SOI Continues to Fluctuate David McRae, Queensland Climate Change Centre of Excellence, 1 July 2008
There was a marginal shift in the monthly SOI value from May (minus 3.5) to June (plus 2.9). This places the SOI in a "Rapidly Rising Phase". Based on this phase and historical rainfall data there is a 50 to 70% chance of getting median rainfall throughout most of Queensland during July to September. For example Roma has a 70% chance of getting above its July to September median rainfall of 75 mm, Miles and Emerald have a 65% chance of getting above their July to September median rainfall of 85 mm and 55 mm respectively and Gympie has a 65% chance of getting above its July to September median rainfall of 115 mm.
The exception is for the central north of the state where there is a lower 30 to 50% chance of getting median rainfall for July to September. While rainfall probabilities for the central north are not as high as the rest of the state it is worth noting that especially for northern Queensland we are in our 'dry season'. Therefore significant or 'drought breaking' rain is not usual during this period regardless of the seasonal outlook.
For those who like to follow the relationship between the SOI and rainfall patterns in more detail have a look at what happened in your area during July to September in the following years since 1920; 1920, 1922, 1929, 1934, 1945, 1947, 1950, 1951, 1953, 1957, 1958, 1966, 1967, 1970, 1973, 1986, 1996, 1998, 2001, 2005 and 2007. Find out your average rainfall for July to September and see how many times rainfall was well below, well above or close to average during July to September in the listed years.
The question remains though as to whether this upward trend of the SOI will continue or is just a short-term fluctuation. As stated regularly, to increase the chance of an overall improvement in seasonal conditions across eastern Australia especially leading into spring and summer, it helps if the SOI has moved into consistently positive values for a couple of months at least.
As always when using a climate forecast you should remember that the probability or percent chance of something occurring is just that - a probability. For example if there is a 70% chance of recording more than 100 mm there is also a 30% chance of recording less than 100 mm i.e. 70-30; 30-70. It does not mean that you will get 70% more than 100 mm or 100 mm plus another 70%.
For more information on sea surface temperature maps, the SOI or rainfall outlook maps try www.longpaddock.qld.gov.au To find more information on rainfall probabilities rainfall figures for your area use Rainman Streamflow. Daily updates on the SOI are available on (07) 46881439.