Southern Coastal 12-month Rainfall Forecast

10 5 0
May 17
Jun 17
Jul 17
Aug 17
Sep 17
Oct 17
Nov 17
Dec 17
Jan 18
Feb 18
Mar 18
Rainfall deciles
10 Well above normal
8-9 Above normal
4-7 Near normal
2-3 Below normal
1 Well below normal

Issue Notes

ENSO status: Neutral.
Indian Ocean Dipole (IOD): Neutral.

Sea surface temperatures (SSTs) and atmospheric indicators across the equatorial Pacific Ocean remain within neutral ENSO boundaries though they are on the El Nino side of neutral.

The Nino3.4 value remained within neutral at 0.18 in March, though with a warming tendency. The Southern Oscillation Index (SOI) recorded a value of 5.1 during the same month, also within neutral.

Current consensus continues to suggest neutral conditions over the coming months, with a gradual warming of the sea surface of the tropical Pacific, possibly reaching El Nino thresholds by late winter/early spring. The Southern Annular Mode (SAM) has been exhibiting a mostly neutral-to-positive trend, with models suggesting a positive trend towards mid-April.

Climate forecasts continue to favour drier than average conditions for the southern half of Australia during autumn and early winter. During this period, southern Australia sees an increase in the number of rain-bearing cold fronts traversing the south. However, a stronger than average high pressure ridge over Australia is looking to keep these at bay. Models also continue to favour above average rainfall for the tropics during the final weeks of the wet season and early in the dry season.

January to June is usually the wettest half of year for the eastern states and warmer than average sea surface temperatures are likely to maintain high levels of moisture in the atmosphere leading to average-to-above average rainfall. East of the divide, late tropical cyclones and east coast lows could bring extreme rainfall events with significant rainfall in short periods of time.

In the longer term, models are hinting to a possible El Nino scenario or at least El Nino side of neutral, which increases the risk of drier than average conditions for late winter and spring.

Issued Apr 7

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.

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