12-Month Rainfall Forecast
Perth 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 9 Mar 2020
ENSO status: Neutral IOD status: Neutral SAM status: Trending neutral Sea surface temperatures (SSTs) across the equatorial central Pacific remained stable throughout February with the Nino3.4 index registering a value of 0.3. On the other hand, the Southern Oscillation Index (SOI) recorded a value of -2.2 in February. The current outlook suggests slightly warmer than average SSTs will persist across the equatorial Pacific Ocean through the austral autumn, potentially cooling further by the austral winter. Four out of seven international models maintain a slightly warmer than average equatorial Pacific Ocean through the first half of winter. To the west of Australia, the Indian Ocean Dipole (IOD) remains neutral, normal for this time of the year. All six international models maintain neutral IOD values through to the austral winter. In terms of precipitation across Australia, the current outlook favours below average rainfall for western, central and southeastern parts of the country. Across NSW and Qld, the outlook is for average-to-above average, mainly across the Qld coasts. This excludes any extreme weather events such as tropical cyclones or East Coast Lows. Across the north, the North Australian Monsoon (NAM) has been quiet for most of the wet season, with the current prognosis suggesting below average rainfall for March but above average in April. Looking further ahead, with no significant signature from ENSO, there are no significant climate drivers tilting the balance towards a drier or wetter end of autumn/winter. The current consensus, however, suggests that average to above average rainfall is a possibility across northern, western and parts of the inland south east.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.