Great Southern 12-month Rainfall Forecast

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

Sea surface temperatures (SSTs) and atmospheric indicators across the equatorial Pacific Ocean remain within La Nina thresholds, although these have been weakening over the past month.

The Nino3.4 value lingered between -0.6 and -0.7 in January. On the other hand, the Southern Oscillation Index (SOI) recorded a value of 8.9 in January.

Current consensus continues to suggest weak La Nina thresholds likely to continue throughout the remainder of the Austral summer. Only three out of eight international climate models currently maintain La Nina thresholds through to April.

Cooler than average SSTs are present off Western Australia, across tropical Australia and the Coral Sea. Warmer than average SST are still present over the Tasman Sea, particularly off Tasmania where SSTs are up to four degrees warmer than the norm, and through the Great Australian Bight.

Climate forecasts are favouring above average rainfall for parts of southeastern and western Australia through the February to April period. Above average rainfall is particularly favoured for eastern Tasmania and WA‚??s northern costs. The odds of above average rainfall across the eastern states, look to decrease early in spring as La Nina diminishes over the Pacific.

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

Along the eastern seaboard, warmer than average SSTs maintain a high risk of intense, but localised rainfall across southeastern QLD, NSW and VIC. These could see an enhanced risk of severe storms through the remainder of summer due to higher than average moisture in the atmosphere.

Issued Feb 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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