Northwest 12-month Rainfall Forecast

10 5 0
May 18
8
Jun 18
9
Jul 18
8
Aug 18
5
Sep 18
6
Oct 18
7
Nov 18
6
Dec 18
7
Jan 19
9
Feb 19
6
Mar 19
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
Indian Ocean Dipole (IOD): Neutral

Sea surface temperatures (SSTs) and atmospheric indicators across the equatorial Pacific Ocean have returned to neutral values, with little signs of a La Nina or El Nino developing over the coming months.

The Nino3.4 index continued to linger around -0.6 in March, as it did in February. On the other hand, the Southern Oscillation Index (SOI) recorded a value of 10.5 in March.

Current consensus suggest the Pacific will continue to see La Nina side of neutral until early winter, finally flicking towards the El Nino side of neutral later in winter. Long range forecasts are hinting to the possibility of an El Nino events towards the end of 2018, though this far out in time the confidence on this assumption is low.

Warmer than average sea surface temperatures (SST) continue to be the dominant feature across the waters surrounding Australia, particularly off eastern Tasmania and northern NSW. SSTs over the north have experience significant warming over the past few weeks due to the lack of monsoonal activity and cloud cover.

Climate forecasts are favouring average-to-above average rainfall for northern parts of Australia and parts of the continental far south east and eastern Tasmania during the April to June period. Warm SSTs off the eastern seaboard are likely to increase the risk of East Coast Lows over the coming months. This could bring intense, but short episodes of heavy rainfall to southeastern Qld, NSW and eastern Vic. On the other hand, odds are favouring below average rainfall during the second half of autumn across WA and central parts of Australia.

In the longer term, a few seasonal models indicate a possible negative Indian Ocean Dipole (IOD) is on the cards in late winter. Typically negative IOD events bring above-average late winter/spring rainfall to southeastern parts of Australia. At the same time, there is some indications that the Pacific Basin could see a return to El Nino-like conditions during the second half of 2018. If this occurs, odds will favour a dier than average outlook for the eastern half of the country (especially northeastern NSW and Qld) later in winter and through spring.

Issued Apr 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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