|WA Seasonal Rainfall Outlook: probabilities for July to September 2008, issued 26th June 2008|
Drier conditions more likely from northwest to southeast WA
The Western Australian outlook for total rainfall during the September quarter shows the odds favouring drier than average conditions over a large area from northwest to southeast WA.
The pattern of seasonal rainfall odds across WA 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% across a large area of WA, extending from the northwest to the southeast. In other words, there is a 60 to 75 % chance of below median rainfall in these parts (see map).
So for every ten years with ocean patterns like the current, about six or seven July to September periods are expected to be drier than average across this band, while about three or four are wetter. It should be noted, however, that this is a seasonally drier time of year in northwest WA.
Over the rest of the state, the chances of exceeding the seasonal median rainfall are 40 to 50%, meaning the odds of being wetter than normal are almost the same as the odds 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 generally weakly consistent across the state, reaching moderate in patches (see background information). This outlook needs to be used with caution in areas where skill is lower.
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.
|More information on this outlook is available from 8.30am to 4.30pm (WST) Monday to Friday by contacting the Climate Services Centre in the Bureau's Perth Office: (08) 9263 2222.|
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 25th July 2008