Central 12-month Rainfall Forecast

10 5 0
Jun 19
7
Jul 19
5
Aug 19
3
Sep 19
5
Oct 19
5
Nov 19
6
Dec 19
6
Jan 20
8
Feb 20
7
Mar 20
5
Apr 20
3
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 Watch)

Sea surface temperatures (SSTs) remain on the warm side of neutral across the equatorial Pacific, albeit having exhibited a slight cooling over the past 2 weeks.

The Nino3.4 index was maintained at 0.9 during most of April. On the other hand, the Southern Oscillation Index (SOI) recorded a value of -1.3 in April, dropping below El Nino territory.

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

The climate outlook for winter favours average-to-below average rainfall across southern Qld, most of NSW, Vic, Tas and the southern costs of SA and WA. There are some early indications of a Positive IOD establishing itself in the Indian Ocean later in winter, with five out of six international models exhibiting a moderate-to-strong event by August/September. During Positive IODs, central and southeastern Australia tends to see a reduction in rainfall amounts towards the second half of winter and early spring.

The lingering warmer than average SSTs across the Tasman Sea increases the risk of extreme weather events such as East Coast Lows (ECLs), which activity reaches its peak in June. These can bring intense periods of rainfall east of the Great Dividing Range, leading to flash flooding.

Issued May 15

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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Wintry week ahead for southeastern Australia

13:59 AEST A procession of cold fronts will cause snow, hail, thunderstorms, rain and damaging winds in parts of southeastern Australia during the next several days.

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