|WA Seasonal Rainfall Outlook: probabilities for February to April 2009, issued 22nd January 2009|
Better than even chance of above average rainfall for late summer to mid-autumn
The outlook for total late summer to mid-autumn (February to April) rainfall shows a moderate to strong bias in the odds favouring higher than normal rainfall over much of WA, particularly in the west.
The pattern of seasonal rainfall odds across Western Australia is mainly a result of a cooling trend in the Indian Ocean in the last few months of 2008.
The chance of exceeding median rainfall during February to April is between 65 and 75% over most of western WA, decreasing to 55 to 65% in the north, east and the far southwest. This means that for every ten years with ocean patterns like the current, about six or seven years are expected to be wetter than normal for most of WA, while about three or four years are expected to be drier.
Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During the February to April period, history shows the effect to be moderately consistent through WA, except for southern parts of the Kimberley, northern parts of the Interior and the far southwest of the state where the effect is only weakly or very weakly consistent (see background information).
The central and eastern equatorial Pacific Ocean cooled further during December. This brings the Pacific Ocean into line with atmospheric indicators, a number of which have been approaching La Niña levels since October 2008. However, most current model outlooks, and a build-up of warmer sub-surface water in the western equatorial Pacific, suggest that the cooler conditions in the Pacific may not persist beyond summer 2009. The most likely scenario is for the central and eastern Pacific to warm over the coming months and hence remain neutral. The SOI remains positive at approximately +13 for the 30 days ending 19 January. 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 detailed forecast maps, including the probabilities of seasonal rainfall exceeding given totals, can be found here.
|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 20th February 2009