Peninsula 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. Negative possible.
SAM status: Negative.

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. Ocean warming has been occurring near Indonesia, with far more modest warming near Africa, lowering the IOD index. Northerly winds have weakened off the Australian continent, but cloud patterns are more inline with a positive phase. All this indicates that a negative IOD could be developing, but the atmosphere and oceans are currently decoupled. 4 out of 5 of the international models (all except BoMs model) are forecasting a negative IOD by October. A negative IOD increases the number of northwest cloudbands that cross Australia during winter and spring, increasing rainfall across central and southeastern parts of Australia.

The Southern Annular Mode (SAM) is negative after a Sudden Stratospheric Warming event over Antartica in mid-July. This event has significantly altered the shape of the polar vortex, and directly leads to a negative SAM phase. Given the magnitude of this change, weather in Australia (and the rest of the southern hemisphere) will likely go through periods of wetter (trough) and drier (ridge) patterns every 2-4 weeks during winter and spring. A negative SAM increases the frequency of cold fronts. During the cool months, this increases rainfall through southwest WA, SA, inland and southern NSW, Vic and Tas, and dries out eastern Qld and NSW.

Rainfall outlooks are showing above average rainfall for southwest WA, southern SA, inland and southern NSW, Vic and Tas in August and September. From October, wetter than average conditions are expected for eastern Australia, and drier for inland parts of WA.

Issued Jul 25

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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