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Adelaide 12-month rainfall forecast

10 5 0
Jul
16

3
Aug
16

2
Sep
16

2
Oct
16

4
Nov
16

6
Dec
16

6
Jan
17

3
Feb
17

4
Mar
17

8
Apr
17

1
May
17

9

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

The 2015/2016 El Nino is now officially over, with the tropical Pacific Ocean now within a neutral state. Sea surface temperatures (SSTs) across the equatorial Pacific have cooled to neutral values and have remained within these parameters for the past four weeks. Atmospheric indicators have also returned to neutral with: trade winds and cloudiness near the Dateline and the Southern Oscillation Index (SOI) back to within average conditions. The Nino3.4 value dropped below El Nino thresholds in May, reaching +0.4. On the other hand, the Southern Oscillation Index (SOI) recorded a positive value (+0.3) towards the end of May, the first time it has been positive since June 2015. 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, most international climate models suggest there is a 50% to 70% risk of a La Nina developing during the winter. These same numerical models also indicate a negative Indian Ocean Dipole (IOD) is likely to develop in the coming months. The weekly Dipole Mode Index (DMI) has dipped below negative IOD thresholds already for two weeks in a row. Although, it will take several more weeks of similar IOD index values before a negative IOD can become established. Taking these two climate drivers into consideration, the odds favour neutral-to-above average rainfall across the country in the coming months. Moreover, SSTs surrounding Australia remain significantly warmer than average, providing more available moisture to any weather system menacing Australia. In the longer term, La Nina tends to bring an earlier onset of the northern Australian Wet Season and an earlier onset of the North Australian Monsoon with higher than average numbers of tropical cyclones across the Australian region. 

Issued 08 Jun 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.