12-Month Rainfall Forecast
Riverland 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 5 Aug 2020
ENSO status: La Nina Watch IOD status: Neutral SAM status: Positive Sea surface temperatures (SSTs) across the equatorial central Pacific show a slight decrease in the intensity of cooling compared to June, with the Nino3.4 index registering a value of 0.0 in July. The Southern Oscillation Index (SOI) recorded a value of 4.2 in the same month, a swing into positive values compared to June. The current outlook suggests that cooler than average SSTs will continue across the equatorial pacific through the end of winter and potentially reach La Nina thresholds by mid-spring. All eight international models continue to suggest a cooler side of neutral for the region throughout the remainder of the Austral winter and through spring, with four of these indicating La Nina by Spring. The current International Research Institute for Climate and Society (IRI) outlook suggests a 54:41 chance for La Nina:Neutral conditions by the end of the Austral spring. To the west of Australia, the Indian Ocean Dipole (IOD) remains neutral. Two of the six international models indicate a negative IOD becoming established during the Austral spring, with the remaining four indicating a neutral phase. Of the four neutral models, including the BOM, three are trending slightly negative within the neutral zone by early spring, with only the Canadian model trending positive. In terms of precipitation over Australia, the current short-term outlook favours above average rainfall across most of Australia over the next fortnight, with the exception of western TAS, far southern VIC, and northern QLD and the NT. These expected wetter than average conditions will likely be due to the passage of moisture troughs inland and fronts, in part driven by the positive SAM. Models are then indicating a continued trend to above average rainfall for inland SA, all but the far north of the NT, western, central and southern QLD, northern and western NSW and parts of southern WA. Parts of WA’s Pilbara and Kimberley however are showing signs of below average rainfall into early spring.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.