Warm start to summer for NSW

There is a moderate shift in the odds towards higher than average maximum temperatures for the late spring to mid-summer period (Nov-Jan) across much of New South Wales, the Bureau of Meteorology announced today. Furthermore, 2005 is set to be one of NSW's warmest years on record after the State-average maximum temperature for the January to September period was the third highest on record.

For the November to January period, the chance that seasonal maximum temperatures will be higher than the median is between 60 and 65% across most of New South Wales west of the Great Dividing Range (see map). So in years with ocean patterns like the current, about six November to January periods out of every ten are expected to be warmer than average over most of the State, with about four out of ten being cooler. The outlook pattern for maximum temperatures has been more heavily influenced by recent higher than average temperatures in the Pacific Ocean. Outlook confidence is related to the consistency of the influence of Pacific and Indian Ocean temperatures on seasonal temperatures. During November to January, history shows this influence on maximum temperatures to be moderately consistent across NSW (see background information). The chance that seasonal minimum temperatures will be higher than the median is between 60 and 70% across NSW, approaching 75% over parts of the Northwest Slopes and northern border regions (see map). History shows the oceans' influence on minimum temperatures during November to January to be moderately consistent over most of NSW.

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THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 16^th NOVEMBER 2005

Corresponding rainfall outlook

Maximum temperature departures from average for July to September 2005 - base period 1961-1990

Minimum temperature departures from average for July to September 2005 - 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/).