Arnhem 12-month Rainfall Forecast

10 5 0
Feb 18
7
Mar 18
6
Apr 18
8
May 18
7
Jun 18
1
Jul 18
10
Aug 18
9
Sep 18
5
Oct 18
6
Nov 18
6
Dec 18
8
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.

The Nino3.4 value lingered between -0.6 and -0.8 during December. On the other hand, the Southern Oscillation Index (SOI) recorded a value of -1.4 in December.

Current consensus continue to suggest weak La Nina thresholds likely to continue throughout the Austral summer. Seven out of eight international climate models currently maintain La Nina thresholds through January and February. Only five models maintain such conditions through to March.

SSTs have warmed-up significantly once again over the Arafura and Timor seas thanks to little monsoonal activity and cloud cover. Across southeastern Australia, an oceanic heat wave continues to dominate the Tasman Sea, with SSTs four to five degrees warmer than average.

Climate forecasts continue to favour above average rainfall for parts of eastern and and western Australia during the reminder of the summer. Above average rainfall is particularly favoured over eastern Vic, northeastern NSW, southeastern Qld and eastern parts of Tasmania.

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.

Along the eastern seaboard, warmer than average SSTs maintain a high risk of intense, but localised rainfall events over the coming months, with increase risk of severe storms during the peak of the season due to higher than average moisture in the atmosphere.

Issued Jan 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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