Central 12-month Rainfall Forecast

10 5 0
Jun 17
8
Jul 17
2
Aug 17
4
Sep 17
9
Oct 17
9
Nov 17
4
Dec 17
2
Jan 18
3
Feb 18
6
Mar 18
2
Apr 18
2
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 remain within neutral ENSO boundaries. However, these are now exhibiting some El Nino characteristics.

The Nino3.4 value remained within neutral at 0.3 in April, though continuing to show a warming tendency. The Southern Oscillation Index (SOI) recorded a value of -6.3 during the same month, also within neutral. The negative value shows the high variability of the SOI during recent weeks due to the increased atmospheric instability across the central Pacific.

Current consensus continues to suggest El Nino side of neutral over the coming months, with three out of 8 international models reaching an El Nino threshold by July and 5 by September. Australia‚??s model (POAMA) however, is now downplaying the risk of an El Nino event later this year.

The Southern Annular Mode (SAM) has continued to exhibit a mostly neutral-to-positive trend, with models suggesting an easing positive trend through the remainder of May.

Climate forecasts continue to favour average to drier than average conditions for the southern half of Australia during May and first half of winter.

During this period, southern Australia sees most of its rainfall through rain-bearing cold fronts traversing the south. However, a stronger than average high pressure ridge over Australia is looking to keep these at bay. Models also continue to favour average to above average rainfall for the tropics during the early dry season.

Despite the overall drier than average conditions for the south, east of the divide, east coast lows can always bring extreme rainfall events with significant rainfall in short periods of time.

In the longer term, a possible El Nino scenario, or at least El Nino side of neutral, will increase the risk of drier than average conditions for the late winter and spring. Moreover, climate models are also indicating the risk of a positive Indian Ocean dipole towards the end of winter and through spring, increasing the risk of drier than average conditions for southern Australia.

Issued May 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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