12-Month Rainfall Forecast
Sydney 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 21 Jun 2022
ENSO status: Neutral, However, some models suggest La Niña may re-form in spring. IOD status: Neutral, however the index has been below zero for the past six weeks. SAM status: Negative, but is expected to rise to positive values before returning to neutral levels late in June. The 2021-22 La Nina event has ended, however four out of seven models suggest a return of La Nina in spring. Even though this event has ended, a La Nina-like pattern could persist in winter with warm sea surface temperatures surrounding Australia. There is still some uncertainty between forecast models, especially beyond the Southern Hemisphere’s spring. However, new data released by the U.S. Climate Prediction Centre continues to point towards the prospect of a third consecutive La Niña later this year. While the IOD is currently officially neutral, however the index has been below zero for the past six weeks. All climate models are forecasting a negative IOD event this winter and spring, with several forecasting stronglynegative IOD values in August. A negative IOD increases the chance of above average winter and spring rainfall and cloud across parts of southern Australia. The rainfall outlooks are reflecting this, with above average rain forecast during winter for much of Australia, except southwest WA, TAS and parts of VIC. If a negative IOD does develop this winter, it will increase the risk of flooding over large areas of inland Australia, especially areas that had a wet summer and autumn.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.