12-Month Rainfall Forecast
Central West 12-month Rainfall Forecast
Rainfall deciles10Well above normal8 - 9Above normal4 - 7Near normal2 - 3Below normal1Well below normal
Issue NotesIssued 13 May 2020
ENSO status: Neutral IOD status: Neutral SAM status: Trending neutral Sea surface temperatures (SSTs) across the equatorial central Pacific remained stable throughout April with the Nino3.4 index registering a value of 0.5. On the other hand, the Southern Oscillation Index (SOI) recorded a value of -0.5 during the same month. The current outlook suggests slightly warmer than average SSTs will persist across the equatorial Pacific Ocean during the end of autumn and early winter, potentially cooling by the second half of the austral winter. Eight out of eight international models continue to suggest a cooling trend of the NINO3.4 region throughout the Austral winter, with all models on the cooler side of neutral by September. The current International Research Institute for Climate and Society (IRI) suggests a 40:35 chance for neutral:La Nina conditions by the Austral spring. To the west of Australia, the Indian Ocean Dipole (IOD) remains neutral, normal for this time of the year. All six main international models, however, continue to hint at a possible negative IOD establishing from mid-to-late winter. In terms of precipitation across Australia, the current medium term outlook continues to favour above average rainfall for most of the country. This outlook is mainly driven by warmer SSTs across northwestern Australia, enhanced soil moisture across some parts of the continent and early indications of a cool phase in the Pacific Ocean, which increases the moisture content across the northern tropics. Across parts of southern Vic, NSW and Qld (east of the Great Dividing Range), southwestern WA and Tasmania the outlook is for average-to-slightly below average rainfall over the coming months as these regions transition to their â€˜drierâ€™ part of the year (eastern seaboard) or tend to be less influenced by the IOD (SW WA and Tasmania) but more on the phases of the Southern Annular Mode (SAM). This doesnâ€™t include any severe weather events like east coast lows.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.