12-Month Rainfall Forecast
Flinders 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 11 Nov 2020
ENSO status: La Niña IOD status: Neutral SAM status: Positive La Niña is well underway in the central and eastern equatorial Pacific waters, with significant cooling observed during the last few months. The atmospheric and ocean conditions have both exceeded La Niña thresholds, increasing the confidence of La Niña influencing the outlook until at least the remainder of summer. The majority of models predict that La Niña will peak in December or January, with all models predicting the event will last until January, 6 of 8 models to February and 5 of 8 to March. Given we have observed both the atmosphere and the oceans responding to La Nina, the models have strengthened the event, with around half of the models predicting a strong event. The 2010-2012 La Niña event is different to this year, due to the influence of a negative Indian Ocean Dipole (IOD) in 2010-2012, which enhanced the rainfall across Australia. To the west of Australia, the Indian Ocean Dipole (IOD) is neutral, which is likely tocontinue through to at least autumn. Two of six international models indicate a negative IOD during November, before becoming 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. La Niña increases the likelihood of above normal precipitation in spring and summer across much of Australia. The remainder of spring looks wetter than normal for much of Australia, except for parts of western WA and western TAS, where normal precipitation is expected. 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.