Lower Western 12-month Rainfall Forecast

10 5 0
May 18
5
Jun 18
6
Jul 18
6
Aug 18
4
Sep 18
7
Oct 18
4
Nov 18
6
Dec 18
8
Jan 19
5
Feb 19
8
Mar 19
8
Apr 19
9
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 values.

The Nino3.4 index lingered at around -0.3 in April. On the other hand, the Southern Oscillation Index (SOI) recorded a value of 4.5 in April.

Current consensus suggest neutral ENSO conditions through the southern hemisphere winter. Long range forecasts continue to hint at the possibility of an El Nino event towards the end of 2018, though the confidence on this assumption is low.

Warmer than average sea surface temperatures (SST) dominate the eastern seaboard as well as the northwest. SSTs over the northern Coral Sea have experienced some cooling over the past few weeks.

Climate forecasts continue to favour average-to-above average rainfall for northern parts of Australia as well as the far southeast including Tasmania, Victoria and southern NSW. Warm SSTs off the eastern seaboard maintain a high risk of East Coast Lows over the coming weeks. This could bring intense, but short episodes of heavy rainfall to southeastern Qld, eastern NSW and eastern Vic. On the other hand, odds are favouring below average rainfall during the coming months across western WA, southern SA and western Victoria.

In the longer term, there remains a risk for a possible negative Indian Ocean Dipole (IOD) to develop later in winter. Typically negative IOD events bring above-average late winter/spring rainfall to southeastern parts of Australia. Moreover, there is also some indications that the Pacific Basin could see a return to El Nino-like conditions during the second half of 2018. If this occurs, odds will favour a drier than average outlook for the eastern half of the country through spring.

Issued May 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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