The Southern Oscillation Index (SOI) fell substantially, from +23.9 in April to +2.1 in May. This change currently places the SOI in a Rapidly Falling Phase.
Based on previous years when the SOI has been in a 'Rapidly Falling' phase at the end of May, the probability of above- or below- median rainfall for the next three-month period (June to August) (PDF, 328K, last updated 01:19PM, 6 October 2011)* is normal (40 to 60 per cent) for most of Queensland. A 40 to 60% probability of above- or below- median rainfall means that a ‘Rapidly Falling’ phase of the SOI in May has little bearing on rainfall from June to August. The exception is for some regions of southern Queensland where there is a lower 20 to 40% probability of getting above median rainfall for June to August.
In assessing this information, it is worth considering other years with a ‘Rapidly Falling’ SOI phase at the end of May. Since 1950, these include: 1953, 1957, 1958, 1972, 1999, 2001, 2002, 2006, 2008 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% probability of above-median rainfall, then there is also a 30% chance of below-median rainfall. It does not mean that rainfall will be 70% 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.
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