" Adelaide long range forecast - 12 month rainfall forecast
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Adelaide 12-month rainfall forecast

10 5 0
Sep
16

7
Oct
16

2
Nov
16

8
Dec
16

8
Jan
17

3
Feb
17

4
Mar
17

6
Apr
17

1
May
17

8
Jun
17

9
Jul
17

2

Rainfall deciles
10 Well above normal
8-9 Above normal
4-7 Near normal
2-3 Below normal
1 Well below normal

Adelaide district forecast
Adelaide 28-day rainfall forecast
Issue Notes

Neutral ENSO conditions continue across the Pacific Ocean. A strong negative Indian Ocean Dipole (IOD) continues to dominate the Indian Ocean basin. Sea surface temperatures (SSTs) and atmospheric indicators like the trade winds and cloudiness near the Dateline remain within average ENSO boundaries. The Nino3.4 value continued to cool down, dropping below 0 in July reaching -0.2. On the other hand, the Southern Oscillation Index (SOI) recorded a value of 4.2 in July. Both, well within neutral boundary values. During the months following the breakdown of an El Nino event, Australia tends to see higher than average rainfall across the eastern and southern states. Moreover, some international climate models suggest there is a 50% risk of a La Nina developing during the winter. Although the outlook for the formation of a La Nina has continued to weaken during the past month, all international models continue to agree that ENSO is likely to remain on the La Nina-side of neutral for the remainder of 2016. Over the west, a strong negative Indian Ocean Dipole (IOD) continues to dominate the Indian Ocean basin. The weekly Dipole Mode Index (DMI) eased to -0.76 during the first week in August, after reaching a peak of -1.30 in mid-July. A negative IOD typically brings above-average rainfall to southern Australia and exacerbates the risk of above-average rainfall from La Nina. Taking these two climate drivers into consideration, the odds continue to favour above-average rainfall across central and eastern parts of the country over the next few months. As we head into the second half of spring and early summer, the signal for above-average rainfall is looking to diminish considerably as the negative IOD decays and due to the uncertainty about La Nina. Over northern Australia, La Nina tends to bring an earlier onset of the northern Australian Wet Season and an earlier onset of the North Australian Monsoon (NAM) with higher than average numbers of tropical cyclones across the Australian region. 

Issued 11 Aug 2016

Forecast    Current Conditions    Synoptic Chart    Satellite    Radar   

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.