WA Seasonal Temperature Outlook: probabilities for Winter 2010, issued 25th May 2010
The WA outlook for temperatures over the winter period (June to August) shows a strong tendency in the odds favouring warmer than normal daytime temperatures over WA.
The pattern of seasonal temperature odds across Australia is due to recent higher than average temperatures in both the Pacific and Indian Oceans.
The chance that the average June-August maximum temperature will exceed the long-term median maximum temperature, is greater than 80% for much of WA, grading to 60 to 65% for small coastal patches in the north and west Kimberley (see map). This means that for every ten years with ocean patterns like the current, more than eight winter periods would be expected to be warmer than average across much of WA, whilst in the Kimberley six to seven years out of ten would be expected to be warmer than normal in terms of maximum temperature.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During winter, history shows this effect on maximum temperatures to be moderately consistent over most of WA grading to strongly consistent in the west Kimberley (see background information).
The chance that the average minimum temperature for June to August will be above the long-term median minimum temperature is between 55 and 65% in the north and east of WA, to above 80% in central western parts (see map). This means for every ten years with similar ocean temperatures to current, roughly seven to eight would be expected to be warmer than average in terms of overnight temperature across much of south and west WA.
History shows the oceans' effect on minimum temperatures in winter to be moderately consistent over most of WA, apart from areas of the Kimberley where the effect is only weakly or very weakly consistent.
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 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 23rd June 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 or from SILO (Seasonal Climate Outlook Products).
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 25 El Niño events since 1900 are summarized on the Bureau's web site (El Niño - Detailed Australian Analysis).
© Australian Government, Bureau of Meteorology