National Seasonal Rainfall Outlook: probabilities for October to December 2012, issued 19th September 2012

Drier conditions likely in the southeast

The national outlook for October to December indicates that:

• a drier than normal season is likely for large parts of southeast Australia
• a wetter than normal season is likely for western WA and southwest Queensland

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 between 25 and 40% over southeast SA, most of Victoria and Tasmania (see map above). In other words the chances of below normal rainfall are between 60 and 75%. Such odds mean that for every ten years with similar ocean patterns to those currently observed, about three to four years would be expected to be wetter than average over these areas, while about six to seven years would be drier than average.

In contrast, the chances of receiving above normal rainfall are higher than 60% over most of western WA and parts of southwest Queensland. However, it should be noted that rainfall is commonly low over southwest Queensland and much of the Pilbara and Gascoyne districts in WA at this time of year.

Over the rest of the country, the chances of a drier or wetter October to December period 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 much of the country, with the exception of northeast NSW, southwest WA, and small parts near the southern Queensland border, 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.

 

Click on the map above for a larger version of the map. Use the reload/refresh button to ensure the latest forecast map is displayed.

 

The following climate meteorologists in the National Climate Centre can be contacted about this outlook: Andrew Watkins on (03) 9669 4360, Catherine Ganter on (03) 9669 4679, Elise Chandler on (03) 9669 4748.

 

Regional versions of this media release are available: | Northern Aust | Southeastern Aust | WA |

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 24^th October 2012

Corresponding temperature outlook

August 2012 rainfall in historical perspective

June to August 2012 rainfall in historical perspective

 

Background Information

• 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)