Central 28-day Rainfall Forecast

period
28-day
12-month

Sun

Mon

Tue

Wed

Thu

Fri

Sat

11
med

12
med

14
med

15
med

16
high

17
low

20
low

21
high

22
high

23
high

24
low

29
low

30
high

Oct 1
high

2
high

3
med

4
med

5
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 5-wave pattern with 3 main troughs and 2 minor ones. The main troughs are near the longitudes of the eastern Indian Ocean, the eastern Pacific Ocean, and the Greenwich Line. The lesser troughs are in the central Indian Ocean and mid- Tasman Sea. The pattern is only slowly 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 of weeks with the Indian Ocean trough moving the the Great Australian Bight. The persistence of the unstable pattern has led to some changes in timing of events.
Summary:
Over southern and eastern Australia the major spring cold front events likely to bring widespread rain are now expected about 10-15 September, 21-23 September, 30 September to 03 October and 18-20 October. Events limited to more southern regions are likely about 04-05 October and 09-11 October. Rain events originating in the tropics and moving south are possible about 13-14 September, 29 September to 03 October and 16-19 October.
Over Western Australia the strongest winter cold fronts should occur about 11-15 September, 17-18 September, 25-27 September, 12-14 October and 22-24 October. A lesser event is likely about 06 October. Rain events moving in from the north are possible about 13-14 September, 17-18 September, 25-27 September and 12-14 October.

Issued Sep 8

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.