12-Month Rainfall Forecast
Roper-McArthur 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 2 Aug 2022
ENSO status: Neutral, However, some models suggest La Niña may re-form in spring. IOD status: Negative, all models suggest this event will continue until late spring. SAM status: Neutral, neutral to positive levels are forecast in August The 2021-22 La Nina event has ended according to BOM, however two out of seven models suggest a return of La Nina in late winter, with four in October and five in November. Even though this event officially has ended, a La Nina-like pattern is likely to persist for the remainder of 2022. However, new data released by the U.S. Climate Prediction Centre continues to point towards the prospect of a third consecutive La Niña later this year. Their predictions give La Niña a 62 percent chance of occurring by early spring and a 65 to 66 percent chance in late spring and early summer.Looking further ahead, the majority of forecast models predict that the La Niña pattern in the Pacific Ocean should break down towards the end of summer, most likely returning to a neutral state early in 2023. The Bureau of Meteorology has declared that a negative IOD event is underway to the northwest of Australia. This declaration comes after sea surface temperatures in the Indian Ocean have remained near or exceeded the negative IOD threshold for the last eight weeks. This is now the 2nd consecutive year to be declared a negative IOD year, following a relatively weak event in 2021. This is the first time we have seen two consecutive negative IOD years since reliable records of the IOD began in 1960. Unlike last year’s event, this year’s negative IOD is expected to be strong and last through the remainder of winter and spring. A negative IOD increases the chance of above average winter and spring rainfall and cloud across parts of southern Australia. The rainfall outlooks are reflecting this, with above average rain forecast during winter for much of Australia, except drier than average conditions are forecast in southwest WA and parts of TAS. During spring much of Australia isforecast to average to above average. Early indications of summer rainfall look average to above average for much of Australia, except TAS which looks drier. The negative IOD could increase the risk of flooding over large areas of inland Australia. With two La Niña seasons already in the bag and the prospect of a third La Niña now a looming possibility, this is likely to have a compounding effect that may exacerbate the impacts we normally see in Australia. So, while individual La Niña events usually cause more rain and flooding in northern and eastern Australia, any La Niña-fuelled rainfall this year will be falling onto already saturated ground and into full dams. This makes flooding a heightened risk, especially for areas that just 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.