West & Sth Coast 12-month Rainfall Forecast

10 5 0
Aug 17
Sep 17
Oct 17
Nov 17
Dec 17
Jan 18
Feb 18
Mar 18
Apr 18
May 18
Jun 18
Jul 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 increased to 0.4 in July. The Southern Oscillation Index (SOI) recorded a value of 8.1 during the same month.

Current consensus suggests El Nino side of neutral during the remainder of the year, but three out of eight international models are indicating a flip to the cooler side of neutral (La Nina) by December. None of the eight models are indicating a fully fledged El Nino.

As per previous month's, climate forecasts continue to favour drier than average conditions for the southern half of Australia during the last month of winter and first half of 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 to medium term.
Sea Surface Temperatures (SSTs) across the North West Shelf and the far eastern Indian Ocean have remained 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.

The only noticeable exception is parts of northern Australia and Tasmania, which could see above average rainfall through this period. Warmer than average sea surface temperatures across northern and eastern Australia are likely to bring an earlier onset of the wet season across the NT and Qld with neutral odds for WA. Along the eastern seaboard, rainfall odds are close to neutral, warmer than average SSTs could lead to intense rainfall events with localised rainfall events over the coming months.

Despite most models not predicting a fully fledge Positive Indian Ocean dipole, all eight models do have the IOD on the positive side of neutral for the remainder of the year.

Issued Aug 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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