Autumn rainfall outlook positive for much of eastern states

Thursday, February 28, 2008

A wetter than normal autumn is forecast over large parts of Queensland, especially in the north, over much of NSW and in parts of SA.
But in the WA wheatbelt and in much of Tasmania, chances of median rainfall from March-May fall below 50pc.

The pattern of seasonal rainfall odds across Australia is a result of:

• Cooler than average waters across the equatorial eastern Pacific in association with La Niña.

• Continuing higher than average temperatures in the central to south-east Indian Ocean.

The chances of exceeding the median rainfall in the autumn in the three months March to May are:

• 60-70pc over most of north Queensland.

• 60-65pc in a band extending from central SA to the far west of NSW.

• 55-60pc in south-west Qld and much of western NSW.

• 50-55pc in south-east Qld, north-east NSW, and much of the wheatbelt in Vic.

• But most of the WA wheatbelt and Tasmania have below 50pc chance of exceeding median rainfall.

(See attached BOM map).

BOM says confidence is these forecasts is related to how consistently the Pacific and Indian Oceans affect Australian rainfall.

During autumn, history shows this effect to be moderately consistent across much of the north and west of Australia.

Elsewhere, the effect is only weakly or very weakly consistent.

So BOM warns that swings in the chances of below or above average rainfall outside of Queensland need to be viewed with caution - the confidence level is lower.

A La Niña event is in progress across the Pacific Basin and computer models indicate it is likely to persist until the end of autumn in the southern hemisphere.

The 30-day value of the Southern Oscillation Index (SOI) was +20 as at February 23.

The national outlook for average autumn maximum temperatures (March to May) shows a moderate shift in the odds favouring cooler than normal conditions in southern Queensland and northern NSW.

SOURCE: National Climate Centre, Bureau of Meteorology and Farm Weekly.