12-Month Rainfall Forecast
-
Central Plateau 12-month Rainfall Forecast
Dec
24Jan
25Feb
25Mar
25Apr
25May
25Jun
25Jul
25Aug
25Sep
25Oct
25Nov
25
10
50
766465657875Rainfall deciles
10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normalIssue Notes
Issued 10 Dec 2024ENSO status: La Niña Watch. IOD status: Neutral. SAM status: Negative. Trending positive. The El Niño Southern Oscillation (ENSO) is in a La Niña Watch. The current technical definition of a La Niña is only a small chance to be met, although ocean and atmospheric conditions do resemble a La Niña pattern. Trade winds are still stronger than average along the western equatorial Pacific, and Relative-Nino Index (factors in equatorial ocean temperatures elsewhere around the world, compensating for the background warming signal) is below the threshold. A La Niña typically leads to cooler temperatures northern Australia during summer, while producing higher temperatures for western/southwestern WA, Vic and western Tas. The Indian Ocean Dipole (IOD) is in a neutral phase. Recent conditions have been like a negative IOD, but it is too late in the year for this climate driver to develop. Once the monsoon establishes over northern Australia, the IOD has no impact on Australia’s weather. The Southern AnnularMode (SAM) is in a brief negative phase, having switched a positive phase in November. Forecasts predict it will become neural in mid-late December and become positive in January and February. A positive SAM decreases the frequency of cold fronts, with more regular high pressure systems and easterly winds. During summer, this increases rainfall over eastern Australia, and reduces rainfall over southeastern SA, western Vic 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 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.