NW Slopes & Plains 12-month Rainfall Forecast

10 5 0
Mar 18
Apr 18
May 18
Jun 18
Jul 18
Aug 18
Sep 18
Oct 18
Nov 18
Dec 18
Jan 19
Feb 19
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, although these have been weakening over the past month.

The Nino3.4 value lingered around -0.6 in February. On the other hand, the Southern Oscillation Index (SOI) recorded a value of -6.0 in February.

Current consensus suggest the weak La Nina experienced over the past months is in decline with a return to neutral conditions within the next couple of months. Only one out of eight international climate models currently maintain La Nina thresholds through to May.

Cooler than average SSTs continue to feature off Western Australia, the Gulf of Carpentaria and east of the Cape York Peninsula. Warmer than average SST remain a dominant feature over the Tasman Sea, particularly off eastern Tasmania, and through the Great Australian Bight. The past month has also seen some warming across the Coral Sea.

Climate forecasts are favouring below average to average rainfall for central Australia and parts of the south east during the March to May period. This will signify a reduction of cold fronts and cut-off lows clipping the WA's southern coasts, SA and western parts of Vic. Above average rainfall is particularly favoured for eastern Tasmania and WA's northern costs. Across NSW and southeastern Qld, the odds are favouring neutral rainfall during autumn. However, warm SSTs off the eastern seaboard is likely to increase the risk of East Coast Lows as we head into the peak of their season. This could bring intense but short episodes of heavy rainfall to southeastern Qld, NSW and eastern Vic.

As for the winter outlook, its is hard to say with high precision as we are in the middle of the so-called 'spring predictability barrier', which is when seasonal outlook models have a harder time making accurate forecasts.

BoM's seasonal model (POAMA) is hinting at negative Indian Ocean Dipole (IOD) conditions for that period. Typically negative IOD events bring above-average winter and early spring rainfall to northwestern, central and 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 drier than average outlook for the eastern half of the country later in winter and through spring.

Issued Mar 9

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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Darkness descending on Australia

16:55 AEDT The March Equinox occurs in Australia on Wednesday morning, marking the end of the astronomical summer in the southern hemisphere.

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