ACT 28-day Rainfall Forecast

period
28-day
12-month

Sun

Mon

Tue

Wed

Thu

Fri

Sat

27
low

28
low

29
low

30
high

31
low

Sep 1

8
low

9
med

10
low

18
low

19
med

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 central Indian Ocean, the Date-line, just east of South America with lesser troughs near the Great Australian Bight and west of South America. The pattern is quite meridional (predominantly north/south orientation) especially in the Australian region. Consequently highs and lows in that area should continue to move rather slowly from west to east over the next couple weeks before the pattern resumes it more normal 4-wave set-up.
Summary:
Over southern and eastern Australia the major winter cold front events likely to bring widespread rain are expected about 10-13 August, 18-20 August, 26-30 August, 06-09 September, 16-21 September and 27-28 September. An event limited to more southern regions is likely about 02-03 September. Rain events originating in the tropics and moving south are possible about 27-30 August, 07-10 September and 18-20 September.
Over Western Australia the strongest winter cold fronts should occur about 23-26 August, 02-05 September, 12-14 September and 24-26 September. Lesser events are likely 17-18 August and 30-31 August. Rain events moving in from the north are possible about 24-25 August, 03-05 September, 13-14 September and 25-26 September.

Issued Aug 11

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.