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12-Month Rainfall Forecast

  1. Lower Western 12-month Rainfall Forecast

    Aug
    20
    Sep
    20
    Oct
    20
    Nov
    20
    Dec
    20
    Jan
    21
    Feb
    21
    Mar
    21
    Apr
    21
    May
    21
    Jun
    21
    10
    5
    0
    7
    7
    8
    7
    8
    7
    9
    8
    5
    6
    8

    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 17 Jul 2020

    ENSO status: La Nina Watch IOD status: Neutral SAM status: Trending neutral to positive Sea surface temperatures (SSTs) across the equatorial central Pacific have continued to exhibit some cooling over the past month, with the Nino3.4 index registering a value of -0.1 in June. The Southern Oscillation Index (SOI) recorded a value of -9.6 during the same month. The current outlook suggests cooler than average SSTs will continue across the equatorial Pacific Ocean during winter, potentially reaching La Nina thresholds during spring. All eight international models continue to suggest a cooler side of neutral for the region throughout the remainder of the Austral winter and through spring. The current International Research Institute for Climate and Society (IRI) outlook suggests a 55:50 chance for La Nina:Neutral conditions by the end of the Austral spring. To the west of Australia, the Indian Ocean Dipole (IOD) remains neutral. Three of the six international models indicate a negative IOD becomingestablished during the Austral winter or early spring, with the remaining three indicating a neutral phase. Of the three neutral models, two are trending slightly negative within the neutral zone by early spring, while one model (BoM’s outlook) is trending positive. The BoM model does trend to neither positive or negative by late spring. In terms of precipitation across Australia, the current short-term outlook continues to favour below average rainfall during July, except for NSW and Qld east of the divide where there are indications of above average rainfall. The below average rainfall forecast for July is driven by the breakdown of what was likely to be a negative IOD until a late season tropical cyclone (Mangga) cooled waters off the northwest coast. The above average rainfall forecast for eastern NSW and Qld is driven by the likelihood of coastal troughs, rainfall in the wake of cold fronts and moist trade winds in the northeast. Models then suggest above average rainfall for most of the easterntwo-thirds of the country, and central western and inland southern WA, for late winter and early spring. Southeast SA, much of Victoria, western and central Tasmania and southwest WA are not showing any signals of above or below average rainfall during this period, most likely due to a trending positive SAM. The SAM has been mainly negative in June and early July. The northern Australian dry season is continuing, with northern WA likely to see less rainfall than northern NT and QLD due to dry winds.

    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.