Southeastern Aust Seasonal Rainfall Outlook: probabilities for October to December 2012, issued 19th September 2012
The southeast Australian outlook for mid-spring to early summer (October to December) indicates that:
This outlook is a result of warmer than normal waters in the tropical Pacific Ocean and warmer than normal waters in the Indian Ocean.
The chances of receiving above median rainfall during the October to December period are less than 40% over the Eyre Peninsula, southeast SA, most of Victoria and Tasmania. Probabilities drop below 30% in lower southeast SA, southwest Victoria extending through to the Port Phillip region, and northeast Tasmania. In other words, the chances of a drier than normal season across these areas are between 60 and 75%. Such odds mean that for every ten years with similar ocean patterns to those currently observed, about six to eight years would be expected to be drier than average over these areas, while about two to four years would be wetter.
Across NSW, far eastern Victoria and the northern pastoral areas of SA, the odds of a wetter or drier October to December are roughly equal.
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 October to December, history shows the effect to be moderately consistent over most of southeast Australia, with the exception of parts of northern and eastern NSW, southern parts of Tasmania and southern coastal parts of SA, where the effect is only very weakly consistent (see background information). Users should exercise caution when using this outlook in areas of low skill.
Oceanic indicators remain close to El Niño thresholds. However, atmospheric indicators, such as the Southern Oscillation Index (SOI) and trade wind strength, remain near-normal. Climate models surveyed by the Bureau of Meteorology suggest sea surface temperatures in the tropical Pacific Ocean will maintain values around typical El Niño thresholds before easing towards more normal values by the end of 2012 or early 2013. Observations also suggest that current Indian Ocean temperature patterns are typical of those historically associated with decreased spring rainfall over parts of southern, central and northern Australia.
Climatologists will continue to monitor conditions and outlooks closely for any further developments over the coming months, with information on the likelihood of El Niño available fortnightly at the ENSO Wrap-Up.
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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:
|Sydney -||(02) 9296 1555|
|Adelaide -||(08) 8366 2664|
|Melbourne -||(03) 9669 4949|
|Hobart -||(03) 6221 2043|
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 24th October 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