Mixed seasonal rainfall odds for the March quarter

National Seasonal Rainfall Outlook: probabilities for January to March 2010, issued 21st December 2009

Mixed seasonal rainfall odds for the March quarter

The national outlook for total rainfall over the March quarter (January-March), shows a moderate shift in the odds favouring a drier than normal season through much of Queensland, the northeast half of NSW and southwest WA. In contrast, the odds point to above average falls in a band extending from southern SA across parts of Victoria to northern Tasmania.

The pattern of seasonal rainfall odds across Australia has been heavily influenced by the El Niño pattern of higher than average temperatures across the central to eastern Pacific Ocean. Although weaker, the contribution from above normal Indian Ocean temperatures has reinforced the Pacific influence.

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

The chances of exceeding the median rainfall for January to March are between 30 and 40% over a large area covering much of eastern and central Queensland and the northeast half of NSW (see map). This means that for every ten years with ocean patterns like the current, about six or seven March quarters are expected to be drier than average in this region, while about three or four are wetter. However, the skill of these outlooks is not high over the more western inland areas of Queensland and NSW (roughly west of the 35% contour), so caution is advised when applying the outlook in those regions.

There are similar odds in southwest WA, although the January to March period is usually rather dry in that part of the country.

Contrasting this, the chances are between 60 and 65% for above average January to March falls in a band stretching from the Eyre Peninsula in SA, to central Victoria and northern Tasmania. Once again caution is advised due to the low levels of skill for the outlooks at this time of year.

Across the rest of the country, the chances of exceeding the median March quarter rainfall are between 40 and 60%, meaning that above average falls are about as equally likely as below average falls.

An expanded set of seasonal rainfall outlook maps and tables, including the probabilities of seasonal rainfall exceeding given totals (e.g. 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 quarter, history shows the effect to be moderately consistent through eastern parts of NSW and Queensland, large parts of the NT and over much of southern and western WA. Elsewhere the effect is only weakly or very weakly consistent (see background information).

An El Niño event persists across the Pacific Basin, with most leading climate models suggesting tropical ocean temperatures will remain above El Niño thresholds until at least early 2010. El Niño events are usually (but not always) associated with below normal rainfall in the second half of the year across large parts of southern and inland eastern Australia. However, the influence from El Niño often weakens in the second half of summer. The SOI is approximately −9 for the 30 days ending 16 December. 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.

 

The following climate meteorologists in the National Climate Centre can be contacted about this outlook: Grant Beard on (03) 9669 4527, Karl Braganza on (03) 9669 4344, 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 19th January 2010

Corresponding temperature outlook

November 2009 rainfall in historical perspective

September to November 2009 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 (Seasonal Climate Outlook Products).

  • 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 25 El Niño events since 1900 are summarized on the Bureau's web site (El Niño - Detailed Australian Analysis).

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