Northwest 12-month Rainfall Forecast

10 5 0
Oct 18
6
Nov 18
5
Dec 18
5
Jan 19
6
Feb 19
6
Mar 19
5
Apr 19
6
May 19
7
Jun 19
9
Jul 19
9
Aug 19
9
Sep 19
8
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)
Indian Ocean Dipole (IOD): Neutral

Sea surface temperatures (SSTs) and atmospheric indicators across the equatorial Pacific Ocean remain within neutral values. However, these continue on the El Nino side of neutral with significant warming of the subsurface waters across the equatorial Pacific.

The Nino3.4 index lingered between 0.3 and 0.4 during August. On the other hand, the Southern Oscillation Index (SOI) recorded a value of -6.9 in August.

Current consensus suggest neutral ENSO conditions will continue through the southern hemisphere spring. However, the risk of an El Nino developing later in 2018 is about 70% according to the International Research Institute for Climate and Society. The Australian Bureau of Meteorology maintains its outlook to an El Nino WATCH, reflecting the increased likelihood of the event. In the long run, eight out of eight international models have El Nino thresholds being met by November.

Warmer than average SSTs remain across the Gulf of Carpentaria and the Coral Sea, but the Tasman Sea has finally experienced some cooling over the past few weeks. SSTs off the northwest continue to be cooler than average. Three out of six international models indicate positive IOD conditions in the Indian Ocean during the early spring, with all six on the Positive side of neutral. Under this scenario, cooler than average SSTs off the North West Shelf usually result in a reduction in moisture advection over Australia, reinforcing the potential El Nino during spring.

Climate forecasts are favouring below average rainfall for southeastern Australia through the September to November period.

8-month serious-to-severe rainfall deficiencies continue to be observed across NSW, northern and eastern Victoria, parts of southern Qld and eastern South Australia.

In the longer term, the risk of an El Nino towards the end of 2018, favours a drier than average outlook for the eastern half of the country for summer.

This could lead to:
- A significant reduction of rainfall during the eastern states' wet season,
- A later onset of the North Australian Monsoon,
- A reduction of tropical cyclone activity across northern Australia during the summer (leading to a reduction in rainfall for western Qld).

Issued Sep 11

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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