Southwest 12-month Rainfall Forecast

10 5 0
Dec 17
Jan 18
Feb 18
Mar 18
Apr 18
May 18
Jun 18
Jul 18
Aug 18
Sep 18
Oct 18
Nov 18
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: La Nina
Indian Ocean Dipole (IOD): Neutral

Sea surface temperatures (SSTs) and atmospheric indicators across the equatorial Pacific Ocean have now reached La Nina thresholds.

The Nino3.4 value decreased from -0.3 in October to -0.6 in November. On the other hand, the Southern Oscillation Index (SOI) recorded a value of 11.8 during November.

Current consensus suggests weak La Nina thresholds are likely to continue throughout the Austral summer. Seven out of eight international climate models, currently maintain La Nina thresholds until February, bringing a high risk of a weak La Nina being declared in the Pacific Ocean this season.

Across the far eastern Indian Ocean, SSTs have cooled down significantly thanks to enhanced tropical activity throughout the region late in November and early December. Further south, an oceanic heat wave drifted eastwards across the Tasman Sea, from Tasmania to New Zealand, bringing SSTs four to five degrees warmer than average to the region.

Climate forecasts are favouring above average rainfall for parts of eastern and southeastern Australia early in the summer. Above average rainfall is particularly favoured over eastern Vic, southeastern NSW as well as southeastern Qld and eastern parts of Tasmania.

Areas over central and southern Australia, could also see higher than average rainfall, as near stationary trough over the continent stream tropical moisture from the North West Shelf towards the southern coasts.

Most of eastern Qld, Top End and the Kimberley have already seen the onset to their Wet Season, with over 50mm of rainfall accumulated since September 1st.

Along the eastern seaboard, warmer than average SSTs maintain a high risk of intense, but localised rainfall events over the coming months.

Issued Dec 12

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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A stormy outlook for Darwin

13:23 AEDT Darwin is on track to reach their December average rainfall, which is already sitting at the halfway mark.

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