WA Seasonal Temperature Outlook: probabilities for Summer 2010/2011, issued 23rd November 2010
The Western Australia outlook for minimum temperatures averaged over summer (December-February) favours warmer than normal night-time temperatures for western, central and southern WA, whilst in northern parts of the state, cooler nights are favoured. In terms of daytime temperatures, the northeast Interior is more likely to be warmer than normal.
The pattern of seasonal temperature odds across Australia is a result of recent warm conditions in the Indian Ocean as well as cool conditions in the equatorial Pacific Ocean associated with the current La Niña event.
The chance that the average December to February maximum temperature will exceed the long-term median maximum temperature, is between 35 and 40% in the northeast Interior. In other words, for every ten years with ocean patterns like those currently observed, about six to seven summer periods would be expected to be cooler than average, with about three to four being warmer. Most of WA however shows odds of between 40 and 60%, indicating that the chance of being above or below median is equally likely.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During summer, history shows this effect on maximum temperatures to be moderately consistent across much of WA, but generally weakly or very weakly consistent in parts of the Kimberley and eastern Interior (see background information).
The average minimum temperature for December to February is favoured to be above the long-term median minimum temperature in much of western, central and southern WA, with probabilities of exceeding the median in the southern Gascoyne region between 75 and 80%. Conversely , cooler night-time temperatures are favoured in much of the Kimberley and the adjacent Interior, with the odds of exceeding the median between 30 and 35% in the southeast Kimberley and northeast Interior.
History shows the oceans' effect on minimum temperatures during summer to be moderately consistent across most of WA.
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More information on this outlook is available from 8.30am to 4.30pm (WST) Monday to Friday by contacting the Climate Services Centre in the Bureau's Perth Office: (08) 9263 2222.
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 17th December 2010
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. 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 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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