|WA Seasonal Rainfall Outlook: probabilities for Spring 2006, issued 23rd August 2006|
Wetter spring favoured in southwest WA
There is a moderate shift in the odds towards a wetter than normal spring (Sep-Nov) in southwest WA, the Bureau of Meteorology announced today. However, over most of the State the chances are generally close to 50% for accumulating at least median rainfall over September to November.
The pattern of seasonal rainfall odds across Western Australia is mainly a result of higher than average temperatures in the Indian Ocean, which has been warming strongly in recent months.
For the September to November period, the chances of above median rainfall are between 60 and 65% over a large part of southwest WA (see map). So in years with ocean patterns like the current, about six springs out of ten are expected to be wetter than average in this region of the State, with about four out of ten being drier.
Across the rest of WA, the chances are mostly between 40 and 55% for the spring rainfall total to exceed the long-term median. However, through northwest and parts of northern Western Australia, spring is a seasonally dry time of year and heavy rain during this period is uncommon.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During spring, history shows this effect to be moderately consistent in only the southwest and far northwest of the State (see background information). Elsewhere the effect is largely very weakly consistent.
The Southern Oscillation Index (SOI) was negative for the third straight month in July with a value of −9. This came after readings of −6 in June and −10 in May. The approximate SOI for the 30 days ending 20th August was −16. If history is any guide, the SOI will probably stay negative for the rest of the year.
Although a late-developing El Niño event is still a possibility this year, especially considering the SOI, the consensus from computer models is for continued neutral ENSO conditions for the remainder of 2006, but on the warm side of average. Continued Pacific warmth and negative SOI values are generally associated with increased likelihood of below average rainfall in eastern and northern Australia. For routine updates and comprehensive discussion on the latest data relating to ENSO, together with details on what the phenomenon is and how it has affected Australia in the past, 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 9.00am to 5.00pm (WST) Monday to Friday by contacting the Climate and Consultancy section in the Bureau's Perth Office: (08) 9263 2222.|
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 26th SEPTEMBER 2006