|National Seasonal Rainfall Outlook: probabilities for July to September 2008, issued 26th June 2008|
Drier conditions indicated from northwest WA to southeast Australia
The national outlook for total rainfall over the mid-winter to early spring period (July to September), shows a shift in the odds favouring drier than average conditions over a broad band from northwest WA to southeastern Australia.
The pattern of seasonal rainfall odds across Australia is a result of higher than average temperatures in the Indian Ocean surrounding the west coast of WA, and a warming trend in recent months over the equatorial Pacific.
The chances of exceeding the median rainfall over July to September are between 25 and 40% over broad zone extending from northwest to southeast WA, across most of SA, parts of far southwest Queensland, far western NSW as well as western and northeast Victoria (see map). This means the chances of below normal falls are between 60 and 75% in these areas.
So for every ten years with ocean patterns like the current, about six or seven September quarters are expected to be drier than average across this band, while about three or four are wetter. However, it should be noted that this is a seasonally drier time of year in northwest WA. In addition, confidence is not high in WA or Victoria so this outlook needs to be used with caution in those areas.
Over the rest of the nation, mid-winter to early spring rainfall totals have a 40 to 50% chance of exceeding the three-month median. So the chances of being wetter than normal are about the same as the chances of being drier.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During July to September, history shows this effect to be moderately consistent across most of Queensland and the NT, as well in large parts of NSW and SA. The effect is generally weakly consistent elsewhere across the country, reaching moderate only in patches (see background information).
The equatorial Pacific Ocean has been gradually warming during autumn and early winter resulting in the decay of the 2007/08 La Niña event, with neutral conditions currently prevailing. Computer models indicate a continuation of the warming trend, with neutral conditions the most likely outcome during the forecast period. The 30-day value of the Southern Oscillation Index (SOI) was zero as at 23rd June. 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: Clinton Rakich on (03) 9669 4671, David Jones on (03) 9669 4085, Lyn Bettio on (03) 9669 4165.|
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 July 2008