|Southeastern Aust Seasonal Rainfall Outlook: probabilities for October to December 2008, issued 25th September 2008|
Wetter conditions indicated for northern districts but drier in southeastern Australia
The outlook for October to December rainfall indicates a moderate to strong shift towards wetter than normal conditions over much of northern NSW and northeast South Australia. Conversely, the outlook shows a moderate to strong shift towards drier conditions for Tasmania, southern Victoria and the far southeast of South Australia.
The pattern of seasonal rainfall odds for southeastern Australia reflects a strong contribution from the Indian Ocean where continued warmth exists in the central and southeastern Indian Ocean, especially off the west coast of Western Australia. However, the Pacific Ocean remains largely in a neutral state and so is not contributing strongly to the rainfall outlook.
The chances of October to December totals exceeding the 3-month median rainfall are 60-65% over northern NSW and northeastern South Australia (see map). This means for every ten years with ocean patterns like the current, about six or seven years are expected to be wetter than average while about three or four years are drier. The rest of NSW, northern Victoria and much of South Australia indicate neutral conditions for the period, which means the chances of being wetter than normal are about the same as the chances of being drier.
In contrast, the chances of exceeding the median rainfall over the October to December period drop to 30 to 40% over Tasmania, most of southern Victoria and the southeast of South Australia. This means the chances of below average rainfall are around 60 to 70% in these areas. So in years with ocean patterns like the current, about six or seven October to December periods out of ten are expected to be drier than average in these parts of southeastern Australia, while about three or four are wetter.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During October to December, history shows this effect to be moderately consistent over the western half of NSW, the eastern half of SA, Victoria and central Tasmania. However, confidence is only weakly to very weakly consistent over the northeast quarter of NSW and the western half of SA (see background information). In these areas where outlook confidence is not high caution should be used when interpreting these outlooks.
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.
More information on this outlook is available Monday to Friday from 9.00am to 5.00pm local time by contacting the Bureau's Climate Services sections in Queensland, NSW, SA, Victoria and Tasmania at the following numbers:
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 28th October 2008