12-Month Rainfall Forecast
Northwest 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 7 Apr 2021
ENSO status: Neutral. IOD status: Neutral, potentially trending negative during winter. SAM status: Positive, trending negative during mid April. La Niña has concluded, with neutral conditions expected until at least the end of Autumn. A few models are predicting La Niña conditions to return later this year, while the majority of models predict neutral conditions. While La Niña has returned to neutral, the oceans around northern Australia are warmer than normal. This is expected to generate above normal rainfall for much of Australia during the next month or so. Some models are suggesting a negative Indian Ocean Dipole (IOD) may develop during winter, which would increase rainfall across parts of Australia during winter. During Autumn, many parts of Australia are likely to be wetter than normal, due to warmer waters off the northern coast of Australia. Southern parts of SA, VIC, most of TAS and southwest WA are likely to observe normal precipitation for the time of year. During winter, much of the Australia is likely to be wetter than normal, apart from far southwestern WA, southern VIC and much of TAS. The negative IOD may be playing a part in this, potentially feeding moisture into cold fronts sweeping across Australia's snowfields. Therefore a normal to above normal snow season is forecast.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.