West & Sth Coast 12-month Rainfall Forecast

10 5 0
Oct 17
6
Nov 17
5
Dec 17
5
Jan 18
5
Feb 18
6
Mar 18
5
Apr 18
4
May 18
3
Jun 18
5
Jul 18
5
Aug 18
7
Sep 18
6
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 -0.3 in August. The Southern Oscillation Index (SOI) recorded a value of 6.9 during the same month.

Current consensus continues to suggests La Nina side of neutral is likely during the remainder of the year. All eight international models are now indicating a cooler side of neutral (La Nina) by throughout the Austral summer, with four models reaching La Nina thresholds in December.

September saw a continuation of the weather patterns that led to warmer than average temperatures during winter. Clearer skies and increased solar irradiance lead to above average daytime temperatures, with close to normal overnight temperatures across the east.

The current climate forecast favours warmer than average daytime and nighttime temperatures across northern and far southeastern Australia for the remainder of spring and beginning of summer. Cooler than average temperatures are only favoured across southern parts of WA, SA and western parts of Vic and NSW. Further east, temperatures are likely remain average to above average for the remainder of spring and early summer.

This forecast continues to favour an earlier onset of the warm season across the north and east. Due to recent rainfall deficiencies across southern Australia, low moisture levels in the soil and drier vegetation early heat waves are a high risk for eastern and southeastern Australia.

However, with the prospects of an intensification the cooler side of neutral in the Pacific (La Nina side) there is some significant risk of increased rainfall towards the end of spring and early summer along the eastern seaboard. This in turn could affect temperatures providing some relief to the warming effect. This could see warmer than average temperatures contracting towards the coasts in the northeast and southeast, were higher humidity values could increase the discomfort levels.

Issued Oct 16

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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