Central West 12-month Rainfall Forecast

10 5 0
Aug 19
Sep 19
Oct 19
Nov 19
Dec 19
Jan 20
Feb 20
Mar 20
Apr 20
May 20
Jun 20
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

Sea surface temperatures (SSTs) showed a cooling trend over the central Pacific during June, though remain on the warmer side of neutral across the equatorial Pacific.

The Nino3.4 index lingered between 0.6 and 0.5 through the month of June.The Southern Oscillation Index (SOI) recorded a value of -10.4 in June, remaining within El Nino territory.

Current consensus suggests warmer than average SSTs will remain across the equatorial Pacific Ocean through the remainder of winter. Moreover, seven out of eight international models maintain a warmer than average Pacific Ocean through the remainder of 2019, with a 50-60% chance of an El Nino developing again later this year.

The climate outlook for winter and early spring continues to favour an average-to-below average rainfall outlook across the southern half of Australia. A Positive IOD is now established over the Indian Ocean, with five out of six international models exhibiting a moderate-to-strong event lasting until late spring. During Positive IODs, central and southeastern Australia tend to see a reduction in rainfall during the second half of winter and the first half of spring.

SSTs along the eastern seaboard remain significantly warmer than average, 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 Jul 10

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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The week ahead for NSW

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