SOI Remains in Near Zero Phase Dave McRae, Queensland Climate Change Centre of Excellence, 04/08/09.
Despite the value of the SOI fluctuating during the last few weeks, based on the shift in monthly value of the SOI from minus 2.8 for June to plus 2 for July, the SOI has remained in a "Consistently Near Zero" phase.
An analysis of historical rainfall and a Consistently Near Zero SOI phase at the end of July indicates a 40 to 50% chance of getting above median rainfall throughout most of Queensland during August to October. This is a marginal improvement on the July to September seasonal outlook.
The exception is for the region running from the central Queensland coast to the peninsular and adjacent inland area where there is a lower 20 to 40% chance of getting above median rainfall. The latest rainfall probability maps are available at www.longpaddock.qld.gov.au
It is worth noting that August and September have the lowest median monthly rainfall totals for most locations in Queensland. For example median rainfall for August and September at Blackall is 9 mm and 6 mm, at Emerald is 11 mm and 9 mm, at Goondiwindi is 26 mm and 34 mm, at Toowoomba is 33 mm and 41 mm and Mareeba is 2 mm and 2 mm.
To follow the relationship between the SOI and rainfall patterns in more detail have a look at what happened in your area during August to October in the following years since 1930 that have had a 'Near Zero' SOI phase at the end of July; 1930, 1932, 1935, 1942, 1944, 1953, 1957, 1958, 1959, 1961, 1962, 1966, 1967, 1969, 1971, 1980, 1983, 1991, 2000, 2001, 2002, 2005, 2006 and 2008.
For example at Dalby, well below median rainfall for August to October in those years was recorded 3 times, close to median rainfall was recorded 16 times and above median rainfall was recorded 5 times. Therefore rainfall at Dalby during August to October is more likely to be closer to the long term median than well above or below median. Refer to Rainman StreamFlow for more information.
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%.
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