Central 12-month Rainfall Forecast

10 5 0
Sep 17
Oct 17
Nov 17
Dec 17
Jan 18
Feb 18
Mar 18
Apr 18
May 18
Jun 18
Jul 18
Aug 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.

The Nino3.4 value decreased to -0.2 in August. The Southern Oscillation Index (SOI) recorded a value of 3.3 during the same month.

Current consensus suggests La Nina side of neutral is likely during the remainder of the year, with only two out of eight international models indicating a warmer side of neutral (El Nino) by January. Out of these two,l only one is indicating a fully fledged La Nina by January, with the remainder models maintaining neutral values.

Sea Surface Temperatures (SSTs) across the far eastern Indian Ocean remain cooler than average, reducing the amount of moisture available in the atmosphere. The current SST pattern in the Indian Ocean suggest a Positive Indian Ocean Dipole event, which tends to bring below average rainfall to central and southeastern Australia during the tail end of winter and first half of spring. However, SSTs over the North West Shelf have experienced a slight warming over the past few weeks, increasing the atmospheric humidity over the region.

Climate forecasts are favouring average-to-below average rainfall for most of central and southern Australia during early spring. A stronger than average high pressure ridge over southern Australia has restricted the northward intrusion of significant rain-bearing cold fronts through the winter months, a scenario likely to continue in the short term. Above average rainfall is however, favoured for coastal areas of WA and SA as cold fronts brush southern Australia. Further ahead, odds are now favouring average-to-wetter than average rainfall for the second half of spring and into early summer for parts of eastern Australia.

Warmer than average sea surface temperatures across northern and eastern Australia are increase the risk of an earlier onset of the wet season across the NT and Qld with neutral odds for WA. Along the eastern seaboard, warmer than average SSTs could lead to intense but localised rainfall events later in spring and early summer. During neutral ENSO years, SE Qld and NE NSW tend to see above average thunderstorm activity during the warmer months.

Issued Sep 12

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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