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Lower West 28-day Rainfall Forecast
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- 28-day
- 12-month
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| 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 now a 5-wave pattern. The main troughs are near the longitudes of the eastern Indian Ocean, the Date-line, and the central Atlantic Ocean with lesser troughs in the western Indian Ocean and east of South America. The pattern appears to be re-organising to a more normal 4-wave set-up. It is still quite meridional (predominantly north/south orientation). Consequently highs and lows in the Australian region should continue to move rather slowly from west to east over the next couple weeks.
Summary:
Over southern and eastern Australia the major spring cold front events likely to bring widespread rain are expected about 06-09 September, 16-21 September, 26-28 September and 04-05 October. Events limited to more southern regions are likely about 26-30 August and 03-04 September. Rain events originating in the tropics and moving south are possible about 27-29 August, 17-20 September, 25-27 September and 04-07 October.
Over Western Australia the strongest winter cold fronts should occur about 12-14 September, 23-25 September. 01-03 October and 08-10 October. A lesser event is likely about 06-08 September. Rain events moving in from the north are possible about 27-28 August, 13-14 September, 24-25 September and 02-03 October.
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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