NW Pastoral 12-month Rainfall Forecast

10 5 0
No forecast available for this district.
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 Niņa Watch.
IOD status: Neutral. Positive less likely
SAM status: Neutral, favouring positive in winter.

The El Niņo Southern Oscillation (ENSO) is in a La Niņa Watch, meaning there is approximately a 50% chance of a La Niņa developing this year. Some ocean cooling is occurring in the eastern Pacific, with 3 out of 7 global models forecasting a La Niņa to develop in 2024. However, the Nino3.4 region is still warm from last years El Niņo, meaning if La Niņa were to develop, it would likely be declared no earlier than October. Warmer than normal waters are situated in the Coral Sea, providing moisture and humidity to the east coast. The atmosphere continues to be neutral. A La Niņa typically increases rainfall over central Australia, Qld and NSW, but has little effect on southern WA, SA, Vic and Tas during winter.

The Indian Ocean Dipole (IOD) is in a neutral phase. A burst of tropical activity in May has significantly disrupted the ocean component of the IOD, with the index now very close to zero. The atmosphere is still mostly set up, so a positive IOD could still develop this year. However, if a La Nina-like pattern continues to develop in the Pacific, warm water would likely push into through the Indonesian flow-through, disrupting any positive IOD signal. Therefore, it is now more likely than not the IOD stays in a neutral phase this year. A positive IOD reduces the number of northwest cloudbands that cross Australia during winter and spring, reducing rainfall across central and southeastern parts of Australia.

The Southern Annular Mode (SAM) is neutral, after a 3-week negative spell during late May and early June. The SAM is expected to favour a positive phase during July and August. A positive SAM increases easterly winds across Australia. This leads to increased rainfall and thunderstorm activity in the east, while reducing cold fronts and rainfall over southern Australia, particularly for southwest WA.

Rainfall outlooks are showing slightly below average rainfall over winter (mostly June and July) for WA, and most of SA, Vic and Tas. Conversely, above average rainfall is expected for eastern parts of Qld and NSW, and eastern Tas. Outlooks turn close to average in August throughout mainland Australia, with Tas still showing drier than normal.

Issued Jun 12

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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Welcome rain spreading across parched SA

16:06 AEST A much-needed burst of wet weather will sweep across the southern half of SA during the next two days, although the useful showers will be accompanied by blustery winds and a wintry chill.

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