Commentary on rainfall probabilities based on phases of the SOI
Seasonal climate outlook message for July 2015 to September 2015
Commentary on rainfall probabilities based on 'phases' of the SOI
The monthly value of the Southern Oscillation Index (SOI) was -13.1 in May and -10.3 in June. According to the SOI Phase system, this places the SOI in a ‘Consistently Negative’ phase.
Based on previous years when the SOI has been in a ‘Consistently Negative’ phase at the end of June, the probability of above-median rainfall for the next three-month period (July to September) is less than 50 per cent for most of Queensland and less than 30 per cent for many southern regions. While the probability of exceeding median rainfall exceeds 50 per cent for parts of the Gulf, median July to September rainfall there is typically low.
Previous years with a ‘Consistently Negative’ SOI phase at the end of June include 1905, 1911, 1912, 1914, 1940, 1946, 1972, 1977, 1987, 1993, 1994, 1997, 2002 and 2006. The rainfall probabilities stated above are based on the 1900 to 1998 period (that does not include 2002 and 2006).
When using a climate outlook it should be remembered that the probability, or per cent 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 should note that, while climate outlook schemes cannot provide outlooks with absolute certainty, users who follow a skilful scheme should benefit from doing so in the long-term. Thus, users should consider the historical track record of any scheme, and such information is becoming increasingly available.
Further seasonal climate outlook information for Queensland is available in the monthly climate statement produced by the Department of Science, Information Technology and Innovation and at www.bom.gov.au
Last updated 18 March 2011