|National Seasonal Rainfall Outlook: probabilities for October to December 2008, issued 25th September 2008|
Mixed December quarter rainfall outlook
The national outlook for rainfall over the December quarter (October to December) shows a moderate to strong shift in the odds favouring wetter than normal conditions over much of the north and west of the continent. It should be noted though, that the December quarter is a seasonally dry time of year in northwest WA, with heavy rain being uncommon. Conversely, there is a moderate shift in the odds favouring drier conditions in southeastern Australia, including Tasmania.
The pattern of seasonal rainfall odds across Australia is mainly a result of continued warmth in the central and southeastern Indian Ocean, especially off the west coast of WA.
The chances of exceeding the median rainfall for October to December are between 70 and 75% for central Queensland and western parts of WA, rising to 75 to 80% in the Gascoyne region of WA (see map). Over much of the rest of central to western WA the odds for exceeding the median rainfall are between 60 to 70%, as they are over northern NSW, the eastern NT and most remaining areas of Queensland. 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.
In contrast, the chances of exceeding median rainfall for October to December are between 30 and 40% across Tasmania, southern Victoria (except east Gippsland) and southeast SA. In coastal regions of southwest Victoria the chances drop a little below 30%. So over these parts of southeastern Australia there is a 60 to 70% chance of below average falls over the December quarter.
Across the rest of the country, including northern Victoria, southern NSW, 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 totals in these regions.
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).
The tropical Pacific Ocean remains ENSO-neutral. There has been some slight cooling at the surface and slightly stronger cooling of the subsurface in the central Pacific recently. The Southern Oscillation Index (SOI) has responded to this cooling and has risen to a value of approximately +17 for the 30 days ending 22nd September. Computer models indicate that neutral conditions are likely to continue for the next few seasons. 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: Lyn Bettio on (03) 9669 4165, David Jones on (03) 9669 4085, Brad Murphy on (03) 9669 4409.|
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 28th October 2008