South West 12-month Rainfall Forecast

10 5 0
Jul 20
Aug 20
Sep 20
Oct 20
Nov 20
Dec 20
Jan 21
Feb 21
Mar 21
Apr 21
May 21
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: Neutral
SAM status: Trending positive

Sea surface temperatures (SSTs) across the equatorial central Pacific exhibited significant cooling over the past month, with the Nino3.4 index registering a value of -0.1in May. On the other hand, the Southern Oscillation Index (SOI) recorded a value of 2.8 during the same month.

The current outlook suggests cooler than average SSTs will take hold across the equatorial Pacific Ocean during winter, potentially reaching La Nina thresholds by mid-spring. All eight international models continue to suggest a cooler side of neutral for the region throughout the remainder of the Austral winter and through spring. The current International Research Institute for Climate and Society (IRI) outlook suggests a 45:40 chance for La Nina:Neutral conditions by the end of the Austral spring.

To the west of Australia, the Indian Ocean Dipole (IOD) remains neutral, normal for this time of the year. However, out of season Tropical Cyclone Mangga brought some significant cooling to the sea surface temperatures to the northwest of Australia late in May. This disrupted the warming trend in the region that would have led to a negative IOD by mid-winter. Nonetheless, all but one of the six main international models (BoMs outlook), continue to hint at a possible negative IOD establishing later in winter/first half of spring.

In terms of precipitation across Australia, the current short-term outlook now favours below average rainfall for most of the country for the remainder of June and early July. This outlook is mainly driven by cooler SSTs across northwestern Australia, reducing the amount of available moisture in the atmosphere. As discussed earlier, tropical cyclone Mangga was a key player in the cooling of the SSTs off the northwest coast.

Most models, however, are indicating a positive IOD is still on the cards later in winter and through spring, bringing a wetter outlook for most of the southern half of the country from mid-to-late July.

Across parts of southern Vic, NSW and Qld (east of the Great Dividing Range), southwestern WA and Tasmania, the outlook continues to favour average-to-slightly below average rainfall over the coming months as these regions transition to their ?drier? part of the year (eastern seaboard) or tend to be less influenced by the IOD (SW WA and Tas) but more on the phases of the Southern Annular Mode (SAM).

Issued Jun 15

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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