Sydney 12-month Rainfall Forecast

10 5 0
Jul 18
7
Aug 18
7
Sep 18
5
Oct 18
4
Nov 18
7
Dec 18
6
Jan 19
4
Feb 19
10
Mar 19
5
Apr 19
6
May 19
6
Jun 19
7
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, the past weeks have seen an significant warming of the equatorial Pacific, both across the surface and the top 200 metres of the water column.

The Nino3.4 index experienced a significant jump from 0 in May to +0.3 in June. On the other hand, the Southern Oscillation Index (SOI) recorded a value of -5.5 in June.

Current consensus suggest neutral ENSO conditions are likely to persist through the southern hemisphere winter. However, the risk of an El Nino developing later in 2018 has considerably increased. The Australian Bureau of Meteorology has updated its outlook to an El Nino WATCH, reflecting the increased likelihood of the event. In the long run, five out of eight international models have El Nino thresholds being met by November. Moreover, the International Research Institute for Climate and Society, is now indicating a 60% probability of having an El Nino by the end of the Austral spring.

Although areas of warmer than average SSTs remain across Australia's east, the past month has seen considerable cooling across the Tasman and Coral seas. Off the northwest coast SSTs have continued to see a cooling trend. Five out of six international models are indicating positive IOD conditions in the Indian Ocean during early spring. 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-like conditions from the Pacific.

Climate forecasts are favouring below average rainfall for most of Australia's eastern and southern halves through the remainder of winter and early spring.

Despite some good rainfall in recent weeks, most of western NSW, far north western Victoria and eastern SA continue to see 5-month rainfall deficiencies at serious-to-severe levels.

In the longer term, there is risk that El Nino-like conditions during the second half of 2018, could 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.

Issued Jul 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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