National Seasonal Rainfall Outlook: probabilities for Autumn 2012, issued 22nd February 2012
The national outlook for March to May 2012 shows the following:
At this time of year, the outlook model has low skill outside the tropics and southwest WA; 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 are above 60% over much of the continent's eastern half, rising to between 70 and 75% in a broad area west of Townsville. Such odds mean that for every ten years with similar ocean patterns to those currently observed, about six or seven years would be expected to be wetter than average over these areas, while about three or four years would be expected to be drier during autumn. However, this outlook should be used with caution over southern Queensland, SA and NSW due to low confidence levels.
Conversely, the chances of receiving above normal rainfall are between 30 and 40% over southwest WA, southern Victoria and Tasmania. In other words, the chances of below normal rainfall is between 60 and 70%. Once again, we advise caution in the southeast due to low confidence.
Over the rest of the country, the chances of a drier or wetter autumn 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 the March to May period, history shows the effect to be moderately consistent over the northern parts of the country, western Tasmania and southwest WA. Elsewhere, the effect is only weakly to very weakly consistent (see background information).
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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The following climate meteorologists in the National Climate Centre can be contacted about this outlook: Andrew Watkins on (03) 9669 4360, Grant Beard on (03) 9669 4607, Catherine Ganter on (03) 9669 4679.
Regional commentary is available from the Climate Services Sections in the Bureau's Regional Offices:
|Queensland -||(07) 3239 8660|
|New South Wales -||(02) 9296 1555|
|Victoria -||(03) 9669 4949|
|Tasmania -||(03) 6221 2043|
|South Australia -||(08) 8366 2664|
|Western Australia -||(08) 9263 2222|
|The Northern Territory -||(08) 8920 3813|
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