Mount Lofty Ranges 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: El Niņo, reached a strong event.
IOD status: Positive IOD, staying strong.
SAM status: Neutral, trending negative.

The El Niņo Southern Oscillation (ENSO) is in an El Niņo, with all international models indicating this event will continue until at least April 2024. Ocean warming is continuing to occur in the eastern and central Pacific and has reached the threshold for a 'strong event'. The Southern Oscillation Index (SOI) has reduced further and is comfortably below the El Niņo threshold, especially over the last 90-days. El Niņo generally decreases spring rainfall over eastern Australia, with some reductions in rainfall for SA, southern WA and the Top End of the NT. During summer, El Niņo typically delays the first monsoon onset and reduces the number of tropical cyclones impacting Australia, but otherwise has little effect on Australia during January and February.

The Indian Ocean Dipole (IOD) is in a positive phase, and is the 2nd strongest on record, behind only 2019. This event is maintaining its current strength, but may start to weaken around the end of November with tropical activity near Jakarta. Even if it does weaken at that time, the IOD typically takes 3-6 weeks to decay, so could persist into the start of 2024. However, the first monsoon onset over northern Australia will still sever its effect on Australia. A positive IOD greatly reduces the formation of northwest cloudbands, leading to reductions in spring rainfall over northern WA, central Aus, and the southeast. A positive IOD often has a compound effect with El Niņo, reducing rainfall even further.

The Southern Annular Mode (SAM) is currently neutral. It is expected to favour a negative event for the remainder of November, although models suggest a return to a positive event in early-mid December. Longer-term, since negative SAM events become more likely during El Niņo, models suggest it becoming strongly negative around February. A negative phase tends to increase cold front activity over southern Australia, increasing rainfall over southern Vic, western Tas, and southeast SA, while decreasing it for eastern NSW and Qld. The opposite occurs in a positive event, with easterly winds more likely, increasing rainfall and thunderstorm activity in the east, but reducing it in southern and Western Australia.

Rainfall outlooks are predicting above average rainfall for eastern NSW & Qld, below average rainfall over the Tas and far northern Australia, and average elsewhere in November. Large rainfall deficits are expected over northern Australia from December, likely with a delayed initial monsoon onset. Much of the rest of the country is expected to be near average, but eastern NSW is expected to be wetter than normal with increased storm activity. Generally over summer, models are suggesting drier than normal for northern Aus and WA, wetter than normal for western Tas, and near normal elsewhere.

Issued Nov 21

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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