Southern Tablelands 12-month Rainfall Forecast

10 5 0
Nov 19
3
Dec 19
4
Jan 20
7
Feb 20
5
Mar 20
5
Apr 20
7
May 20
7
Jun 20
8
Jul 20
6
Aug 20
4
Sep 20
4
Oct 20
3
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) across the equatorial central Pacific warmed up slightly over the past few weeks. As a reflection, the Nino3.4 index lingered between 0.5 and 0.7 throughout the month of October. The Southern Oscillation Index (SOI) recorded a value of -5.6 during the past month.

The current outlook consensus suggests warmer than average SSTs will persist across the equatorial Pacific Ocean through the coming months, but remaining within neutral values. Five out of eight international models maintain a warmer than average Pacific Ocean through the end of 2019 and first quarter of 2020, however, none of these reaching El Nino thresholds. The IRI ENSO forecast continues to suggest 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, with the highest positive value since the early 2000s. All six international models maintain a strong positive event until December.

In terms of rainfall, there is little variation to the outlook issued last month. The climate outlook for October continues to favour below average rainfall for most of Australia, in particular the eastern half of the continent. During Positive IODs, central and southeastern Australia tend to see a reduction in rainfall during spring, due to a reduction of moisture streaming from the northwest.

For northern Australia, the current outlook is for a late onset of the wet season across the Northern Territory and Queensland. Thus far, Darwin and South Eastern Qld have seen the onset of the wet season this year though a week later than usual.

Further south, a negative trend in the Southern Annular Mode (SAM) will continue to enhance frontal activity over the far south during October and early. This is likely to offset the Positive IOD in far southern Victoria and western Tasmania, which could see average-to-above average rainfall during October and early December.

Looking further ahead, with no significant signature from ENSO, there are no significant climate drivers tilting the balance towards a drier or wetter summer. The current outlook suggest neutral odds.

Issued Nov 8

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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