12-Month Rainfall Forecast
Sydney 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 13 Jul 2021
ENSO status: Neutral, however two models suggest La Niña thresholds may be met again from September. IOD status: Neutral, although it was negative for six weeks between mid-May and late-June SAM status: Negative. The El Niño–Southern Oscillation (ENSO) is currently neutral. However, there are growing signs that La Niña could redevelop in the Pacific Ocean later this year, prompting the US Climate Prediction Centre to issue a La Niña Watch this week. Two models forecast temperatures reaching the La Niña threshold during September. The Indian Ocean Dipole (IOD) is currently neutral, although we saw six consecutive weeks below the negative IOD threshold between mid-May and late-June. The IOD index needs to stay below -0.4C for eight consecutive weeks to be officially declared as a negative IOD event. The IOD ducked back into neutral territory thanks to recent warming in the western Indian Ocean. While a negative IOD has not been officially declared, the negative weekly values may enhance rainfallacross central and southern Australia. Three of the five models are forecasting the negative IOD to develop in winter, with another one indicating development in early spring. Most of the models suggest that the negative IOD will continue through until mid spring. During winter, much of the Australia is likely to be wetter than normal, apart from far southern WA and SA, southern VIC and southwestern TAS. The weekly negative IOD values may 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 southwestern TAS and WA. 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.