SOI fluctuates in value Dave McRae, Qld Climate Change Centre of Excellence, 05/05/09.
The monthly value of the SOI has risen in value from minus 1.3 for March to plus 8.0 for April placing the SOI in a Rapidly Rising phase.
Based on historical rainfall records and a Rapidly Rising SOI phase at the end of April, there is a reasonable 60 to 80% chance of getting above median rainfall across parts of north-west, central, south and south-east Queensland during May to July. Across the rest of the state there is a 40 to 60% chance of getting above median rainfall for May through to the end of July.
It will be interesting to see if these positive values are maintained over coming months. If the 30 day average of the SOI where to fall into strongly negative values and remain there through to the end of autumn, it would be a warning sign for a likely dry winter/spring. 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.
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 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 May to July in the following years since 1950 that have had a Rapidly Rising SOI phase at the end of March: 1954, 1961, 1984, 1985, 1989, 1990 and 1999.
Find out your average rainfall for May to July and see how many times rainfall was well below, well above or close to average during May to July in the listed years. For more information on historical rainfall data for your region try Rainman Streamflow or www.longpaddock.qld.gov.au
Autumn is the key time for the establishment of climate patterns such as El Nino and La Nina. Therefore QCCCE climate staff will continue to closely monitor what happens over the next few months. Updates on the development of these climate patterns can be found at www.longpaddock.qld.gov.au or www.bom.gov.au/climate/enso or www.cpc.ncep.noaa.gov