12-Month Rainfall Forecast
-
West Coast 12-month Rainfall Forecast
Jul
25Aug
25Sep
25Oct
25Nov
25Dec
25Jan
26Feb
26Mar
26Apr
26May
26
10
50
6777697101064Rainfall deciles
10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normalIssue Notes
Issued 19 Jun 2025ENSO status: Neutral. IOD status: Neutral. Negative possible. SAM status: Neutral. Trending positive. The El Niño Southern Oscillation (ENSO) is in a neutral pattern. Neutral conditions are likely to persist through winter, and the vast majority of international climate models expect neutral conditions in 2025. The Indian Ocean Dipole (IOD) is in a neutral phase, but the Indian Ocean overall is much warmer than average. The IOD is forecast to tend towards a negative phase, with some clear building of ocean heat starting near Java, and increased convection near Sumatra. Slightly more than 50% of international models forecast a negative IOD to develop this year. A negative IOD typically increases the amount of northwest cloudbands that cross Australia, increasing rainfall from northwest WA through to southeastern Australia during winter and spring. The Southern Annular Mode (SAM) is in a neutral phase, meaning cold fronts are generally reaching as far north as you would typically expect at this time of year. Forecasts are indicating a positive phase to be more dominant during Winter (at least June and July). During winter, a positive SAM increases rainfall over most of Qld, NSW and the Nullarbor, while decreasing rainfall for SA, Vic, Tas, and southwest WA. Rainfall outlooks are showing below average rainfall over eastern Australia and increased rainfall for WA in June. In July and August, models show above average rainfall over WA, central Aus, Qld and NSW, and the Alps (August only), with below average rainfall for SA, Vic and Tas.
Forecast ExplanationNotes 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.
