% chance for a lower than normal seasonal average. This means that for every ten years with ocean patterns like the current, six to eight would be expected to be cooler than average in these areas, while two to four would be expected to be warmer. The odds swing the other way across northern NSW, where the chances are between 60 and 70% for the average minimum temperature to exceed the seasonal median. Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During the June quarter, history shows shows this effect on minimum temperatures from April to June to be moderately consistent over most of NSW, Victoria and southern SA. However, over parts of the NSW South Coast, the northern half of SA, coastal Victoria and all of Tasmania the effect is only weakly to very weakly consistent during this time of year (see background information). The outlook for June quarter maximum temperatures also shows a pattern of contrasting probabilities, with a 60 to 65% chance for a higher than normal seasonal average in a band from western SA to western Victoria. Odds in northeast NSW are shifted in the opposite sense: a higher than normal seasonal average has a 35 to 40% chance of occurring, meaning there's a 60 to 65% likelihood of a cooler than normal season. This is consistent with the rainfall outlook which favours a wetter three months in the same area. History shows the oceans' effect on maximum temperatures in the April to June period to be moderately consistent over much of the northeast quarter of NSW, parts of western Victoria, western and northern SA and northern Tasmania. However, for the remainder of southeast Australia confidence is only weakly to very weakly consistent, thus the outlook for these areas needs to be used with caution.

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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:

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 24^th April 2009

Corresponding rainfall outlook

Maximum temperature departures from average for December 2008 to February 2009 - base period 1961-1990

Minimum temperature departures from average for December 2008 to February 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/).