12-Month Rainfall Forecast
Upper Western 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 9 Sep 2020
ENSO status: La Niña Alert IOD status: Negative SAM status: Negative trending positive Sea surface temperatures (SSTs) across the equatorial central Pacific have continued to cool through August, with the Nino3.4 index registering a value of -0.5. The Southern Oscillation Index (SOI) recorded a value of 9.8 in the same month, a significant increase compared to July, tipping over the La Niña threshold of +7. The current outlook suggests that cooler than average SSTs will continue across the equatorial pacific during the Austral spring, with a high chance of a weak La Niña forming. All eight international models continue to suggest further cooling during spring, with three of these indicating La Niña conditions in September. A further two exceeding the La Niña threshold in October, with six of the eight indicating La Niña by November. The current International Research Institute for Climate and Society (IRI) outlook suggests a 57:41 chance for La Niña:Neutral conditions by the end of the Australspring. To the west of Australia, the Indian Ocean Dipole (IOD) has shifted into a negative phase. Four of the six international models indicate a negative IOD to be sustained for the duration of the Austral spring, with the remaining two indicating a neutral phase. Both of the models that indicate a neutral phase trend towards the negative side of neutral. In terms of precipitation over Australia, the current short term outlook favours above average rainfall over the interior over the remainder of the first month of spring, particularly eastern WA and the Kimberley, western NT and western SA Above average rainfall is also favoured, although not as pronounced, over inland NSW and QLD and parts of the NSW and VIC coastal area. Drier than average conditions are expected across western WA as well as parts of northern coastal QLD and parts of the southeast. The remainder of spring is then likely to be wetter than average for NSW, QLD, VIC, SA, TAS, the NT and eastern WA, whilst drier than averageconditions look to persist further west. There are then indications of increased rainfall over WA toward the end of spring and into early summer, with the eastern two thirds of the country remaining wetter than average.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.