Southeastern Aust Seasonal Rainfall Outlook: probabilities for January to March 2009, issued 17th December 2008

Wet start to the year favoured for northeastern NSW

The outlook for rainfall during the first quarter of 2009 (January to March) shows a moderate shift in the odds favouring wetter than normal conditions over northeastern NSW. For the remainder of southeastern Australia the odds favouring a wetter or drier first quarter of 2009 are even.

Over much of northeastern NSW the chances of January to March totals exceeding the 3-month median rainfall is around 60% (see map). This means for every ten years with ocean patterns like the current, about six years are expected to be wetter than average while about four years are expected to be drier. Over the remainder of NSW, South Australia, Victoria and Tasmania the chances of exceeding the seasonal median are between 45% and 55%. This means the chances of being wetter than normal in these areas are about the same as the chances of being drier.

The pattern of seasonal rainfall odds across Australia is mainly a result of continued warmth in the central Indian Ocean. The Pacific Ocean has not contributed significantly to this pattern (see below however, for a full discussion of the current state of the Pacific Ocean and ENSO).

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

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During January to March, history shows this effect to be moderately consistent over coastal NSW and weak to moderately consistent over the northeastern inland of NSW, parts of Tasmania, as well as the Victorian coastal districts west of Port Philip and the west coast and southern part of the northwest pastoral districts of SA. However, confidence is only weakly to very weakly consistent over much of the south and southwestern NSW, most of Victoria, SA and parts of Tasmania (see background information). In these areas where outlook confidence is not high caution should be used when interpreting these outlooks.

The tropical Pacific Ocean remains ENSO-neutral. Sea surface temperatures across most of the equatorial Pacific are near normal, however, cool sub-surface temperatures persist from the dateline to the South American coast and the trade winds are now stronger than normal across most of the basin. The SOI remains positive at approximately +13 for the 30 days ending 14 December. Most international computer models indicate that neutral conditions are likely to continue through summer and autumn, however, two models predict cooling. For routine updates and comprehensive discussion on any developments please see 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 22nd January 2009

Corresponding temperature outlook

November 2008 rainfall in historical perspective

September to November 2008 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 or from SILO (www.bom.gov.au/silo/products/SClimate.shtml).

  • Probability outlooks should not be used as if they were categorical forecasts. More on probabilities is contained in the booklet The Seasonal Climate Outlook - What it is and how to use it, available from the National Climate Centre. 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 23 El Niño events since 1900 are summarized on the Bureau's web site (www.bom.gov.au/climate/enso/).