|National Seasonal Rainfall Outlook: probabilities for August to October 2009, issued 24th July 2009|
A drier outlook for E Aus, but wetter conditions favoured in SW WA
The national outlook for total rainfall over the late winter to mid-spring period (August to October), shows moderate to strong shifts in the odds favouring a drier than normal season across much of eastern Australia. On the other hand, the odds are shifted in favour of a wetter than normal season in southwest WA.
The pattern of seasonal rainfall odds across Australia is a result of recent warm conditions in the Indian Ocean and warming in the Pacific. The Pacific influence dominates the outlook in eastern Australia, while the Indian Ocean has had a greater influence on the probabilities in WA.
For the August to October period, the chance of exceeding median rainfall is below 40% in a broad area extending over most of Queensland, the far east of the NT, the eastern half of SA, far western NSW, the western half of Victoria and into northern Tasmania (see map). Within this zone, the chances drop to around 20% in north-central Queensland. This means that for every ten years with ocean patterns like the current, about two to four years would be expected to be wetter than average in these parts of eastern Australia during this three-month period, with about six to eight being drier.
In contrast, over southwest WA the chances of exceeding the seasonal median rainfall are between 60 and 65%.
Over most remaining parts of the country, the chance of a wetter than average season is between 40 and 50%. In other words the chances of above normal falls are about the same as the chances of below normal.
New: 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 August to October, history shows this effect to be moderately consistent across most of Queensland, the east and north of the NT and the northern inland of NSW. Elsewhere, it is generally weakly consistent, reaching moderate only in patches, including in southeast SA, western and central Victoria and northern Tasmania (see background information). Therefore, this outlook should be used with caution across central SA and the far southwest of NSW where the effect is very weakly consistent.
An El Niño event looks to be developing across the Pacific: the latest outputs from computer models indicate it will reach peak intensity late in the year. 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. The SOI is approximately +11 for the 30 days ending 21 July. 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 detailed forecast maps, including the probabilities of seasonal rainfall exceeding given totals, can be found here.
|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, Robyn Duell on (03) 9669 4671.|
Regional commentary is available from the Climate Services Sections in the Bureau's Regional Offices:
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 26th August 2009