12-Month Rainfall Forecast
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Barkly 12-month Rainfall Forecast
Apr
24May
24Jun
24Jul
24Aug
24Sep
24Oct
24Nov
24Dec
24Jan
25Feb
25Mar
25
10
50
878977789777Rainfall deciles
10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normalIssue Notes
Issued 23 Apr 2024ENSO status: Inactive/Neutral. El Niño ended. IOD status: Neutral, becoming strongly positive. SAM status: Leading to neutral and returning positive. The El Niño Southern Oscillation (ENSO) is presently in an inactive phase, for the first time since August 2021. Ocean cooling continues over the eastern Pacific Ocean as sea surface temperatures approach near-normal conditions. Warmer than normal waters are present across the western Pacific Ocean, including the Coral Sea with added moisture and humidity to influence weather conditions in the eastern regions of Australia. The atmosphere continues to be firmly neutral after weakening early in the year with 3 out of 7 global models forecasting a La Niña to develop this coming winter season. A neutral ENSO typically has no significant impact on Australian rainfall during winter. The Indian Ocean Dipole (IOD) is in a neutral phase, although record-high ocean temperatures across the northwest Indian Ocean have the IOD index just tipping into a positivethreshold value. Persistent southeasterly winds over the eastern Indian Ocean during the previous month are forecast to ease with below-average wind speeds to develop off northwest Australia and the eastern Indian Ocean during the winter season. All international models are suggesting the IOD will strengthen into a strong positive phase over the next 2-3 months, with significant upwelling to start near Indonesia possibly following May. A positive IOD reduces the number of northwest cloudbands that cross Australia during winter and spring, thus reducing rainfall across central and southeastern parts of Australia. The Southern Annular Mode (SAM) is currently positive (mid-April) with the outlook favouring a neutral phase forecast in May and then the likelihood of returning to a positive phase during winter. A positive SAM increases easterly winds, increasing rainfall and thunderstorm activity in the east. A positive SAM also reduces the impact of cold fronts and decreases the rainfall over southernAustralia. Rainfall outlooks are showing above-average rainfall over winter for WA and central and northern SA, and eastern regions of QLD and NSW. Conversely, below-average rainfall is expected for western TAS, southern SA, and most of VIC.
Forecast ExplanationNotes 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.
