12-Month Rainfall Forecast
Flinders 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 15 Jun 2021
ENSO status: Neutral. IOD status: Neutral, although during the past three weeks it has been negative. SAM status: Positive. The El Niño–Southern Oscillation (ENSO) is currently neutral, which is predicted to continue until at least October. The Indian Ocean Dipole (IOD) is currently neutral. although the last 3 weekly values have been negative.The negative value would need to continue for another 5 weeks for a negative IOD to be declared. While a negative IOD has not been officially declared, the negative weekly value may enhance rainfall across central and southern Australia. A negative IOD is forecast during winter or early spring by 3 out of 5 climate models. However, the confidence in the IOD forecast at this time of the year is low, specifically because the models are showing a range of outcomes from mid winter onwards. During winter, much of the Australia is likely to be wetter than normal, apart from far southern WA and SA, southern VIC and much of TAS. The weekly negative IOD valuesmay be influencing this outlook, potentially feeding moisture into cold fronts sweeping across central and southern Australia. Therefore a normal to above normal snow season is forecast. Springs rainfall at this stage is forecast to be above normal for much of the Australia, apart from western TAS. Again, the negative IOD values may be playing a role in this outlook.Forecast Explanation
Notes on the concept of deciles
If all the data in a record are ranked from lowest to highest they can then be divided into 100 equal blocks. These blocks are known as percentiles. The values that fall into the lowest 10% range (from 0 to 10%) are said to be in the first decile, those in the group 10+% to 20% are in the second decile, and so on. Those in the group 90+% to the maximum value recorded are in the 10th decile. The 50% value is a special one known as the 'median'. It is noteworthy since there is the same number of records above and below its value.
Deciles have been found to be very useful for analysing rainfall in particular as its distribution is not the normal bell-shape distribution but is skewed towards many low values with only a few high values. The deciles can be described in qualitative terms. A table is provided in the accompanying results.