|National Seasonal Rainfall Outlook: probabilities for Spring 2009, issued 26th August 2009|
A mixed spring rainfall outlook
The national outlook for spring (September to November) rainfall, shows moderate to strong shifts in the odds favouring a drier than normal season across parts of eastern and southern 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 both the Pacific and Indian Oceans. The Pacific influence (El Niño) dominates the outlook in eastern Australia, while the Indian Ocean has had a greater influence on the probabilities in WA.
For the September to November period, the chance of exceeding median rainfall is below 40% across the northern half of Queensland, southern NSW west of the Great Divide, much of SA and Victoria, and large parts of Tasmania (see map). Within these areas, the chances drop to around 25% in parts of Queensland and in south-central SA. This means that for every ten years with ocean patterns like the current, about three or four years would be expected to be wetter than average in these parts of eastern and southern Australia during spring, with about six or seven 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 spring, history shows this effect to be moderately consistent across most of the eastern mainland states and the NT, as well as northern Tasmania, southwest and far northwest WA, and parts of SA (see background information).
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 −7 for the 30 days ending 23 August. 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, David Jones on (03) 9669 4085.|
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 22nd September 2009