Nth Cst Tableland 12-month Rainfall Forecast

10 5 0
Dec 19
Jan 20
Feb 20
Mar 20
Apr 20
May 20
Jun 20
Jul 20
Aug 20
Sep 20
Oct 20
Nov 20
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
SAM status: Negative

Sea surface temperatures (SSTs) across the equatorial central Pacific continued to exhibit a slight warming during November. The Nino3.4 index lingered between 0.6 and 0.7 throughout the month of November. However, this has dropped to 0.4 since early December. On the other hand, the Southern Oscillation Index (SOI) recorded a value of -9.3 in November.

The current outlook continues to suggest warmer than average SSTs will persist across the equatorial Pacific Ocean through the coming months, but remaining within neutral values. Seven out of eight international models maintain a warmer than average equatorial Pacific Ocean through the first quarter of 2020, none of these reaching El Nino thresholds. The IRI ENSO forecast continues to suggest a less than 40% chance of another El Nino developing over the next 6 months, with a neutral ENSO the most likely outcome.

To the west of Australia, the Indian Ocean Dipole (IOD) remains in the positive side of neutral, though it has significant strength after peaking in October . All six international models maintain a positive event until January.

In terms of rainfall across Australia, there is little variation to the outlook issued last month across the west. However, the climate outlook for the eastern states has seen a significant drying over the past 2 months. The current outlook now favours a clearer below average rainfall signal for the eastern states. This however, excludes any extreme weather events such as tropical cyclones in Qld or East Coast Lows.

For northern Australia, the current outlook favours a late onset of the North Australian Monsoon (NAM) across the Northern Territory and Queensland. Usually, Darwin tends to see the onset of NAM around christmas day.

Further south, a negative trend in the Southern Annular Mode (SAM) continues to enhance frontal activity over southern SA, Victoria, Tasmania and southern NSW. This has offset the Positive IOD in far southern Victoria and western Tasmania, which continue to see average-to-above average rainfall odds through December and early January.

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 suggests neutral odds for the second half of summer.

Issued Dec 11

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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