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Adelaide 12-month rainfall forecast

10 5 0
Aug
14

2
Sep
14

7
Oct
14

2
Nov
14

7
Dec
14

5
Jan
15

4
Feb
15

5
Mar
15

4
Apr
15

2
May
15

6
Jun
15

8

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

While tropical Pacific Ocean temperatures have recently reached weak El Niņo thresholds, atmospheric conditions remain neutral. It is expected that these atmospheric indicators will shift in the coming months with further warming of the central and eastern tropical Pacific expected. There is still considered to be greater than a 70% chance of an El Niņo during the second half of 2014. During June 2014, sea surface temperatures in much of the central and eastern tropical Pacific Ocean reached weak El Niņo levels, with averaged Nino3.4 values of +0.5 for the month. For El Niņo to be declared, three-month average sea surface temperatures in the east central equatorial Pacific Ocean need to be 0.5° C or more above normal. While the sea surface temperatures have increased in the Niņo index regions, they also have remained above average through the western tropical Pacific, near Australia, which is uncharacteristic of an El Niņo. In addition to warming in the east central equatorial Pacific, atmospheric conditions also need to meet certain criteria for an El Niņo event to be declared. This being that trade winds need to be consistently weaker than average with a three-month average Southern Oscillation Index (SOI) of -8 or lower. As a result of the anomalously warm waters persisting in the western tropical Pacific, the atmospheric indicators have only shown a weak response. The lack of a clear atmospheric response to the positive sea surface temperatures indicates ENSO-neutral, though the tropical Pacific has continued to evolve toward El Niņo. The Indian Ocean Dipole (IOD) is currently neutral. Model outlooks suggest the IOD is most likely to remain neutral through winter and spring. The likelihood of a positive IOD event increases with El Niņo, however at this point there is less than a 30% chance of a positive IOD occurring in 2014. For the remainder of winter, eastern and southeastern parts of the nation are expected to experience near average rainfall. Current forecasts are indicating the potential for above average rainfall for parts of eastern Australia, however this is largely being influenced by warmer than average seas off the east coast. It is expected that the developing El Niņo and shifting atmospheric conditions will negate the effects of these warm waters, with closer to average values more likely. El Niņo will exert more influence during spring, with a trend to drier conditions for large parts of the nation. For much of eastern and southeastern Australia, particularly across the interior, rainfall totals are expected to remain well below median during spring and persist at below median levels during summer. For southern parts of the nation, the correlation to below median rainfall during an El Niņo is lower, however near to below average rainfall is expected. For Western Australia, there is no strong correlation with ENSO and rainfall patterns will be influenced mainly by warm waters over the southeastern Indian Ocean. During the rest of winter, which are the wettest months of the year, near-to-above median rainfall is favoured. Rainfall totals will be variable during the spring months, due to the lack of a defined climate driver to influence rainfall distribution. 

Issued 08 Jul 2014

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.