SOI worth watching Dave McRae, Qld Climate Change Centre of Excellence, 05/10/09.
The monthly value of the SOI rose from minus 3.8 for August to plus 3.6 for September which places it in a "Rapidly Rising" phase. Based on this SOI phase and historical rainfall records, throughout most of Queensland there is a 50 to 60% chance (with a few locations having up to a 70% chance) of getting above median rainfall during October to December.
For example Roma has a 70% chance of getting above its October to December median rainfall of 165mm, Goondiwindi has a 60% chance of getting above its October to December median rainfall of 170mm and Dalby has a 50% chance of getting above its October to December median rainfall of 220mm.
However it's worth remembering that an El Nino like sea surface temperature pattern can be still found in the Pacific. Therefore it will be interesting to see what trend SOI values take through to the end of the month. Negative SOI values (say below minus 5) are normally associated with El Nino events. SOI values are currently being influenced by the warmer than average sea surface temperatures to the north of Australia.
A key point to remember with any probability based forecasts is that they are just that - probabilities and not definitive forecasts. Therefore the opposite always applies eg 70-30; 30-70.
Taroom for example, has around a 70% chance of getting above 205mm for October to December. This also means that there is a 30% chance of NOT getting above 205mm. Another way of looking at this is that in around seven years out of ten historically with the current SOI phase, Taroom has received at least 205mm during October to December. Therefore in three years out of ten with the current SOI phase, Taroom has gotten less than 205mm during October to December.
For those interested in following historical patterns more closely, some of the years that have had the same SOI pattern at the end of September include 1931, 1936, 1949, 1970, 1979, 1983, 1989, 2001 and 2005. Compare the rainfall recorded during October to December in those years with your long term average October to December rainfall.
The 30day average of the SOI as of the 5th October is plus 2.7. You can receive a text message with the latest SOI values sent to your mobile phone. To subscribe to this free service, call me on (07) 4688 1459 or e-mail email@example.com The latest rainfall probability maps and SOI values are also available at www.longpaddock.qld.gov.au