|WA Seasonal Rainfall Outlook: probabilities for October to December 2009, issued 22nd September 2009|
Wetter than normal season favoured for western and central WA
The WA outlook for rainfall over the December quarter (October to December) shows a tendency for a wetter than normal season over 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 Western Australia is mostly a result of recent warm conditions in the central to southeastern Indian Ocean.
The chances of exceeding the median rainfall for October to December are between 60 and 75% for a large part of western and central WA (see map). This means that for every ten years with ocean patterns like the current, about six or seven years are expected to be wetter than average over these regions, while about three or four years are drier.
For the rest of the state, including the Southwest Land Division, the chances of exceeding the median rainfall for October to December are between 50 and 60%, meaning that above average falls are about as equally likely as below average totals 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 most of WA (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.
|More information on this outlook is available from 8.30am to 4.30pm (WST) Monday to Friday by contacting the Climate Services Centre in the Bureau's Perth Office: (08) 9263 2222.|
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 23nd October 2009