Lower Western 12-month Rainfall Forecast

10 5 0
Nov 18
Dec 18
Jan 19
Feb 19
Mar 19
Apr 19
May 19
Jun 19
Jul 19
Aug 19
Sep 19
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)
Indian Ocean Dipole (IOD): Positive

Sea surface temperatures (SSTs) and atmospheric indicators across the equatorial Pacific Ocean remain within neutral values, however, SSTs across the equatorial Pacific have experienced a marked warming over the past weeks.

The Nino3.4 index lingered between 0.2 and 0.5 during September, but jumped to 0.5 towards the end of the month. On the other hand, the Southern Oscillation Index (SOI) recorded a value of -10 in September.

Current consensus suggest neutral ENSO conditions will continue over the coming weeks. The risk of an El Nino developing in December 2018 is about 70% according to the International Research Institute for Climate and Society. The Australian Bureau of Meteorology has elevated its outlook to an El Nino Alert, reflecting the increased likelihood of the event. In the long run, the general consensus is that if an event does occur, it is likely to be marginal and short lived.

The warmer than average SSTs observed over the Coral and Tasman seas during winter have seen some significant cooling, in line to what we would expect during an El Nino event. SSTs off the northwest continue to be cooler than average, heralding a positive Indian Ocean Dipole (IOD) phase. Under this scenario, cooler than average SSTs off the North West Shelf usually results in a reduction of moisture advection over Australia, leading to below average rainfall across central and southeastern Australia during spring.

With a positive IOD and an El Nino on the cards, climate forecasts are favouring below average rainfall for southeastern Australia through the October to December 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 despite the recent rainfall events over the eastern states.

In the longer term, the risk of an El Nino during summer favours a drier than average outlook for the eastern half of the country during the December to January period. If a marginal El Nino does eventuate, rainfall values could return to normal towards the end of summer or early autumn. Higher humidity levels towards the end of summer/early autumn could bring some good rainfall to the eastern states and perhaps enhanced thunderstorm activity, leading to flash floods.

This could lead to:

- A significant reduction of rainfall during the eastern statesā?? 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 Oct 17

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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