Central 12-month Rainfall Forecast

10 5 0
May 19
Jun 19
Jul 19
Aug 19
Sep 19
Oct 19
Nov 19
Dec 19
Jan 20
Feb 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 (El Nino Alert)

Sea surface temperatures (SSTs) warmed up ever so slightly in March, placing the Nino3.4 Index well into El Nino thresholds. Based on this, the North American Weather services have issued an El Nino advisory, effectively declaring it an El Nino albeit weak. Due to the higher thresholds used by BoM, the Bureau maintains an alert.

The Nino3.4 index reached a peak of 0.9 during the week starting on the 24th of March and has lingered at 0.8 since then. On the other hand, the Southern Oscillation Index (SOI) recorded a value of -6.8 in March, the second month in a row with values into the El Nino territory.

Current consensus suggests weak El Nino conditions will continue through autumn and winter, with six out of eight international models maintaining El Nino thresholds through to at least August with a 60-70 % chance of an El Nino event lasting through winter.

Due to the recent Tropical Cyclone activity off northern WA, SSTs have cooled down significantly along Australia's western seaboard but remain warmer than average along the eastern seaboard.

The climate outlook favours average-to-below average rainfall across Qld, NSW and Vic through the southern hemisphere autumn (in particular the first half) and again towards the second half of winter. There are some early indications a Positive IOD could establish itself in the Indian Ocean later in winter with four out of six international models hinting so. During Positive IODs, central and southeastern Australia tends to see a reduction in rainfall amounts towards the second half of winter and early spring.

With warmer than average SSTs across the Tasman Sea, extreme weather events such as East Coast Lows (ECLs) can still bring intense periods of rainfall east of the Great Dividing Range leading to flash floods. May to July are the peak months for ECLs.

Issued Apr 12

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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Behind the ongoing El Nino threat

16:33 AEST There have been signs that El Nino could soon develop for the last several weeks.

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