Commentary on rainfall probabilities based on phases of the SOI
Seasonal climate outlook message for September 2015 to November 2015
The monthly value of the Southern Oscillation Index (SOI) was -13.8 in July and -19.0 in August. According to the SOI Phase system, the SOI remains in a ‘Consistently Negative’ phase.
Based on previous years when the SOI has been in a ‘Consistently Negative’ phase at the end of August, the probability of above-median rainfall for the next three-month period (September to November) is less than 40 per cent for most of Queensland. While the probability of exceeding median rainfall is close to 50 per cent for some western parts of the state, median September to November rainfall there is typically low.
Previous years with a ‘Consistently Negative’ SOI phase at the end of August include 1905, 1911, 1914, 1918, 1919, 1923, 1925, 1940, 1941, 1946, 1951, 1965, 1972, 1976, 1977, 1982, 1987, 1993, 1994 and 2006. The rainfall probabilities stated above are based on the 1900 to 1998 period (that does not include 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