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

10 5 0
Apr
15

4
May
15

3
Jun
15

9
Jul
15

6
Aug
15

4
Sep
15

6
Oct
15

4
Nov
15

6
Dec
15

6
Jan
16

4
Feb
16

7
Mar
16

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

The tropical Pacific Ocean has continued to observe some warming across the region, with the last week in March showing significant warming over the central and eastern parts of the basin. This continuous warming, coupled with increasing temperatures in the subsurface are tilting the balance towards a possible El Nino as early as winter 2015. The sea surface temperatures (SSTs) across the equatorial region around the Date Line continue to show the greatest warming anomaly, about 1-2 degrees warmer than the norm. Over the past few weeks, far eastern parts off the Basin (off the coast of Ecuador) have also shown similar warming. The monthly NINO3.4 value showed a slight warming, ending up at +0.6 degrees in March. Subsurface temperature anomalies along the equatorial Pacific (top 300m of the water column between the Date Line and 100 degrees west) have also continued to warm during March. This warming continued to be most significant over the central Pacific with a current temperature anomaly greater than 2-3 degrees above the average. The overlaying atmosphere has shown some inclination towards El Nino values, with the Southern Oscillation Index (SOI) dipping below -10 during the second half of March. This was due to a burst of westerly winds across the equatorial Pacific thanks to the presence of twin tropical cyclones at either side of the western Equatorial Pacific. Although both El Nino-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are expected to remain within neutral values over the coming months there is an increasing chance that an El Nino and positive IOD will develop during winter. Both El Nino conditions and a positive IOD tend to occur in combination and the fact that the majority of climate models are indicating this, increases our confidence levels in these two events. Usually, but not always, an El Nino event and/or a positive IOD lead to decreased rainfall over eastern and southern Australia during winter and spring. In the shorter term, however, sea surface temperatures (SSTs) across Australia remain quite warm. These SSTs are likely to result in increased moisture levels in the atmosphere. As such, long range forecast models are favouring near-to-above average rainfall through eastern and southwestern parts of Australia, particularly near the coast where we find a strong Eastern Australian and Leeuwin Currents advection warm tropical waters towards the poles. Forecasts are for near-to-below median rainfall through Victoria and South Australia, where intra-seasonal variability of the Southern Annular Mode is likely to keep fronts further south than usual early in the autumn but return to wetter conditions towards the end of Autumn/early winter. In the longer term, the prospects of an El Nino looming in the Pacific brings a significant risk of below average rainfall well into the Austral spring, mainly through the eastern and southern states. However, not every single El Nino is the same and their effects vary year to year. 

Issued 09 Apr 2015

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.