|National Seasonal Rainfall Outlook: probabilities for October to December 2007, issued 25th September 2007|
Mixed rainfall odds for the December quarter
The national outlook for total rainfall over the December quarter (October to December), shows contrasting odds between the west and southeast of the country. Higher seasonal falls are indicated in western WA while a drier than average season is more probable in the far southeast of the country. 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 mainly a result of continuing higher than average temperatures over parts of the tropical and sub-tropical Indian Ocean.
The chances of exceeding the median rainfall for the October to December period are between 60 and 70% over much of western to central WA (see map). So for every ten years with ocean patterns like the current, about six or seven December quarters are expected to be wetter than average in this part of the country, while about three or four are drier. A small part of north-central Queensland also has a 60% chance of a wetter than average season.
In contrast, much of Tasmania and the far southeastern coastal fringe of the mainland have a 30 to 40% chance of exceeding the three-month median rainfall. This means that below-normal falls have a 60 to 70% chance of occurring. Over most remaining parts of the country the chances of accumulating at least average rain for the December quarter are between 50 and 60%.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During the December quarter, history shows this effect to be moderately consistent across much of the country (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 present in central and eastern areas. The 30-day value of the Southern Oscillation Index (SOI) was +6 as at 22nd 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.
|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 OCTOBER 2007