Central Wheatbelt 12-month Rainfall Forecast

10 5 0
Aug 17
5
Sep 17
4
Oct 17
2
Nov 17
7
Dec 17
8
Jan 18
9
Feb 18
6
Mar 18
8
Apr 18
5
May 18
8
Jun 18
4
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.

The Nino3.4 value remained unchanged at 0.5 in June. The Southern Oscillation Index (SOI) recorded a value of -10.4 during the same month, exhibiting a high variability due to the higher than average high pressure over Australia.

As opposed to earlier in the year, El Nino odds continue to exhibit a significant downward trend. Current consensus continues to suggest El Nino side of neutral during the remainder of the year, but only 1 out of 8 international models is indicating a possible El Nino event by November. Usually, at this time of the year, predictability of El Nino events increases once we have surpassed the Spring Predictability Barrier.

On the other hand, Australia's model (POAMA), continues to be an outlier indicating a flip to cooler than average conditions across the Pacific (La Nina side of neutral) by November.

Overall, climate forecasts continue to favour drier than average conditions for the southern half of Australia during the remainder of winter and early spring. A stronger than average high pressure ridge over southern Australia has restricted the northward intrusion of significant rain-bearing cold fronts, a scenario likely to continue over the coming weeks at least.

Moreover, Sea Surface Temperatures (SSTs) across the North West Shelf and the far eastern Indian Ocean have experienced a rapid cooling over the past few weeks reducing the amount of moisture in the atmosphere. The current SST pattern in the Indian Ocean suggest a Positive Indian Ocean Dipole event, which tends to bring below average rainfall to central and southeastern Australia during the tail end of winter and first half of spring.

Despite the overall drier than average conditions for the south, east of the divide, east coast lows (ECL) or just localised low pressure systems can always bring extreme rainfall events with significant rainfall in short periods of time.

Issued Jul 7

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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