About the Masked Rainfall Percentages Maps


The maps displayed in the drought section of the Weekly Rainfall Update are masked rainfall percentages maps. These maps show the percentage of mean rainfall that has been received for specified periods. The time periods of the maps and the grey masking is determined by the most recent Drought Statement.

At the beginning of each month drought periods are defined in the Drought Statement. These are periods of time where areas of Australia have been considered to have suffered from serious or severe rainfall deficiencies. The term serious and severe are defined as rainfall so low that it falls within the bottom 10% of records (Read more about rainfall deficiencies).

In the Weekly Rainfall Update, these drought periods defined in the drought statement are evaluated on a weekly basis, to see if rainfall that occured during the previous week has had an impact on the rainfall deficits. This can be done by looking at the masked rainfall percentages map. Only the regions that have experienced serious or severe rainfall deficits in the most recent drought statement are displayed on these maps; all other regions are shaded in grey. The maps have the same start date as the drought periods discussed in the drought statement, but the end date is always the Tuesday on the date of issue of the latest Weekly Rainfall Update. The maps show how much rainfall, as a percentage of the mean rainfall for that period, was received in the rainfall deficient regions. Percentages that are below 100% are below the mean rainfall for that particular period. Read more about percentages of mean rainfall.

Example Maps


Figure 1. Rainfall deficiencies: 6 months Figure 2. Masked rainfall percentages for the 6-month period in Figure 1 to date

Figure 1 shows the rainfall deficiencies map for the 6 months starting 1 January 2010 ending 30 June 2010, while Figure 2 shows the masked rainfall percentage map also starting on the 1 January 2010, but ending on Tuesday 19 July. Figure 1 shows that in the 6 months ending 30 June 2010 much of western WA recorded rainfall so low that serious to severe rainfall deficits are now evident rainfall that in the lowest 10% of records for that period). Figure 2 shows that by Tuesday 19 July large areas of the central WA coast, and much of the Pilbara and Gascoyne districts recorded less than 60% of their mean rainfall for the January to June period (i.e. the rain which fell during July up to the 19th has been added to the total for January to June). Some sites in the Pilbara and Gascoyne had recorded less than 30%. Further south, much of the southwest of WA has recorded between 70 to 90% of their mean rainfall for the period, with several sites recording a little less than 70%.

© Australian Government, Bureau of Meteorology