Peninsula 12-month Rainfall Forecast

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

Sea surface temperatures (SSTs) and atmospheric indicators across the equatorial Pacific Ocean remain within neutral values. However, these are now on the El Nino side of neutral with warmer than average ocean waters across the equatorial Pacific.

The Nino3.4 index lingered between 0.4 and 0.5 during July. On the other hand, the Southern Oscillation Index (SOI) recorded a value of 1.6 in July.

Current consensus suggest neutral ENSO conditions will continue through the remainder of the southern hemisphere winter and early spring. However, the risk of an El Nino developing later in 2018 is close to 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, six out of eight international models have El Nino conditions by December.

Although areas of warmer than average SSTs remain across the Gulf of Carpentaria, the Coral Sea and parts of the Tasman Sea, below average SSTs continue to be observed over the North West Shelf and the Great Australian Bight. Five out of six international models continue to indicate positive IOD conditions in the Indian Ocean during early spring, with three meeting the threshold. Under this scenario, cooler than average SSTs off the North West Shelf could lead to a reduction in moisture advection over Australia, reinforcing the potential El Nino during the end of winter and early spring.

Climate forecasts are favouring below average rainfall for southeastern Australia through the last few weeks of winter, spring and early summer.

7-month serious-to-severe rainfall deficiencies continue to be observed across most of NSW, northern Victoria and eastern South Australia. Areas of serious-to-severe rainfall deficiency also remain over southern WA and southern Qld.

In the longer term, there is risk that El Nino-like conditions towards the end of 2018, favour a drier than average outlook for the eastern half of the country through late spring and 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 Aug 10

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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Wetter than average August for Perth

17:03 AEST Even though we are not yet half way through August, Perth has already recorded higher than average rainfall.

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