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Sydney 28-day Rainfall Forecast
- period
- 28-day
- 12-month
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med
med
med
low
low
high
high
low
low
high
high
low
| Chance of rainfall within district | |||
|---|---|---|---|
| nil < 25% |
low 25% to 50% |
med 50% to 75% |
high ≥ 75% |
Issue Notes
The hemispheric long wave pattern is still a 4-wave pattern. The blocking feature previously present near South Africa has now mostly dissipated but a new one is forming east of New Zealand. The main troughs are near the longitudes of the eastern Indian Ocean, New Zealand, just west of South America and central Atlantic Ocean. Highs and lows in the Australian region should now move fairly steadily from west to east over the next few weeks.
Summary:
Over southern and eastern Australia the major winter cold front events likely to bring widespread rain are expected about 19-22 July, 06-09 August, 15-18 August and 25-28 August. Events limited to more southern regions are likely about 15-18 July, 28-29 July and 02-04 August. Rain events originating in the tropics and moving south are possible about 08-10 August and 26-29 August.
Over Western Australia the strongest winter cold fronts should occur about 15-16 July, 24-27 July, 02-06 August, 12-15 August and 22-25 August. Lesser events are likely 15-20 July and 31 July to 01 August. Rain events moving in from the north are possible about 18-20 July. 03-04 August and 23-24 August.
Forecast Explanation
This forecast is produced by looking at the movement and strength of the long wave pattern, and its effect on short wave pressure systems such as lows and cold fronts. The daily movement of long and short wave patterns is monitored and projected in order to determine regions where long and short wave troughs are likely to coincide over the coming month.
If long and short wave troughs coincide or amplify there is an increased likelihood of a rain event across a large area. Where long and short wave patterns do not coincide, but rather cancel one another, the likelihood of a rain event is reduced.
Rain events are presumed to move steadily from west to east and to track the general movement of the long wave pattern, thus providing an indicator of where and when rain is most likely.
Regions where troughs are expected to amplify are assigned a high rainfall probability. Whereas regions in which the long and short wave troughs cancel each other have a low rainfall probability. On days where there are no long or short wave troughs affecting a particular region, it is likely to be dry.
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