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Southeast Coast 12-month Rainfall Forecast
10
5
0
| Rainfall deciles | |
|---|---|
| 10 | Well above normal |
| 8-9 | Above normal |
| 4-7 | Near normal |
| 2-3 | Below normal |
| 1 | Well below normal |
Issue Notes
ENSO status: La Niņa-like conditions.
IOD status: Neutral.
SAM status: Negative. Trending slightly negative.
The El Niņo Southern Oscillation (ENSO) is in a La Niņa-like pattern. Recent conditions show some atmosphere-ocean coupling, resulting in the US declaring a La Niņa event has started. A La Niņa typically increases rainfall over northern and eastern Australia, while reducing rainfall over southern coastal Australia and western Tas.
The Indian Ocean Dipole (IOD) is in a neutral phase and does not play a part in Australia's climate during this time of year.
The Southern Annular Mode (SAM) is in a negative phase. Recent forecasts show negative SAM is more likely to dominate for the remainder of summer, due to the polar vortex taking an unusually long time to recover from the stratospheric warming event in winter 2024. During summer, a negative SAM decreases rainfall over most of southeastern Australia, and increases rainfall for far southwest WA and western Tas.
Rainfall outlooks are showing above average rainfall over large parts of the country during summer, with closer to average conditions for the southwest corner of WA, southern SA and Vic, and central parts of Tas.
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.
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