12-Month Rainfall Forecast
Perth 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 11 May 2022
ENSO status: La Niña, it is expected return to neutral state by early winter. IOD status: Neutral, models suggest it could enter a negative phase in the next month. SAM status: Positive and is expected to remain positive for the next month or so. The El Niño–Southern Oscillation (ENSO) is in a La Niña phase and has not changed in strength during the past two weeks. La Niña events typically breaks down in autumn and while this current La Niña is persisting longer than usual, most models suggest it should end by early winter. However, one seasonal model operated by the U.S. National Weather Service’s Climate Prediction Centre (CPC) suggests that a La Niña-like signal could linger in the Pacific Ocean through the middle of the year and possibly remerge in the Southern Hemisphere’s spring. That being said, it is important to note that seasonal forecast models have lower accuracy at this time of year. This is called the autumn predictability barrier. The La Niña forecast will increase the likelihood of above-average rain in northern and eastern Australia during the remainder of Autumn, possibly into winter. Even as La Niña weakens it should continue to influence our climate. all five climate models suggest a negative phase will begin during late autumn or early winter. Climate models are forecasting strong negative IOD values by August, with the negative IOD event possibly lingering into spring. While a neutral IOD has little influence on our climate, a negative IOD increases the chance of above average autumn and winter rain for much of Australia. The rainfall outlooks are reflecting this, with above average rain forecast during winter for much of Australia, except southwest WA, parts of TAS and VIC. If a negative IOD does develop this winter, it will increase the risk of flooding over large areas of inland Australia, especially areas that had a wet summer and autumn.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.