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CW Slopes & Plains 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: Neutral.
IOD status: Neutral. Negative possible.
SAM status: Positive. Trending positive.
The El Niņo Southern Oscillation (ENSO) is in a neutral pattern. Neutral conditions are likely to persist through autumn and early 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 the negative from late autumn, with early signs already being observed (i.e. northeasterly trade winds rather than southeasterly, and increased rain and cloud near Jakarta, and an increase in warm water from the Indonesian flow-through), and is a higher than normal chance to develop in 2025. 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 positive phase, meaning cold fronts are further south than they typically would be at this time of year. Forecasts are indicating a positive phase to be more dominant during Winter (at least June and July). During autumn, a positive SAM increases rainfall over most of eastern Qld and NSW, and decreases rainfall for SA, Vic, western 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 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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NSW floodwaters visible from space as rain slowly clears
14:26 AEST A sunny spell lasting the best part of a week is on the cards for the entire NSW coast from this Saturday, after a week of record May rainfall in many areas. At most, only a brief light shower or two should fall in most coastal spots at any time over the next week, with showers more likely on the South Coast as a cold front sweeps through on Monday.
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