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12-Month Rainfall Forecast

  1. Melbourne 12-month Rainfall Forecast

    Apr
    24
    May
    24
    Jun
    24
    Jul
    24
    Aug
    24
    Sep
    24
    Oct
    24
    Nov
    24
    Dec
    24
    Jan
    25
    Feb
    25
    Mar
    25

    10

    5
    0

    9
    4
    6
    5
    4
    6
    5
    10
    6
    7
    7
    5

    Rainfall deciles

    10
    Well above normal
    8 - 9
    Above normal
    4 - 7
    Near normal
    2 - 3
    Below normal
    1
    Well below normal

    Issue Notes

    Issued 16 Apr 2024

    ENSO status: Inactive. El Niño ended. IOD status: Neutral, becoming positive. SAM status: Positive, favouring positive. The El Niño Southern Oscillation (ENSO) has returned to an inactive phase for the first time since August 2021. Ocean cooling is occurring in the eastern Pacific, with 3 out of 7 global models forecasting a La Niña to develop in 2024. Warmer than normal waters are situated in the Coral Sea, providing moisture and humidity to the east coast. The atmosphere continues to be firmly neutral after weakening early in the year. A neutral ENSO typically has no significant impact on Australian rainfall during winter. The Indian Ocean Dipole (IOD) is in a neutral phase. Ocean temperatures are high across most of the Indian Ocean, and the IOD index is spiking in response. Persistent southeasterly winds have set in over the eastern Indian Ocean, signalling that the atmosphere is starting to respond to the ocean patterns. All international models are suggesting the IOD will strengthen into apositive phase over the next 2-3 months, with significant upwelling to start near Indonesia possibly in May. A positive IOD reduces the number of northwest cloudbands that cross Australia during winter and spring, reducing rainfall across central and southeastern parts of Australia. The Southern Annular Mode (SAM) is positive and will favour a positive phase during autumn and into winter. A positive SAM increases easterly winds, increasing rainfall and thunderstorm activity in the east. A positive SAM also reduces cold fronts and rainfall over southern Australia, particularly for southwest WA. Rainfall outlooks are showing below average rainfall over winter for WA, and the southwest coasts of SA, Vic and Tas. Conversely, above average rainfall is expected for eastern parts of Qld and NSW, and potentially eastern Tas.

    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.