Northern Tablelands 12-month Rainfall Forecast

10 5 0
Sep 19
1
Oct 19
2
Nov 19
3
Dec 19
6
Jan 20
6
Feb 20
6
Mar 20
5
Apr 20
9
May 20
6
Jun 20
8
Jul 20
5
Aug 20
5
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: Positive

Sea surface temperatures (SSTs) continued the cooling trend over the central Pacific during August, with areas in the eastern equatorial Pacific exhibiting below average temperatures for the first time in about two years.

The Nino3.4 index lingered between 0.2 and 0.1 through the month of August. The Southern Oscillation Index (SOI) recorded a value of -4.4 during the past month, which remains in neutral territory.

Current consensus suggests warmer than average SSTs will persist across the equatorial Pacific Ocean through the austral spring, but well within neutral values. Although, three out of eight international models maintain a slightly warmer than average Pacific Ocean through the end of 2019, none of these are likely to reach El Nino thresholds during the 2019/2020 summer. Moreover, the IRI ENSO forecast suggests less than 40% of another El Nino developing over the next 12 months, with a neutral ENSO the most likely outcome.

To the west of Australia, the Indian Ocean Dipole (IOD) remains strongly positive. All six international models maintain a strong positive event until the beginning of the summer.

The climate outlook for spring favours below average rainfall across the southern half, and in particular the eastern half of the continent. During Positive IODs, central and southeastern Australia tend to see a reduction in rainfall during the end of winter and spring due to a reduction of moisture streaming from the northwest.

On the other hand, far southern Victoria and western Tasmania could see above average rainfall during this period as a sudden stratospheric warming (SSW) over Antarctica looks to enhance the negative phase of the southern annular mode (SAM) over the coming months.

Issued Sep 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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