Seasonal Climate Outlook Message for July to October 2011

Commentary on rainfall probabilities based on 'phases' of the SOI

The Southern Oscillation Index (SOI) has been near average over the last two months (May +2.1, June +0.9). According to the SOI Phase system, this places the SOI in a ‘Consistently Near-Zero’ phase.

Based on previous years when the SOI has been in a 'Consistently Near-Zero' phase at the end of June, the probability of above-median rainfall for the next three-month period (July to September) is 30 to 40 per cent for most of Queensland which is slightly lower than normal (50 percent). The exception is for some regions in the north-west of Queensland where there is a slightly higher than normal probability (50 to 60 per cent) of above-median rainfall for July to September. However, median rainfall levels at this time of year in those regions are low (generally less than 10 mm).

In assessing this information, it is worth considering other years with a ‘Consistently Near-Zero’ SOI phase at the end of June. Since 1950, these include: 1954, 1960, 1961, 1969, 1976, 1979, 1980, 1991, 1995, 1999 and 2009.

When using a climate forecast it should be remembered that the probability, or percent chance, of something occurring is just that – a probability. For example, if there is a 70 per cent probability of above-median rainfall, then there is also a 30 per cent chance of below-median rainfall. It does not mean that rainfall will be 70 per cent more than the median.

Users are advised not to rely on a single climate outlook alone and should consider the range of climate risk assessment information that is available.

Further seasonal climate outlook information is available at www.longpaddock.qld.gov.au and www.bom.gov.au