12-Month Rainfall Forecast
Arnhem 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 8 Oct 2020
ENSO status: La Niña IOD status: Negative SAM status: Positive La Niña thresholds have been met due to continued cooling in the central and eastern equatorial Pacific waters in the last month. The atmospheric and ocean conditions have both exceeded La Niña thresholds, with the current Nino3.4 index registering a value of -0.68 and the Southern Oscillation Index is +9.5, now well over the La Niña threshold of +7. The majority of models predict that La Niña will peak in December, with all eight models predicting the event will last until January and 5 of 8 models to February. During this month’s model run, the strength of the La Niña event has increased since last month, however at this stage the event is not expected to be as strong as the 2010-2012 La Niña. To the west of Australia, the Indian Ocean Dipole (IOD) has shifted into a negative phase, which is likely to continue through October. Three of six international models indicate a negative IOD to be sustained for the duration of November, beforebecoming neutral in December. La Niña typically favours periods of positive SAM, which can increase the amount of precipitation along the eastern half of the country, due to prevailing onshore winds. The combination of La Niña and negative IOD conditions increase the likelihood of above normal precipitation in spring and summer across much of Australia. In terms of precipitation over Australia, the remainder of spring looks wetter than normal for much of Australia. However, normal to below normal precipitation is forecast for parts of western WA and western TAS. During summer, much of the country is likely to remain wetter than normal including western WA during these months. Western TAS precipitation outlook continues to be normal through summer. La Niña typically increases the number of cyclones and brings an early start to the northern wet season.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.