Mixed rainfall odds for winter

National Seasonal Rainfall Outlook: probabilities for Winter 2012, issued 23rd May 2012

Mixed rainfall odds for winter

The national outlook for June to August 2012 shows the following:

  • a moderate shift in odds favouring a wetter than average season over large areas of Queensland and parts of northeastern NSW
  • drier than normal season for southwest WA, much of SA, and parts of northwestern Victoria and northwest Australia

The pattern of seasonal rainfall odds across Australia is mostly a result of 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 for winter are over 60% across the eastern two-thirds of Queensland as well as in parts of eastern and northern NSW. Odd exceed 70% in parts of northern Queensland. 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 the winter. It should be noted that rainfall is commonly low over much of tropical northern Australia at this time of the year and contributes only a small fraction of the annual total.

In contrast, the chances of receiving above normal rainfall are between 25 and 40% over southwest WA. Odds are around 40% over southern and central SA, and the far western border regions of Victoria and southwest NSW. In other words, the chances of below normal rainfall are between 60 and 75% in these areas.

Over the rest of the country, the chances of a drier or wetter winter 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 winter period, history shows the effect to be moderately consistent across much of Queensland, the NT and northern NSW, but generally weakly consistent elsewhere across the country, reaching moderate only in patches (see background information). Users should exercise caution when using this outlook in these areas of low skill.

All key indicators remain at neutral (i.e., not El Niño nor La Niña) levels. Climate models surveyed by the Bureau of Meteorology indicate that the tropical Pacific Ocean will continue to warm over the next six months, with most models approaching or exceeding El Niño thresholds during the second half of 2012. Bureau of Meteorology climatologists closely monitor the state of the climate, with information on the liklihood of El Niño or La Niña 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, Robyn Duell on (03) 9669 4671, Catherine Ganter on (03) 9669 4679.

 

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 20th June 2012

Corresponding temperature outlook

April 2012 rainfall in historical perspective

February to April 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)

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