|WA Seasonal Rainfall Outlook: probabilities for Winter 2007, issued 22nd May 2007|
50:50 chances for above average winter rainfall in WA
The Western Australian outlook for total winter rainfall (June to August), shows no strong swings in the odds towards either above-normal or below-normal rainfall. The pattern of seasonal rainfall odds across Australia is a result of recent higher than average temperatures (although with a cooling trend) in the tropical Pacific Ocean, and also in parts of the tropical and sub-tropical Indian Ocean.
If, as computer models predict, the Pacific cools to such an extent that a La Niña forms during the next few months, subsequent issues of the seasonal rainfall outlook are likely to show increased chances of above average rainfall over eastern and northern Australia.
The chances of exceeding the median rainfall for the June to August period are mainly between 45 and 50% WA (see map). So in years with ocean patterns like the current, about five winters out of ten are wetter than average across the State and five out of ten are drier. It should be remembered though, that parts of far northern WA are seasonally dry in winter, and significant falls of rain are uncommon.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During winter, history shows this influence to be generally weakly consistent in Western Australia, reaching moderate only in patches (see background information).
The Southern Oscillation Index (SOI) remained neutral in April with a value of −3 following the −1 in March. The approximate 30-day value as at 19th May was −2.
A large body of cool subsurface water persists in the central to eastern Pacific, thereby raising the potential for a La Niña to develop in 2007. Furthermore, computer models are predicting a La Niña to develop during winter. 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 information on this outlook is available from 9.00am to 5.00pm (WST) Monday to Friday by contacting the Climate and Consultancy section in the Bureau's Perth Office: (08) 9263 2222.|
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 26th JUNE 2007