Drier spring likely for southeastern Australia

Southeastern Aust Seasonal Rainfall Outlook: probabilities for Spring 2012, issued 22nd August 2012

Drier spring likely for southeastern Australia

The Bureau of Meteorology's southeast Australian outlook for spring 2012 indicates a dry season is more likely for most of southeast Australia.

This outlook is a result of emerging warmer than normal waters in the tropical Pacific Ocean and persistent warmer than normal waters in the Indian Ocean.

probability of exceeding median rainfall - click on the map for a larger version of the map

The chances of receiving above median rainfall during spring are below 40% across most of Victoria, southern SA, southern NSW and much of Tasmania, with odds below 30% in southeast SA and northwest Victoria. In other words, the chances of a drier than normal spring are between 60 and 75%. This means that for every ten years with ocean patterns similar to those currently observed, about six to eight years would be drier than normal over these areas, while about two to four would be wetter.

Over northern and eastern NSW, northern SA, southern Tasmania and far eastern Victoria the odds of a wetter or drier season 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 spring, history shows the effect to be moderately consistent over most of southeast Australia, but only weakly to moderately consistent in western SA, northeast NSW, and southern Tasmania (see background information).

Climate indicators remain close to El Niño thresholds. Although El Niño development stalled during the second half of July, over the first half of August several indicators, such as the Southern Oscillation Index (SOI) and trade wind strength, have shown renewed trends that are consistent with the early stages of El Niño. Climate models surveyed by the Bureau of Meteorology continue to show further warming across the tropical Pacific Ocean is likely, with temperatures likely to exceed El Niño thresholds before the end of September.

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.

 

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 19th September 2012

Corresponding temperature outlook

July 2012 rainfall in historical perspective

May to July 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)

Related links

Definitions

Email Alert

    If you would like to subscribe to an email alert for this product please email webclim@bom.gov.au

© Australian Government, Bureau of Meteorology