WA Seasonal Rainfall Outlook: probabilities for Autumn 2012, issued 22nd February 2012
The Western Australian outlook for March to May 2012 shows the following:
At this time of year, the outlook model has low skill in large parts of WA outside the Southwest Land Division (SWLD); 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.
The chances of receiving above median rainfall for autumn 2012 are between 30 and 40% over a large proportion of the SWLD. In other words, the chances of below normal rainfall is between 60 and 70% in parts of this region. Such odds mean that for every ten years with similar ocean patterns to those currently observed, about three or four years would be expected to be wetter than average, while about six or seven years would be expected to be drier during autumn. The remainder of WA outside the SWLD mostly has odds between 40 and 60% of exceeding the median, thus the chances of above average and below average rainfall in autumn are roughly even in the majority of WA, though outlook confidence is generally low outside the SWLD in autumn.
An expanded set of seasonal rainfall outlook maps and tables, including the probabilities of seasonal rainfall exceeding given totals (e.g. chance of receiving at least 200 mm), is available on the "Water and the Land" (WATL) part of the Bureau's website.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During the March to May period, history shows the effect to be moderately consistent over most of the SWLD, apart from an area centred on the Southwest district where the effect is only weakly to very weakly consistent (see background information). In the remainder of WA, history shows the effect to be moderately consistent in parts of the southern Kimberley and Interior, but only weakly to very weakly consistent elsewhere.
La Niña, which has dominated our climate since spring, has shown signs of weakening. Computer models surveyed by the Bureau suggest La Niña will continue declining over the southern hemisphere autumn, as is typical for ENSO events. For routine updates and comprehensive discussion on any developments regarding El Niño and La Niña, please see the ENSO Wrap-Up.
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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 21st March 2012
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)
© Australian Government, Bureau of Meteorology