12-Month Rainfall Forecast
Wimmera 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 16 Sep 2021
ENSO status: Neutral, however five of seven models suggest La Niña thresholds may be met again between October and January. IOD status: Neutral, returning from negative values in the recent weeks. SAM status: Positive. 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, which has prompted the US Climate Prediction Centre to increase its prediction of La Niña occurring this year to 70-80 percent. However their thresholds are lower than the Bureau of Meteorology's. Five of seven models forecast temperatures may reach or exceed La Niña thresholds between October and January. Three of seven predicted an established event, which requires La Nina thresholds to be met for three months. La Niña typically brings above average rainfall in northern and eastern Australia. The Indian Ocean Dipole (IOD) is currently neutral, returning from negative values in the recent weeks. Three of five models areforecasting the IOD event will officially end in October, with all models ending the event in December. A negative IOD generates above average rain to southern and eastern Australia. As such, average to above average rainfall is predicted across much of the country this winter and spring.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.