Peninsula 12-month Rainfall Forecast

10 5 0
Feb 20
Mar 20
Apr 20
May 20
Jun 20
Jul 20
Aug 20
Sep 20
Oct 20
Nov 20
Dec 20
Jan 21
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
IOD status: Neutral
SAM status: Trending neutral

Sea surface temperatures (SSTs) across the equatorial central Pacific remained stable throughout January with the Nino3.4 index registering a value of 0.4 as per December. On the other hand, the Southern Oscillation Index (SOI) recorded a value of 1.3 in January.

The current outlook suggests warmer than average SSTs will persist across the equatorial Pacific Ocean through the austral autumn, cooling even further by mid 2020. Six out of eight international models continue to maintain a warmer than average equatorial Pacific Ocean through the first half of the year, none of these reaching El Nino thresholds.

To the west of Australia, the Indian Ocean Dipole (IOD) remains neutral, which is what you will expect for this time of the year. All six international models maintain neutral IOD values until the austral winter, with indications we could see another positive event late winter/spring. However, the skill of the models is low at this time of the year.

In terms of rainfall across Australia, the current outlook favours average-to-below average rainfall for parts of inland NSW and Qld for the end of summer and autumn. This however, excludes any extreme weather events such as tropical cyclones or East Coast Lows. Across the north, the North Australian Monsoon (NAM) had its latest onset on record in early February at Darwin (by its technical definition) with the current prognosis suggesting another below average wet season for northern Australia. The northern half of the country however, could see a wet end to the wet season compared to the season thus far.

Looking further ahead, with no significant signature from ENSO, there are no significant climate drivers tilting the balance towards a drier or wetter end of autumn/winter.

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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