12-Month Rainfall Forecast
Central Wheatbelt 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 16 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 and is likely to be mostly positive for the coming three months. The Bureau of Meteorology has issued a La Niña Alert, increasing the likelihood (70% chance) that they will officially declare a third consecutive La Niña event later this year. This comes after the latest observations from the Pacific Ocean show that La Niña is now re-emerging after briefly weakening over the last couple of months. The BoM, NOAA and JMA are all giving a 60 to 80 percent chance of La Niña occurring during Southern Hemisphere’s spring. 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 Southern Annular Mode (SAM) index is currently neutral. Values are expected to be positive for the remainder ofAugust, and generally positive throughout spring. A positive SAM has a drying influence for parts of south-west and south-east Australia at this time of year, but increases the likelihood of rainfall in eastern New South Wales, far eastern Victoria, and parts of southern Queensland. The Indian Ocean is currently in a negative IOD phase, which was officially declared earlier this month. This is 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. New data which came out the week ending on the 15th of August shows that the IOD index had dipped to its lowest value since 2016. This means that the negative IOD is becoming entrenched to the northwest of Australia and is likely to enhance rainfall and flooding over our continent in the coming months. Unlike last year’s event, this year’s negative IOD is expected to be strong and last throughthe 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 is forecast 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 befalling 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.