Understanding Probabilities Dave McRae 30/10/06
Given Australia's love of gambling be it on the dogs, ponies, sport or lotto it is interesting that many people are not comfortable with or do not understand probability based climate and weather forecasts. Although that may be the reason that we love gambling - we don't understand what probabilities or the chance of something occurring actually means.
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 example based on historical rainfall figures and a consistently negative SOI phase Clermont currently has a 30% chance of getting its long term October to December median rainfall of 170 mm. Therefore Clermont has a 70% chance of NOT getting its October to December median rainfall of 170 mm.
Another way of looking at this is that 3 times out of 10 historically with the current SOI phase Clermont has recorded above 170 mm during October to December. Therefore 7 times out of 10 historically Clermont has recorded below 170 mm during October to December.
When looking at the seasonal outlook for your area it may make it easier to think of rainfall probabilities in these terms. Probabilities above 80% equal a high chance, probabilities above 60% equal an above average chance, probabilities below 40% equal a below average chance and probabilities below 20% equal a very low chance of that occurring.
So currently Clermont has a below average chance of recording more than 170 mm during October to December. For more on rainfall probabilities for your location refer to Rainman StreamFlow.
Based on the current position of the MJO it would be reasonable to next expect it in early to mid November. Usually during summer stronger MJO signals can be expected. The MJO is a band of low air pressure originating off the east coast of central Africa travelling eastward across the Indian Ocean and northern Australia roughly every 30 to 60 days. Research has shown the MJO to be a useful indicator of the timing of potential rainfall events (but not amounts). For more information try www.apsru.gov.au/mjo/
DPI&F is providing free of charge a SMS text message of the SOI value and the MJO phase. To receive this message once or twice per week contact Neil White 4688 1236 email@example.com Current rainfall probability maps are at www.dpi.qld.gov.au/climate or www.longpaddock.qld.gov.au and SOI updates are available on (07) 46881439. For other enquires contact the DPI&F Information Centre on 132523.