|National Seasonal Rainfall Outlook: probabilities for October to December 2009, issued 22nd September 2009|
Mixed rainfall outlook for the December quarter
The national outlook for rainfall over the December quarter (October to December) shows a moderate shift in the odds favouring a drier than normal season across parts of northeastern and southeastern Australia. On the other hand, the odds are shifted in favour of a wetter than normal season in western and central WA. It should be noted though, that the December quarter is a seasonally dry time of year in northwest WA, with heavy rain being uncommon.
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.
The chances of exceeding the median rainfall for October to December are between 25 and 40% over northeast Queensland, parts of Victoria, southeast SA and Tasmania (see map). This means that for every ten years with ocean patterns like the current, about six or seven years are expected to be drier than average over these regions, while about three or four years are wetter.
Contrasting this, the chance of a wetter than average conditions is between 60 and 75% for western and central parts of WA.
Across the rest of the country, including northern Victoria, NSW, NT, much of SA and eastern WA, the chances of exceeding the median rainfall for October to December are between 40 and 60%, meaning that above average falls are about as equally likely as below average falls in these regions.
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 the October to December period, history shows this effect to be moderately consistent over much of the country (see background information).
An unusual El Niño event persists across the Pacific Basin, with most leading climate models forecasting the event to peak late in 2009. 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 +3 for the 30 days ending 19 September. 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, Elise Chandler on (03) 9669 4748.|
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 23nd October 2009