South West 12-month Rainfall Forecast

10 5 0
Aug 19
7
Sep 19
6
Oct 19
5
Nov 19
4
Dec 19
6
Jan 20
6
Feb 20
6
Mar 20
6
Apr 20
5
May 20
4
Jun 20
8
Jul 20
5
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
IOD status: Positive

Sea surface temperatures (SST's) continued a cooling trend over the central Pacific during July.

The Nino3.4 index lingered at about 0.5 through the month of July. The Southern Oscillation Index (SOI) recorded a value of -5.6 in July, which is now in neutral territory.

Current consensus suggests warmer than average SSTs will remain across the equatorial Pacific Ocean through the austral spring, but well within neutral values. Although, six out of eight international models maintain a warmer than average Pacific Ocean through the remainder of 2019, none of these are reaching El Nino thresholds during the 2019/2020 summer. Moreover, the IRI ENSO forecast suggests less than 50% of another El Nino developing over the next 12 months.

On the other hand, the Indian Ocean Dipole (IOD) remains strongly positive. Six out of six international models maintain a positive event until mid-spring.

The climate outlook for the four weeks left in winter and spring, favours average-to-below average rainfall across the southern half of Australia, in particular over the eastern half of the continent. During Positive IODs, central and southeastern Australia tend to see a reduction in rainfall during the end of winter and the first half of spring due to a reduction of moisture streaming from the northwest.

SST's along the eastern seaboard remain significantly warmer than average (especially off NSW) maintaining a high risk of extreme weather events such as East Coast Lows (ECLs). These can bring intense periods of rainfall east of the Great Dividing Range, leading to flash flooding.

Issued Aug 8

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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