|National Seasonal Rainfall Outlook: probabilities for Spring 2007, issued 23rd August 2007|
Mixed odds for spring rainfall
The national outlook for total spring (September to November) rainfall, shows no strong swings in the odds towards wetter or drier conditions over most of Australia. However, the odds favour a wetter than average spring in southwest WA, while a drier than average spring is the more likely outcome in some relatively small patches in eastern Australia.
The pattern of seasonal rainfall odds across Australia is a result of continuing higher than average temperatures over parts of the south tropical Pacific Ocean, and also in parts of the tropical Indian Ocean.
The chances of exceeding the median rainfall for the September to November period are between 55 and 60% in southwest WA (see map). So in years with ocean patterns like the current, about six springs out of ten are expected to be wetter than average in this part of the country, while about four out of ten are expected to be drier.
In contrast, the southeast of SA together with parts of western and southern Victoria and northern Tasmania have an increased likelihood of a drier than normal spring. This is also the case for the Upper Carpentaria region of northern Queensland. The chances of exceeding the seasonal median in these areas are between 35 and 40%, meaning that below-normal falls have a 60 to 65% chance of occurring. Over the rest of the country the chances of accumulating at least average rain for spring are relatively close to 50%.
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).
There has been further gradual cooling of the equatorial Pacific during the past month, with cool anomalies consistent with a developing La Niña, now present in the east. The 30-day value of the Southern Oscillation Index (SOI) was +1 as at 20th 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.
|The following climate meteorologists in the National Climate Centre can be contacted about this outlook: Grant Beard on (03) 9669 4527, Lyn Bettio on (03) 9669 4165, 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 25th SEPTEMBER 2007